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Hammer’s blowbAK pistol
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  orions_hammer

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Posted: March 06 2008 at 4:39am | IP Logged Quote orions_hammer

Hi!  I'm new here, but I've been lurking intently for the last few months.  Jestism, you've created a beautiful thing with the Challenge!

I've been working on a  very simple little design--a straight-blowback pistol built mostly from scratch but with a few leftover AK parts.  Hence the name "blowbAK"!  Blowback designs are easier to scratch-build, because you don't have bolt lugs, complicated trunnions, gas pistons etc--the back of the cartridge acts like a piston to shove the bolt back and cycle the action.

I've built a little Skorpion-looking prototype in 22lr, feeding from 10/22 magazines (hat tip: dcorb), and it fires!  But my first build is just a bit too rough to cycle reliably with the 22's puny blowback force, so it's manually cycled right now.  I'm planning on doing a second build using PPSh mags in 7.62x25 before the contest deadline of April 1, though!

I'm trying to keep these things buildable with:
  • Hacksaw (or, far better, a metal-cutting bandsaw).  A lot of stuff that looks milled can actually be made with a hacksaw!
  • Drill (or, far better, a small drill press)
  • Bench vise
  • Thread taps, files, hammer, other small tools
No lathe (don't own one yet!), no welding (because I stink at welding!), and ideally no milling (if I can possibly avoid it).  Major parts press, bolt, or rivet together.

Pistol builds are a bit easier to keep US-legal, because both 922(r) (the imported-parts-count statute), and the NFA barrel and overall length limits only apply to rifles and shotguns.  So when you replace your imported pistol grips, FCG parts, too-short barrels, etc with compliance parts on your rifle builds, you can then build pistols out of the leftover parts!

Materials:
  •  AK-47 hammer, trigger, disconnector, FCG pins, and pistol grip.  Imported parts are legal in pistols, so anybody that builds AKs probably has spares lying around; or centerfire sells 'em for $10.  Speaking of legal, the AK FCG has the "positive hammer disconnect" the ATF requires for semi-autos.
  • Thin (like 1mm/.040"/20ga) sheet steel, for making the receiver.  I used a piece of old galvanized sheet metal from a washing machine or something, and it works fine!
  • Thicker (like 2mm/.070"/16ga) sheet steel, for making a magazine latch.
  • Barrel and magazine.  I used a cheapo Numrich 22 barrel and 10/22 magazines; but any non-rifle cartridge will work in blowback mode.
  • 1.25x2" aluminum solid rectangle bar, for making an AK-sized forward trunnion, to hold the barrel and front of magazine in place.  I really prefer cutting and drilling aluminum, and it's lighter than steel too!
  • Steel bar stock, for making a bolt/bolt carrier.
  • Music wire, for making springs (it's amazingly easy to make springs!)
Overview of the build process:
  • Form the receiver.  I just modified an electronic AK receiver template, printed and glued it to the sheet metal, cut/drilled/filed the outline and holes, and bent up on an ordinary AK bending jig.  You don't need lower rails, just the top rails.  If you've done AK builds before, you're snoring already!
  • Cut off a slab of aluminum for the forward trunnion.  Measure and drill a hole for the barrel (you probably do want to measure this!), file the back of the trunnion to clear the front of your magazine, and slide in your barrel.  The hot-trunnion cold-barrel trick works great here, and aluminum is good at galling up and grabbing steel barrels.  Use a dremel cutoff wheel to make some room in the barrel/trunnion face for your extractor.  Cut rail-slots in the trunnion with a hacksaw so you can slide it down your receiver, and bolt it in place.
  • Bend up a trigger guard/magazine holder/ejector, and rivet into the receiver.  You'll do final shaping later, but make sure an ordinary AK pistol grip fits on the back of your trigger guard!
  • Fit a magazine latch into your trigger guard.  This needs a spring, which you can turn by hand.  Futz with the latch until magazines hang in the right place.  Rivet the latch into the guard.
  • Build the bolt.  The bolt can be really simple!  It needs slots in the sides to run along the receiver's top rails (hacksaw time!).  It needs one dimensioned cut along the bottom to scoop rounds off the magazine.  It needs a slot for an extractor, and a hole for the (floating) firing pin.  That's it!  You're done!
Honestly, it's actually less frustrating fabricating new blowback parts than it is to demill an AK parts kit!

(Edit: Thanks to Jestism for pointing out where the "upload photo" button is!)

Here's an overview of the 22 prototype, less the bolt:


Here's the back of the bolt, showing the firing pin sticking up over the AK-style hammer.  The bolt carrier is milled on this prototype, but I'm going to do a laminated bolt for version 2.0:


Here's a closeup of the bolt, unscrewed from the bolt carrier.  The firing pin rides in a hacksawed channel, held back from the breech face with a little 1/8" diameter spring:



Edited by orions_hammer
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Posted: March 06 2008 at 7:17am | IP Logged Quote Guests

Sounds "very" interesting...

As for pictures.. there should be a button right next to the question mark...
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  kmagg

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Posted: March 06 2008 at 8:21am | IP Logged Quote kmagg

Let's see some pictures.  Sounds like a lot of fun!
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Posted: March 06 2008 at 8:24pm | IP Logged Quote Guests

Great pictures....
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  orions_hammer

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Posted: March 07 2008 at 4:29am | IP Logged Quote orions_hammer

Here's the muzzle flash!


And here's a dissassembled pic.  The only thing not shown is the totally-stock AK FCG.  Note my crap-tastic custom-wound recoil spring--it looked a lot better before I tried to flatten it out where it passes the trigger group...


Just to whet your appetite for the 7.62x25 (CZ 52 barrel, PPSh 43 magazines) version, here's a little mockup of version 2 (using a handy stock-AK receiver).  Man, do guns ever look better in black!

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  orions_hammer

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Posted: March 10 2008 at 6:11am | IP Logged Quote orions_hammer

OK, I've got the plans for the second version working in CAD and with little paper mockups.  Designing onscreen or in paper is sure a lot more forgiving than metal!

Here are the real components in an orientation that should feed:


I've been using the vector graphics program inkscape to do the design--it's a pretty easy program to use, but it supports layers and "aliases" (copies that reflect changes to the original), which helps keep ye sane on a complicated drawing.  Red is the receiver (AK-style folded sheet metal), thin black is the bolt (laminated from 1/4" and 3/8" bar steel), blue is the magazine guide and latch, and purple is the trigger guard (both heavy sheet metal).


Here are the above (never-yet-built!) plans in .svg format (zipped, because the forum requires it): v2_plans.zip

I've got some primo metal picked out to build this prototype--it's 0.035" galvanized steel scraps leftover from some duct fabrication!  It's ridiculously floppy in a big sheet, but I'm always surprised at how strong this stuff gets once you put a few folds in it! 

Watch this space for parts, coming soon!
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  dutigaf

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Posted: March 10 2008 at 7:45am | IP Logged Quote dutigaf

very interesting to say the least... keep posting!!
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  Branny

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Posted: March 11 2008 at 12:29am | IP Logged Quote Branny

I’m huge fan of 7.62x25. I have converted my Remington 1858 to that ammunition. It has re chambered cylinder, six separate firing pins and 30 cal barrel liner. 7.62x25 is very hot ammo and I would really want to know what type of locking system you going to use.
Maybe you will let us know that in time but I’m not patient enough.
Keep posting.
Branny.



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  orions_hammer

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Posted: March 11 2008 at 2:20am | IP Logged Quote orions_hammer

Branny, I'm also a fan of the 7.62x25 Tokarev.  In the cheap eastern-bloc military surplus loadings, it's a tiny 30 caliber 85gr pill going 1500fps, which seems like it'd penetrate substantially deeper than most other autoloader pistol calibers.  I'm in Alaska, where we have a rather unique threat environment (wolves, bears, moose, wild dogs, etc) that's tough to service with any handgun.  Conventional wisdom is a hot-loaded 44mag (or higher!) revolver for the bigger threats, but I'd hate to have only 5-6 shots against a wolf pack.  Yes, 85gr at 1500fps is laughably inadequate against bear, but a 32 round(!) PPSh magazine at least brings up your odds!

Regarding locking, I'm going with "straight blowback"--no rollers, no lugs, no nuttin!.  The inertia of the bolt is the only thing holding the chamber closed, which actually works pretty well on pistol rounds.  Rifle rounds don't work so well with this system for two reasons:first, there's just a lot more pressure trying to push the bolt open; second, the pressure continues for longer due to the longer barrel.  I am definitely going to need at least a pound or so of metal in the bolt--I haven't fabricated the above design, so I'm not sure how big/heavy the bolt will be.  The PPSh (open-bolt submachinegun) safely uses a blowback design with 7.62x25, so here's hoping I won't get a kaboom on the first trigger pull!
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  orions_hammer

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Posted: March 11 2008 at 5:23am | IP Logged Quote orions_hammer

I found the plans for a PPSh41 at the biggerhammer site, and staring at their bolt I've decided to tilt the magazine a bit farther than shown in the plans above, which will give me a bit more metal underneath the round on the bolt face.  I notice Vorvon's Urban Brawler (which also uses PPSh mags) tilts the mags pretty steeply.  Then again, I'm using PPSh 43 mags and a CZ-52 barrel, so... hope it works!  The parts feed OK clamped to my desk, so hopefully they'll work in the pistol.

Oh, and today I built up a receiver, magwell, mag catch, and triggerguard-ejector from sheet metal! 


That's a sheet of galvanized steel duct, with a receiver printout glued on with Elmer's.  I trimmed out the receiver with a jigsaw (and fine metal-cutting bit), punched and drilled out the usual holes (man, I'm glad I bought those 5mm & 7mm drill bits!), and finished off the magwell and trigger holes with the jigsaw and a file.  0.035" steel files down very quickly.  A sane person would just buy an AK flat, but my magwell is pretty different, and it only takes half an hour to bang out one of these:


I then hammered over the top rail in my vice, and used my "custom" AK bending jig (made from blue 1 1/4" angle iron originally from a mattress frame!) to finish the thing off.  I don't have any dimples in my flat, so the bending jig can consist of plain angle iron plus two 1/8" flat alignment holes.  You thread pin punches through the holes to align the flat to the jig, fold one side, then flip the flat and fold the other side.  (Or use a real AK jig!)


OK, receiver complete!


And now it's the same story for the mag catch and trigger guard, in 1/16" mild steel sheet.  The thicker steel sure cuts a lot slower with my jigsaw!


Here are all the sheet metal parts, cut and filed off.  I did the magwell with tin snips, which worked fine.


Now for folding! I still hate folding sheet metal, but I'm slowly figuring out how to do it.  Without a hydraulic press, I've been doing these with a big hammer (cough!) and bench vice.  Steel bar stock sure makes nice mandrels for forming the sheet metal around, and the order of folding makes a huge difference in how easy it is to get at the various pieces.  The glued-on template instantly gets scuffed up and nasty, but it makes it so much easier to figure out exactly where to put the bends!  Here are the finished parts:


Receiver is at the top.  From left to right, the mag well (a sheet metal box), mag catch (folded around a 3/8" bar stock mandrel), and finally the trigger guard (folded around a 1/2" bar stock mandrel). 

It's definitely not saying much, but this is the nicest-looking AK trigger guard I've ever fabricated!   The weird vertical thing sticking up is the ejector.  Figuring out which way I made the bends on the trigger guard still makes my head hurt!


Coming soon: aluminum barrel trunnion!  Bar steel bolt!  Drill rod firing pin!  And FIRING!
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  dcorb

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Posted: March 11 2008 at 7:06am | IP Logged Quote dcorb

Good job creating parts out of flat metal  I am impressed. I am anxious to see the trunnion and bolt!

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  bikergunnut

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Posted: March 11 2008 at 9:42am | IP Logged Quote bikergunnut

hammer, i can weigh a cz bolt if you want .
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  orions_hammer

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Posted: March 11 2008 at 3:16pm | IP Logged Quote orions_hammer

Biker, thanks for the offer!  But the CZ-52 pistol is roller-locked (little hardened steel rollers hold the slide to the barrel during the first phase of firing) and recoil-operated, so I suspect the barrel+slide combination would be substantially lighter than a pure blowback design like I'm trying to build.

Now, if you had a PPSh, I'd be (jealous and) very curious what the weight of the bolt is!  From the plans, it looks like the PPSh bolt is maybe 0.5"x1"x4.5" on top plus 1"x1"x2.5" on bottom, for a total of 5 cubic inches of steel (at the most, since I'm ignoring the many holes, slots, and top/bottom overlap).  Steel's a little under 1/3lb per cubic inch, so 5 cubic inches is 1.4lbs (upper weight limit, probably lower in practice).  Of course, in a submachinegun the weight of the bolt not only holds the bolt closed during fire, but also determines the cyclic rate, so the PPSh bolt is probably heavier than needed for safety.  I'm hoping for about a 1-lb bolt (a pistol can't be too heavy, after all!), which will take about 3.5 cubic inches of steel (e.g., 1"x1.25"x2.8").

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  toomanymachines

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Posted: March 11 2008 at 3:52pm | IP Logged Quote toomanymachines

I have a ppsh and a hugo machine pistol  you know the copy of the mp40, and I can tell you that the ppsh is a much lighter bolt.  I can try to weigh the bolt form the m53 as it came from the factory. not a semi conversion. 
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  orions_hammer

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Posted: March 11 2008 at 6:03pm | IP Logged Quote orions_hammer

Actually, regarding the bolt weight question there's a good entry at some cool firearm design forum where Buspete said: "We weighed bolts from a Sten, Uzi, FMK-3, Baretta 38, PPSh41 and PPs43 and all were in the 1.3-1.5 lb range."  This agrees disturbingly well with my density calculations above!

One way to look at it, is it's the same pressure both pushing the bullet down the barrel and the cartridge/bolt out of the chamber.  The bullet weighs 85gr, or 85/7000 = 0.012lbs, and will reach 1500fps travelling 4 inches (the barrel length).  If the cartridge/bolt weighs one pound and the same force acts on it for the same length of time, the bolt will reach 0.012 times the velocity (18fps, a typical healthy slide recoil speed) and hence cover 0.012 times the distance (50 thousandths, less than 1/16") by the time the bullet leaves the barrel.

So I'm not that worried about a kaboom.  The malfunction I'm trying to avoid is what happened with my 22 build, where the cumulative drag of the internal chamber friction (especially in my rough drill-bit-cut chamber!), hammer spring (full-strength AK), recoil spring, and all the roughness in my handmade parts is too much force for the cartridge's short push-back to overcome.  There's a reason the HK G3 and MP5 (roller-delayed blowback) use fluted chambers--it cuts the chamber friction so the blowback force is high enough to cycle the action!
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  Branny

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Posted: March 11 2008 at 9:03pm | IP Logged Quote Branny

Sorry about this post,  wrong tread.



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  bikergunnut

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Posted: March 11 2008 at 9:49pm | IP Logged Quote bikergunnut

hammer, my CZ26 bolt weighs 22.8 ozs . my ppsh bolt has been modified so..
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  dutigaf

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Posted: March 11 2008 at 10:00pm | IP Logged Quote dutigaf

my ppsh bolts all weigh different...........

all are over 1 lb....  all the weights are posted in the ppsh thread
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  orions_hammer

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Posted: March 12 2008 at 12:46am | IP Logged Quote orions_hammer

Thanks for the weight, Biker!  I mistakenly thought "CZ + Tokarev = CZ 52"; didn't even know about the CZ 26.
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Posted: March 12 2008 at 1:32am | IP Logged Quote bikergunnut

that's what the "hot" ammo is made for. you can shoot a baseball sized object (rock) at 100 yrds with the cz52. never use that ammo in anything else .
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