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7.62x39 Rifle Build
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  Tanner Frisby

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Posted: March 05 2013 at 10:09pm | IP Logged Quote Tanner Frisby

(Version 2 and subsequently 2.5)

Hello all,

It has been too long since my last build, and with an upcoming 10 day weekend starting Friday, it's time for a 2 week build (or else school starts up and I might have to set this aside).

I started designing this firearm Monday and hope to have a ready to fire arm by next Monday.

Design:

There was a lot of talk in the VG 1-5 on "how easy it would be to make," yet action has been retarded to words instead of builds. To show that we're not just a bunch of hot air, I decided to design my build around the gas delayed mechanism.

Barrel length: 20.5" stolen from an sks
Overall length: ~29"
"Bullpup" style action
AR 15 recoil spring
Trigger group similar to inabadhood's gif.

With the exception of the bent receiver, I have most of the design stamped out. I have included a 3d pdf. For those not familiar with them:
Open with adobe reader, left click the image to "activate," left click part to select - right click - part options - hide or show all, scroll to zoom, drag to rotate, etc.

Purple: "Receiver": 12 ga sheet and tubing (undefined)
Yellow: Bolt: blade steel (high carbon) (impact plate) .134", .120 sheet iron, aluminum insert (for extractor and firing pin precision)
Dull Pink: Trunion: Aluminum 6061 (cast)
Cyan: 5/8 OD 3/8 ID DOM tubing
Bright Pink: Piston: .25 rod, 3/8 OD .25x ID ERW tubing
Indigo: Bolt p2: Aluminum
Green: AR 15 Spring
Orange: Barrel
Black: AK Magazine (mechanical stub)

PDF

I have access to a printer on Saturdays, so I will have to finish the design by Friday. The machine is not all that great (poorly calibrated) but should get the rough shape right. Tolerances can be as bad as 1mm ( ) and edges are rounded instead of sharp, but it is what it is.

Enter Questions:

1. I designed the trunion to support the barrel for 3.4", is this enough?

2. "Indigo" is .5" thick and affixed through a 3/8" hole (that will probably be set screwed). I can foresee a torque problem with the spring, should I make indigo thicker or should it be alright as is. Should I add a second rod beneath the barrel for support?

3. Bolt weight? Should I target some number or as is?

4. Gas seal? the 3/8 OD fits very snugly inside the 3/8 ID. Very minimal wiggle with 1" depth, but a minimal amount of gas can still flow. Good enough? The .25 rod wiggles and will need some kind of seal. I was thinking a thin foam sheet with a hole firmly pressed against the rod, but am worried about wear and erosion from the gasses. Suggestions?

5. Thread length between piston and bolt as to not get a shearing off? I have it at about 1/2" right now. Enough? Too much?

6. I have 1/8" rod I intend to use as a firing pin. It is mild steel. Any problems? (AR 15 hammer spring)

7. I have never owned an AK 47, nor touched one. The magazine is different from any other I have used. It seem to need to "rotate" into place instead of being "inserted". Is this correct? How is it locked in the back? How is it released? The geometry of it seems weird.

8. Aluminum shrinks when it cools (~1-2%). The lost PLA article guy uniformly enlarged his model 102%. As pertaining to a holes inside the trunion, do I make the OD larger, anticipating it to crush the mold as it shrinks inward to the correct diameter, or do I make the OD smaller, anticipating the aluminum to shrink away from the hole to the correct diameter? The uniform enlargement model suggests the prior.

9. Should the barrel be supported other than just the trunion? Would this decrease accuracy (not that I really care)?

10. I was planning on bending thin sheet metal over this design. What thickness bends easily and has enough rigidity (mainly concerned over the spring area, as the rest is reinforced by the receiver). How do you go about bending cleanly?

11. Gas venting hole is .125" D and 1/16 - 1/8 from the end of the case. The vent holes above it are 3/32 (to provide a little wiggle room). Is the diameter too large? Too small? Goldilocks? Tips for drilling it? Slow speed, lots of fluid and patients? I placed holes through the trunion to help with wander. I bet a barrel is a pain to drill into.

12. Do you see any flaws (other than looks, but can't help those too much)


Answer any and all you can.

Thanks,
Tanner Frisby
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  northumbrian

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Posted: March 06 2013 at 1:26am | IP Logged Quote northumbrian

Cool, looking forward to seeing this
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  Tanner Frisby

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Posted: March 07 2013 at 10:57pm | IP Logged Quote Tanner Frisby

Hello again,

Still designing, but close to finished. I hope to finish designing tomorrow and start printing off the 1:1 cut outs.

Updated Files:

Assemblage

Bolt

And for those who don't have time to deal with the hassle of downloading the .pdf's and figuring out how the file works:



I tried to keep the color scheme the same, but it looks like it is a little different:

Gray: "Receiver": 12 ga sheet iron
Yellow: Bolt: blade steel (high carbon) .134" {Yellow}, .120 sheet iron {Pink}, aluminum insert (for extractor and firing pin precision) {Green}, .125" mild steel firing pin [Gray], 1/8" x 32 screw {Blue}
Dull Pink: Trunion: Aluminum 6061 (cast)
Cyan: 5/8 OD 3/8 ID DOM tubing
Bright Pink: Piston: .25 rod, 3/8 OD .25x ID ERW tubing
Purple: Bolt p2: Aluminum
Dark Green: AR 15 Spring (now uncompressed)
Orange: Barrel
Black: AK Magazine (mechanical stub, revised)
Light Green: Trigger plate: 12 ga mild iron
Red: Hammer: 12 ga iron layered (x2)
Turquoise: Trigger group housing: Cast
Purple - Maroon: Disconector: .134 carbon steel
(accidentally omitted): Hammer Claw (claw opposite to disconector, but I don't know it's name): .134 carbon steel
Brown - Yellow: Magazine release: 3/8 OD .25x ID tubing, 12 ga sheet iron
misc. springs: AR 15 Hammer spring, various hand wound from OD's I have on hand. (some of the springs are ugly or omitted for the sake of time).

Questions that are still pertinent:

3. Bolt weight? Should I target some ideal weight, or is it fine as is?

4. Gas seal? The .25 rod wiggles and will need some kind of seal. I was thinking a thin foam sheet (Walmart craft foam) with a .24x OD hole such that the rod is firmly pressed against the foam at all times, but am worried about wear and erosion from the hot gasses. Suggestions?

8. Aluminum shrinks when it cools (~1-2%). The lost PLA article guy uniformly enlarged his model 102%. As pertaining to a holes inside the trunion, do I make the interior hole's (barrel hole and gas piston hole) OD larger, anticipating it to crush the mold as it shrinks inward to the correct diameter, or do I make the OD smaller, anticipating the aluminum to shrink away from the hole to the correct diameter? The uniform enlargement model suggests the prior.

9. Should the barrel be supported other than just the trunion?

10. I was planning on bending thin sheet metal over this design. What thickness bends easily and has enough rigidity? 0.030 - 0.040? How do you go about bending an arc cleanly? I figure if the sheet is bent around a mandrel, it will "unbend" a little to a larger diameter.

11. The gas venting hole is .125" D leaving the barrel vertically upward and is 1/16" - 1/8" from the end of the case (such that the bolt seals almost immediately). The vent hole above the barrel is slightly oversized for error that could occur when inserting the barrel. Is the 1/8" diameter too large? Too small? Tips for drilling it? Slow speed, lots of fluid and patients? I placed holes through the trunion to help limit wander. I bet a hardened barrel is a pain to drill into.

Thanks,
Tanner Frisby

(Hours to date: ~10 plus 2.5 for post write ups)
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  Mech warrior

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Posted: March 08 2013 at 12:05am | IP Logged Quote Mech warrior

Tanner, here are some opinions for you;

4. Gas seal? the 3/8 OD fits very snugly inside the 3/8 ID. Very minimal wiggle with 1" depth, but a minimal amount of gas can still flow. Good enough? The .25 rod wiggles and will need some kind of seal. I was thinking a thin foam sheet with a hole firmly pressed against the rod, but am worried about wear and erosion from the gasses. Suggestions?

Leather


9. Should the barrel be supported other than just the trunion?

Nah! The trunnion looks stout enough, if your worried, make it a little deeper to support more barrel. If the barrel fits it loosley after threading/pinning/however you attach it, you can always sliver solder it in!


10. I was planning on bending thin sheet metal over this design. What thickness bends easily and has enough rigidity? 0.030 - 0.040? How do you go about bending an arc cleanly? I figure if the sheet is bent around a mandrel, it will "unbend" a little to a larger diameter.

Even .030 will be tough depending on how you bend it! Right now i bend 16 guage with a bender from harbor freight, and it is pretty tough stuff! Granted that is thicker then .030 by more then 2X but needed in my case. You are correct about bending around the mandrel, it will slightly "unbend". Make the mandrel smaller or tweek it after it is off.......or if you have a roller you can roll it.


3. Bolt weight? Should I target some ideal weight, or is it fine as is?

depends, is it going to be locked upon firing or blow back?
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  Mech warrior

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Posted: March 08 2013 at 12:09am | IP Logged Quote Mech warrior

1. I designed the trunion to support the barrel for 3.4", is this enough?

Plenty!!!

I like that your a go getter!
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Measure, think about it, measure again, think about it some more, measure again, then cut......."Oh S***!!!!"

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  northumbrian

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Posted: March 08 2013 at 1:00am | IP Logged Quote northumbrian

Can think of anything more than what Mech Warrior has already said, other than do some research into heat treatment and levels of hardness.
It'll pay off all the time spend reading about it.

But, looking good so far
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  Tanner Frisby

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Posted: March 09 2013 at 11:20pm | IP Logged Quote Tanner Frisby

Hello again,

I finished the theoretical and am now on to the physical. Today (Saturday) was my opening to try to get my parts printed (locally). Sadly, due to technical errors, the printer is not functioning . In all likelihood, I don't think the 2 week goal is still feasible, but I'm not stopping. If the printer isn't working by next Saturday, then I will outsource my parts to the company I normally get prints from (more expensive, ~4 week delay, super high quality, super anti-gun).

Today I printed out the 1:1 pieces, and will start cutting metal tomorrow. I'd like to have all my metal at least rough cut if not fine cut tomorrow, but I've been a hair behind my goals to date. Here is the picture of the 1:1's:



I use a little notation scheme to help me identify what material to cut out of, what sub assembly it is part of and where in the sub assembly it is located.

I really enjoyed seeing gww250's shop in the Mac-in-the-box thread, so here's my battle station:

"Clean" station: used for designing theoreticals.



"Filth" station: Welding, heavy grinding, forging and other gritty activities. I cleaned it up for the sake of the picture. Also houses my stock metal in the corner.



"General Purpose" station: 90% of work is done here. I keep it as clean as the project at hand requires/permits.



Harbor Freight cheapo drill press with a cross vise for limited milling capabilities.



Foreground: secondary work bench, usually housing things that are being worked on, but not being worked on right now.
Midground: Main work bench, if I'm working on it, I'm probably doing it here.
Back ground: Small misc. screws, pen shaped devices (punches, pens, center punch, x-acto knives) etc.



Background of the previous description. Beneath are a bunch of old projects that in all honest, I have no plans of ever going back to.



Tool array: files, clamps, drill bits, you name it. Storage above, more misc tools and items.



Storage: springs on top, sand paper and abrasives in the first drawer, knifes / blades / knife making materials in the second drawer, other hobby storage in the third drawer, misc. gun related parts and devices in the bottom two.

Hours to date: ~19 all included.


The guy who offered to print my parts is very mechanically inclined. He has been helping me out on my projects for about 2 years. We frequently butt heads on what will work and what won't work (from theoretical to actual). He pointed out a few concerns (bolt weight and gas vent hole diameter) as problem points, or "points for controlled testing." He's wagering that this build won't work off of just my intuition for parameters. I'm still going to try it, but I can count the number of times I have proved him wrong on zero hands.

Thanks,
Tanner Frisby
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  Mech warrior

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Posted: March 10 2013 at 12:06am | IP Logged Quote Mech warrior

Thats a good set up you got goin there! About the same tools i have to work with, but recently, i did buy some good american made drill bits after burning out most of the ones in that black box! Those harbor freight ones sharpen ok but don't last, the american made HHS bits can hold an edge a lot longer and sharpen easier.

I sharpen my bits by hand with my grinder.

Don't tell me your not going back to the beltfed shotgun! I still think .50 BMG links might work!
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  northumbrian

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Posted: March 10 2013 at 5:58am | IP Logged Quote northumbrian

It's alway interesting to see another persons workshop, you can often gleam neat little ideas for your own, thanks for the pics.
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  Viper Dude

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Posted: March 10 2013 at 11:01pm | IP Logged Quote Viper Dude

Hello TF,

Sounds llike you are a student on Spring Break.  Hopefully you are an Engineering student as most universities' Engineering Science Depts. have resources to help engineering students achieve their upper level design projects or grad research thesis.

Access to a metal lathe, real milling machine, and machinetool lab can really boost your project along.  Some universities are rabidly anti-2nd Amend so related work must be a bit covert.

The "hacksaw-file-hammer" school of gunsmithing will require a great deal more labor and time to make things. 

I gather that your design uses a piston/op-rod to pull the bolt forward to retard breech action as in the German VG-1-15 of WW-II.  That type usually requires a fluted or oiled chamber or waxed ammo to preclude cartridge case separation.  The typical SKS barrel doesn't have the flutes.  Such a fluted barrel can be found for the 7.62x39mm M-42 round in the rare HK barrel chambered for it by the way. 

One small item... your firing pin should be hardened steel lest it mash on the ends from impacting.  Perhaps a hard concrete nail can be adapted... the kind with the double heads even. 

Above all... have fun !!!!

VD in AZ

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  Tanner Frisby

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Posted: March 10 2013 at 11:18pm | IP Logged Quote Tanner Frisby

@ VD
Yes, I am a student on spring break, but no, I am not a university student, I am still in high school. I will be attending A&M University in College Station, Tx next semester. They made my college selection decision really easy (they were the only university that accepted me ). Any Aggies out there? I should manage some shop time for my paper weight construction, but also remember firearms are forbidden on U.S. campuses, so all finishing of receivers will have to be done off campus (at a kin's workshop a few miles off campus).

This is the first I am hearing about the barrel issues. Can you please elaborate? What is a fluted chamber and what issues lead to the requirement of one?

I get what you are saying about the firing pin being mashed, but is it much of an issue? Assuming that the firing pin never needs to be removed, do you think I mild steel firing pin would have any problem functioning repeatedly?

Thanks for bringing up these points,
Tanner Frisby
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  Viper Dude

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Posted: March 11 2013 at 1:28am | IP Logged Quote Viper Dude

TF,

Mild steel is soft and doesn't heat treat for anything unless it is coated with certain chemicals such as "Casenite" or cyanide or carbon.  Use a pre-hardened concrete nail or adapt an existing firingpin off the shelf.

A "fluted barrel" has small axial gas grooves from the chamber throat back about 2/3 toward the cartridge head.  Those grooves allow high pressure gas to leak around most of the cartridge case so the case doesn't stick too hard at max chamber pressure.

This floating action is done in arms that use hesitation type operation or violent cycling that happens at high residual chamber pressure.  This is an issue with high power ammo used in auto rifles and some MG's.   Subguns generally don't have this problem as they are lower pressure and use short cased ammo.

It has been a very long time since I was a college student.  My grand niece is presently a college freshman and honor student.  Where I went to college we all carried M1 Garand rifles and wore cadet gray uniforms every day.  It was a pro-gun sort of place !!!

Texas A&M is a very good school by the way.  It is a huge place and easy to get lost there.  Stay focussed on your studies and strive for the very best grades.   I had to work really hard to keep my nose above water (ie C-level) as a student.  Later as a grad student things changed and I was able to do well.  Engineering was challenging and cool !!!

Best wishes for next semester (and this one too) !!!

VD in AZ

 

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  northumbrian

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Posted: March 11 2013 at 2:02am | IP Logged Quote northumbrian

Good luck with your studies
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  Fantomen

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Posted: March 11 2013 at 6:06am | IP Logged Quote Fantomen

Hi, I just wanted to say that the spring around the barrel might not be such a good idea.

I'm not an expert, but I think the heat from the barrel might damage the heat treatment of the spring steel, causing the spring, and subsequently the gun to malfunction.


As previously mentioned, I'm not an expert.
It's very possible I'm wrong.
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  Tanner Frisby

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Posted: March 11 2013 at 10:51am | IP Logged Quote Tanner Frisby

I had a slow connection last night and didn't have time to upload pictures:



Sharpie traced outline, not very visible and easily wiped off.



Masking tape transfer. Some permanence and high visibility.



General shape is cut out and edges are filed smooth.



Finished rough cut.


I diverted attention away from the cut outs yesterday to the casting portion. I have never cast in aluminum before, so I will probably not get it right first try. I also don't know what sprue sizes I need and how well casts come out. Before I started metal craft, I use to make Rubik's cube like puzzles. I have an excess of crappy Rubik's cubes, so I took 4 corner pieces to test, and 1 to keep as a control. I varied sprue diameter, piece orientation, quantity and viscosity of the mold.



Left: Large sprue, 1:1:1 sand : gypsum : water. Very fluid pour, felt like pouring water. Highest hopes.

Mid: double stack, Large Sprue - Small Gate, 1:1:0.9. Poured well, can't wait to see how it turns out.

Right: Small sprue, 1:1:0.8 water. Thickest I felt comfortable pouring. low hopes, but lets see.



Stirring was a pain, so I ended up using a tossing method of transferring the mix from one bowl to another until it was uniform. The sand used was very fine, the finest available at my Lowe's.



Mixing



This is the 1:1:1 mix. You can still see grains (and a few bubbles), but it poured very well.

Well, I hope to burn out and pour these practice molds this afternoon, and finish the rough cutting.

~25 hours

Thanks,
Tanner Frisby
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  northumbrian

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Posted: March 11 2013 at 2:20pm | IP Logged Quote northumbrian

I always try to shape a sprue so it tapers down toward the casting, that way there is less cleaning up afterwards.

The best thing about casting aluminium is if you bugger it up, you can re-melt it and try again with relative ease.

but it is good to see you making progress.
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  gww250

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Posted: March 11 2013 at 9:49pm | IP Logged Quote gww250

Great work on everything. If you're ever up Plano way drop me a line and we'll talk about build projects.
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Posted: March 11 2013 at 11:48pm | IP Logged Quote Tanner Frisby

Well, another update from today.

I started off working with the molds again. I remembered reading somewhere that the molds needed to be dehydrated completely before pouring the aluminum into them, or else the aluminum will flash boil any water left in the molds and cause safety issues. I broke out my forge (the only tool I have ever made that works. To those who make their own tools, you have my greatest respect). I put the can in and tuned my forge to its lowest setting, ~ 1 atm, and took a look. It was oozing water. Perfect right? So I step back to pull out the camera and the can explodes. Well, a data point is a data point, however not the kind I was hoping for. The mold seems to have worked very nicely though, filling in all cavities cleanly.









After this incident, I decided to take a more controlled method of curing / drying the molds. Baked at 250 for 10 min, 300 for 10 min, 350 for 10 min, 400 for 20 min, 450 for 20 min, 500 for 2 hours. The best I can tell, the molds only got to 465 F, which I believe is my oven's limit. I was going for a plastic burn out also, but it didn't happen. I checked on it periodically to see how it was doing. I noticed little bubbles floating out of the sprue, going about 1/8" above the top of the sprue (out of the mold) and then hovering laterally. It was the darnedest thing. So I'm looking at it for a while and I notice a splashing sound. Turns out their was some sort of clear liquid over the top of the mold that was overflowing down onto the bottom of the oven. I poured the molds into a can I had laying around and believe the liquid to be the thin straw, as it only came from the thin strawed mold. This provided for a fun clean up.      

(none of the pictures clearly indicate anything except two coffee cans in an oven)

Once the molds had dried, I stuck the two remaining intact molds into my forge on about 15 - 20 psi. (40 max, 1 atm min) The plaster and can were glowing orange after about 2-3 min. I let the molds bake for about 30 min. before shutting off the forge. The two cans had flames coming out of the sprues that persisted for another 20-30 min, letting of environmentally friendly heavy black smoke. Smelled horrendous .





I pulled them out of the forge after they had stopped burning and set them on a fire brick. They were cracked along the top, but they look fine. I dropped one (after absent mindedly picking it up bare handed), but the chip seems only to be superficial.

I also made my crucible. It is about 12 cubic inches, but will probably safely hold about 8. It is made from 2" x 2" square tube that was been wire brushed inside to remove contamination. I welded a bottom onto it and a long arm so that I don't need to where gloves when using it.



I didn't have any compressed air so I was unable to finish the test, but I'm hoping to try to cast tomorrow.

I did finish cut the left side plate, but my angle grinder crapped out on me. I have spares still in their boxes, but I decided to call it quits for the day on metal cutting. My finish cutting method is to take the paper 1:1 and scotch tape it to the metal rough cut around its perimeter, and then grind out any excess metal.



While waiting on the molds to heat up, I did manage what I consider a huge feat. The main thing holding me back from my 2 week goal (besides the ridiculously short time period) is the 3d prints. I design my parts based on the intention of their construction. For example, if a part is to be constructed out of sheet steel, then I will design it to 14 ga, 12 ga, or 1/4". I will not design it to some odd ball not metric not imperial length. For 3d prints, it can be a different story. Still, with access to the printer up in the air, I decided to take my ghetto milling set up for a spin. Using plastic stock of various thicknesses (again from my Rubik's cube puzzle building days) and some excess super glue, I was able to make one of the parts intended for 3d printing. It wasn't easy, but I think I can do the same for the outlying pieces, with the exception of the trunion, which I still hope to receive from my local 3d printer.






Questions:

The bolt is designed to be ~4.5" long. I couldn't find any concrete nails that long. I have designed the firing pin to be 1/8" (adjustable if needed). Is their a standard source for a firing pin of this size? Can you think of another source?

Barrel: I did a little research into barrel fluting and it seems you are spot on. You stated "usually": "That type usually requires a fluted or oiled chamber or waxed ammo to preclude cartridge case separation." What might an exception be and why? Also, is their a feasible way of fluting the barrel with a back yard set up? What does an oiled chamber entail? Oiling before you go to the range or oiling between shots, etc.? I'm not planning on shooting this too much (I'll be lucky if I get 1 magazine emptied per year ) and wouldn't mind a little impediment, given that the design works.

@ Fantomen:
"I'm not an expert, but I think the heat from the barrel might damage the heat treatment of the spring steel, causing the spring, and subsequently the gun to malfunction."

This is a good point which I hadn't thought about. The spring might be subject to air cooling, as it has a lot of empty space around it, but I wouldn't want to rely on it. I guess I'll just shoot slowly and replace the spring if need be. Thanks for pointing this out.


Hours: ~32    ops: This is taking a lot longer than I expected!

Thanks,
Tanner Frisby

P.S. To any moderator reading, I am having trouble with the blush emoticon. Its script is " : o o p s : " but it keeps reading as " : o " and " o p s : ".
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  backbencher

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Posted: March 11 2013 at 11:59pm | IP Logged Quote backbencher

Well, a gas-delayed blowback rifle hasn't been tried since '45, but you definitely seem to have a plan for it.  And you're in high school?  You, son, are going to make a very fine Texas Aggie.

Fluted chambers aid in extraction as the gas from the chamber also expands on the outside of the brass, keeping it from sticking in the violent extraction of the CETME & HK roller-lock delayed blowback designs like the HK-91.  Small flutes are cut from the mouth of the chamber to allow the gas down the side of the brass case.

You could cut flutes in your SKS barrel, or try steel case ammo the 1st time out.  If you plan on progressively limiting your gas port until the rifle operates properly, hopefully you'll have a useable range where it operates and doesn't tear the base of cases off.  You'll probably want to have a case extractor your 1st trip to the range.

Have you considered the fightin' Texas Aggie Corps of Cadets?  The Army Ordnance Corps could use you.

Farmers Fight!

backbencher '92, '09
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  northumbrian

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Posted: March 12 2013 at 2:03am | IP Logged Quote northumbrian

Well, your first castings turned out Ok by the looks of it well done. Your project is certainly taking shape.

Just an observation, at the end of your posts you put up how much time you've spent on it so far, is this a primary concern for you? Do you have a specific time period in which to complete this project?

Otherwise, may I suggest you stop worrying about time and you'll find this project will go a lot smoother.

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