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Hammer’s blowbAK pistol
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  orions_hammer

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Posted: March 13 2008 at 4:30am | IP Logged Quote orions_hammer

OK, I fabbed the front trunnion today!  The front trunnion's job is to hold the barrel to the receiver, and optionally to act as a front sight--I've sketched a little rail on the top for a red-dot sight, although I don't know if I'll use it.

The trunnion started life as a chunk of aluminum bar stock:


I cut two slots in the trunnion (for the receiver's top rails to fit in) using what will be a recurring technique in this build: a metal-cutting bandsaw used as a slotting/milling machine.  A hacksaw would also work, but it would take a *long* time. Frankly, of all the power tools in my shop (aside from my drill press) the most time-saving tool is my metal-cutting bandsaw.  The first time I cut a 1"x1" steel bar with it (clamp bar in bandsaw, turn on, wait five minutes, clink!) I fell in love!


The sawblade's kerf is about 40 thousandths, which is just over the thickness of my receiver, so I only need to make one pass.  Measure carefully!  Once the blade starts cutting, it'll get sucked back into its old groove if you try a new starting spot.


With the slots cut, all we need is a hole for the barrel.  The CZ-52 barrel I'm using for this build has a diameter of 0.467", which is close enough to 15/32 (0.46875") to work nicely.  I started with a 1/4 drill bit in a centerpunched hole:


With the initial hole in the right place (to center the big bit) and going through the metal (so chips can fall out), I stepped up to the 15/32" hole.  After working steel, aluminum sure is easy to cut!


Now I trim down the front of the trunnion to match the receiver's slanted front:


Now drill and tap holes for #10 machine screws to clamp the barrel to the trunnion, and hold the trunnion to the receiver.  Rather than carefully measuring, marking, and fitting, I just drill the receiver and trunnion at the same time!


And now we have a functional barrel trunnion!


A bit more controversially, I've decided to epoxy my magazine well to the receiver.  The right way to attach it would be a milled magwell adapter, or some fiddly folding and spot welds, but I want to keep this build low-tech!


Epoxy seems to either stick very nicely or not at all.  If it won't bond or instantly cracks off, I can figure some more traditional means of mounting.  I realize epoxy (like aluminum) is not one of the three approved materials for gunmaking: mild steel, tool steel, and spring steel!

Speaking of which, I cut four pieces of mild steel stock for the quasi-wraparound bolt.  The long thin pieces will be slotted to run along the receiver rails, and the thicker middle pieces will be drilled and "milled" (on the hacksaw!) for the firing pin and extractor.



Prior to cutting, my four pieces of bolt stock weigh 1.6lbs, so I should have a plenty-heavy bolt!  Actually, I'm already looking at 4lbs loaded weight for the pistol overall, which is closing in on rifle-weight, but darn it 30 rounds of ammo is heavy!
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  orions_hammer

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Posted: March 14 2008 at 6:39am | IP Logged Quote orions_hammer

Today I did most of the machining on the bolt.  First the 1/8" hole for the firing pin:

Next I cut rectangular channels for the extractor, ejector, and relief so the bolt will clear the magazine.  You can do an amazing variety of machining with a bandsaw/hacksaw, but one thing you just can't do is wide slots--you can make side cuts easily, but digging out the trench in between isn't really possible.


Here's the finished bolt face and bottom.  Once I know exactly where the back of the cartridge will sit, I'll add a little pocket for it to sit in.


Now I "lathe" the firing pin tip on my drill press.  Yeah, the firing pin channel really does curve like that--my 1/8" drill bit wandered a bit during the 2 3/8" deep plunge!


I also bandsaw-milled in long narrow slots for the receiver top rails in the longer 1/4" bolt carrier pieces.  With all four bolt pieces basically done, I did a feed test.  It works!


Here's the front view of the basically-finished bolt sections:


I've yet to:
  • Fix the bolt sections together with a few big screws.
  • Attach the extractor to the bolt.
  • Attach the trigger guard to the receiver.
  • Tune & fit.
  • Test fire!
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  gundoctor

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Posted: March 14 2008 at 10:53am | IP Logged Quote gundoctor

Nice work on the bandsaw and the front trunnion design is nice.

For wide slots you can use a drill the width of the slot at the bottom of it and then saw out the sides.  This would leave a radiused slot, but simple enough to make the mating piece radiused as well.
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  orions_hammer

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Posted: March 15 2008 at 3:07am | IP Logged Quote orions_hammer

Gundoctor, the make-slot-via-drill-and-cut-sides technique is a really good idea!  But I sure have a hard time drilling small deep holes without the bit wandering--does anybody know any tricks for keeping drilled holes straight?  A boring head would work for larger-diameter holes, but if you've already got a mill, slots aren't a problem!

Gclark, glad you like my posts!  What I find striking is how *simple* gunmaking can be--compared to building, say, a functioning engine without a lathe.  It makes the hut-dwelling gunmakers of, say, the Khyber Pass in Pakistan a lot more understandable.
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  orions_hammer

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Posted: March 15 2008 at 7:41am | IP Logged Quote orions_hammer

Today, I bolted the bolt sections together:


That's a 1/4" bolt down below the rails, countersunk into the bolt face.  This lower bolt passes through the firing pin channel, and will be used below to hold the firing pin in place.  Up top is a #10-24 machine screw, doubling as a charging handle, and eventually acting as the extractor pivot/retainer.

Here's a closeup of the firing pin, with a little filed-out area to clear the retaining bolt.  My dang firing pin channel is so curved, I ended up switching to a 3/32" diameter pin to get it to move freely--you gotta be 100% sure your firing pin is free to slide back and forth, because if it sticks forward you're in a very dangerous situation!


I did the bolt in sections to avoid having to cut tricky slots in a single hunk of metal, but I like being able to easily just crack the bolt in half and clean out, for example, the firing pin channel.  For corrosive ammo like my surplus 7.62x25, this is important!


I also fit up the (stock AK) hammer and trigger/disconnector.  The hammer needs a bit of trimming to clear the bottom of the bolt (typical!), but it looks like everything's going to work!  I should be able to test fire tomorrow or the next day!

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  dutigaf

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Posted: March 15 2008 at 2:45pm | IP Logged Quote dutigaf

bub,

you got an impressive way of jumping on things quickly.... i like that.

word of caution:  the tokarev round is very HOT!!!

 your balls may be a bit bigger than mine.....as im not sure id trust what your doing with my life.


but you have at it and if it works great maybe then i'll follow you. 
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  orions_hammer

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Posted: March 15 2008 at 4:26pm | IP Logged Quote orions_hammer

dutigaf, glad you like it!  Regarding safety, I'm planning on firing the first few test rounds with a string, and the pistol held in a vice, with me several yards away behind a yard-thick dirt berm.  It's kinda my standard protocol on untested firearms, because I don't like shrapnel.

I've got one box of Bulgarian ammo with headstamp numbers just one digit away from some known-bad Bulgarian Tokarev.  Most of the cases have deeply split necks, and supposedly these run pretty hot.  I figure if my new action can shoot off these safely, I can trust it with (milder) Winchester or handloads.

One nice thing about a blowback firearm is that headspace isn't a potential problem, because the bolt isn't locked to the forward trunnion with lugs--the bolt just runs forward until the cartridge case is jammed solidly into the chamber.  Also, in the event of a case head separation, my trunnion leaves a 1" x 1" hole for gasses to escape upward and 1" x 1/4" hole forward, both away from the shooter.  Finally, the Skorpion/AK design's forward-of-handguard magazine means the chamber is 4+ inches away from your firing hand (unlike a conventional pistol, where the chamber is right on top of your hand!).

Still, my inner lawyer has asked me to remind potential builders of this sort of gun to protect yourself like I do--stay behind several yards of dirt before remotely-firing any experimental gun design, at least for a few dozen rounds of the hottest ammo you can find.
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  gundoctor

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Posted: March 15 2008 at 10:28pm | IP Logged Quote gundoctor

drilling straight hole.  I assume a drill press.  Use a good bit.  center punch it.  Keep the chips clear and steady.   It will be straight.  Most are crooked because the bit wanders a little when starting the hole and starts at an angle.

Test firing.   I would not use a vise--it will put a lot more pressure on the whole thing than is needed or than will happen when you fire it in your hands. The whole gun moves when you hold it, and that takes up some of the recoil, as does your hands and body recoiling.  You are part of your recoil system.  Fixed in a vise may break the grip or make the bolt hit the rear pretty hard.  The opposite happens when someone shoots a semi "limp wristed"--the whole gun recoils so much that the bolt or slide doesn't even cycle.

I usually use some sandbags (under and over) to allow the gun to recoil.  Do not put any rounds in a magazine with a string on the trigger because it might be aiming at you or something you value when the other rounds go off.  If it ran away (i.e. went FA) it would probably kill you.  Don't "jerk" the string either--as you may move the whole thing--and possibly the direction of the muzzle.  Test your system with some dry fires first.

I have shot the tokarev with a M11 9mm bolt and it seems to do fine.  It is hotter (velocity) but the mass of the bullet is lighter yielding about the same impulse to the bolt in my opinion.  The OEM bolts /springs I have from 9mm and 7.62 tok weapons are very similiar in mass /strength as well.

I wonder if your trunnion attachment--which looks like some pretty soft and small chinese machine screws--may be suspect.  I think 2-3 pin about 1/4" or better that were peened on both sides might be more secure, and drilling in the tight place would also let these retain the barrel for you.  If the trunnion doesn't fit tightly at the bottom of your receiver (can't see from pix) I would make a spacer to take up that space as well--it will be a lot stronger if it is bearing there.
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  orions_hammer

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Posted: March 16 2008 at 1:51am | IP Logged Quote orions_hammer

OK!  The good news: it fires!  And even better, the recoil (at least at the trunnion) is fairly small, and the fired (hot Bulgarian) cases even survived as well.  Here's my test setup:


So that's a big bench vise, with the firearm clamped by the forward receiver, sitting outside and ready to fire via string.  A 1/2" polyethylene shield is for shrapnel containment should something go wrong.  The tester (me!) is sitting ten yards away behind a berm of snowy dirt, safely pulling the string.  The vice doesn't move perceptibly during firing.

Here's what the fired cases look like:


You can see:
  • The gap between the made-in-two-halves bolt left an imprint on the primer, which is showing signs of very healthy pressure (it's *hot* ammo!).
  • The case head did not separate.  No bulges are visible.  This means my bolt (which came out at exactly 1.4lbs!) is indeed heavy enough to safely keep the ammo in the chamber during firing.
  • I need to round over the edge of my firing pin hole, to keep the sharp corner from digging into the primer.  Actually, a smaller firing pin hole or bigger firing pin would also help avoid pierced primers, but that's just finish work.
Now the bad news: I did this test without a recoil spring or rear trunnion, because I wanted to figure out the bolt's post-firing velocity (for example, to size the recoil spring) based on the distance it flew.  That idea will probably work great, as soon as I can locate the bolt and dig it out of my snowy, dirty backstop! 

Note to self: when doing an explosively-disassemble-the-gun experiment in the field, at least tie some bright orange streamers around the parts you expect to have to dig out of the hillside!
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  tommerr

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Posted: March 16 2008 at 3:20am | IP Logged Quote tommerr

I smell a very intelligent engineer or a natural born engineer. I look forward to your future posts.

Tom the envious
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  gclark94560
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Posted: March 16 2008 at 4:45am | IP Logged Quote gclark94560

<<"Now the bad news: I did this test without a recoil spring or rear trunnion, because I wanted to figure out the bolt's post-firing velocity (for example, to size the recoil spring) based on the distance it flew.  That idea will probably work great, as soon as I can locate the bolt and dig it out of my snowy, dirty backstop! 

Note to self: when doing an explosively-disassemble-the-gun experiment in the field, at least tie some bright orange streamers around the parts you expect to have to dig out of the hillside!">>

LOL!

I am amazed that you found the case and not the bolt.  Does this mean the project is dead until June when the snow is gone! ? !
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  bikergunnut

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Posted: March 16 2008 at 4:49am | IP Logged Quote bikergunnut

you took video....right??
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  dutigaf

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Posted: March 16 2008 at 5:49pm | IP Logged Quote dutigaf

yea i wouldnt have wanted to be behind that baby...  but the brass does look pretty good.


for recoil spring and such....   how much travel room do you have??
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  orions_hammer

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Posted: March 16 2008 at 10:13pm | IP Logged Quote orions_hammer

Now that I've got daylight, I did a sort of ballistic reconstruction to estimate my bolt's post-firing flight path, and figured out more or less where on the hillside I should start digging.  I found the bolt buried about eight inches into the snowy ashy soil, close to ten feet from where the gun was fired!  That's why the recoil spring and rear trunnion are safety-critical items, folks! 

I tore the bolt down completely so I could clean the frozen dirt out:


I fired off a few more test shots, this time on a frozen creek where I could shoot straight down.  I also tied a little twine around the charging handle so I wouldn't lose the bolt again.  I took video this time, but there's only 0.3 seconds of action.  Here's the frame before trigger-pull:


Now I pull the string.  The muzzle flash is visible in broad daylight!  Also note how the back of the bolt is starting to motion-blur:


Here's 1/30 second later.  The bolt has left the firearm!  The little blur to the right of the bolt is the shell casing--despite not having an extractor, the case ejects itself nicely!  (I never located another shell casing,though!)


And so on.  The bolt flies offscreen two frames later, but was easily recovered via the string.  Little flying flecks of ice go shooting off in all directions.


At about 8 inches per 1/30 second frame, the bolt is moving at about 20ft/s.  The bolt reached about 7ft altitude before falling, which leads to a similar estimated bolt velocity.  Note the residual chamber pressure is likely higher than normal in this case, where the muzzle was only a few inches away from the solid ice!

For the recoil spring I've got a channel a little under 1/4" wide and 3.5" long between the hammer axis pin and the back side of the bolt.  With the bolt fully extended, this spring should stretch to 6.5" long.  I need to dust off my physics book to figure out just how much force the spring needs to apply at that length to stop the bolt just short of the rear trunnion. 

I'm planning on winding my own recoil spring from music wire.  Here's how I made my disconnector spring (AK parts kits often seem to be missing this spring!), starting from 0.024" music wire wound around a 3/64" pin punch.  I learned springmaking from Bazillion's awesome springmaking site


But the bottom line is: it works!  And Tokarev seems to have plenty of power to both smash the AK hammer back down and launch the bolt back at the operator at high velocity too!


Edited by orions_hammer
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  gundoctor

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Posted: March 17 2008 at 3:40am | IP Logged Quote gundoctor

I like the CSI aspect of the test firing to find the bolt.  Any thoughts on boring a hole in the ice and scuba'ing to recover the projectile?  Awesome video of the bolt recoiling straight up. 

A complicated physic calculation with gravity and the ever increasing mass of the string.   If you weigh the bolt, maybe someone has something else that is close to the same weight and give you a starting place on spring.
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Posted: March 19 2008 at 6:35pm | IP Logged Quote Vorvon

Buahahahaha cannot stop laughing. That is something that I would do! I thought you were going to show stop frames of it firing, ice breaks under the heavy vice, and then the vice falling through the ice..... Again something I would do... The build looks excellent! I thought about making a bolt like that. Basic and "easy"!

Mike
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Posted: March 19 2008 at 6:38pm | IP Logged Quote dcorb

Just think what the members that live in a warm climate that are trying to figure out what the hell this ice thing is.
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  orions_hammer

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Posted: March 23 2008 at 5:17pm | IP Logged Quote orions_hammer

(Sorry for the long gap in posts--work's been crazy!)

Doctor: Any thoughts on boring a hole in the ice and scuba'ing to recover the projectile?

Vorvon: ice breaks under the heavy vice, and then the vice falling through the ice...

Y'all seem to be under the impression there's *water* under the creek ice.  Around here, creeks freeze to the bottom--the ice goes all the way down to the dirt, and  the dirt's frozen too

Anyway, back to the gun.
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Posted: March 23 2008 at 6:19pm | IP Logged Quote orions_hammer

I finally cracked open my physics books to try to figure out how strong my recoil spring should be, in order to stop my bolt just short of the rear trunnion.  The bolt's initial velocity is known from the experiments above.  Looks like in general for a mass-on-a-spring system, the oscillation velocity is W = sqrt(k/m) radians/second, where k is the spring constant (stiffness) and m is the (bolt) mass.  We want the amplitude to equal the bolt's travel distance back to the rear trunnion, call it A=0.25ft, so overall the bolt position P(t) = A sin(W t).  The velocity V of a mass-on-spring system is V(t) = A W cos(W t), which at t=0 is just A W.  So:
    A = bolt total travel distance
    A W = bolt initial velocity
    W = sqrt(k/m) = bolt velocity / bolt distance

So overall we've got:
    k = m W^2
    spring stiffness = bolt mass * (bolt velocity / bolt distance)^2

In our case, bolt mass = 1.4lb (or 0.0435 slugs), bolt velocity = 21fps, and bolt distance = 0.25ft.  So our spring's stiffness should equal 300 lb/ft--this is really very stiff!  For example, with the bolt pulled all the way back, at 3 inches, the spring will push back with a 75lb force!

Just for completeness, we figured out earlier that the bolt's velocity should equal:
    bolt velocity = bullet velocity * bullet mass / bolt mass * pressure-area ratio
Where pressure-area ratio is the ratio of the cartridge base area (pushing on the bolt) and the bullet's base area (pushing on the bullet).  For my gun, this bolt velocity estimate comes out at 21fps--just like reality!  

Combining these two equations, we can build a general blowback spring recoil design equation:
    spring stiffness = (bullet velocity * bullet mass * pressure-area ratio / bolt distance)^2 / bolt mass

So faster bullets, heavier bullets, and bigger cases all require dramatically larger recoil springs.  A higher bolt mass drops the required stiffness linearly.  Suprisingly, a longer recoil distance cuts the required spring stiffness substantially--if you double the bolt travel, you can cut the spring stiffness to one-fourth the old value!  

And yup, this equation also tells me I need an insanely stiff 300 lb/ft recoil spring.  Or do I?  Another option is to just let the bolt slam into the rear trunnion at high velocity, and dissipate the bolt's velocity in the collision!  I bought a little sheet of Sorbothane, which I've been very happy with in recoil pads, so I'm going to try putting a little tab of it on my (freshly fabricated) rear trunnion, and fire away...
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Posted: March 23 2008 at 6:31pm | IP Logged Quote orions_hammer

Here are some recoil springs I turned.  The tiny left one is 0.024" music wire, which is too wimpy (when it tried to fight the AK hammer spring and close the bolt, it lost!), and the three on the right are 0.050" music wire, which are all too stiff (I can hardly pull the bolt back with pliers, and if I do, they permanently stretch out).  Springs get stiffer with thicker wire, *fewer* turns (shorter coiled section), or tighter turns (smaller diameter); they get floppier with thinner wire, more turns, and larger diameter.  Sadly, I don't have room to make the thick-wire springs any longer or wider.  What I really need is some intermediate size like 0.040" music wire, and I've ordered some, but it'll be at least a week until it arrives.


On the other hand, this abomination works surprisingly well:


Yes, those are rubber bands.  Yes, it cycles.  Quite well, thank you! 
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