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AR/AK hybrid trunnion and bolt

3K views 28 replies 8 participants last post by  TRX 
#1 ·
If you want to build an AK-ish rifle with minimal AK parts, you're limited to blowback, or you have to use an AK trunnion, bolt, and bolt carrier. Those are all fairly complex parts to DIY.

I was looking at an exploded view of an AR-15 last night. The barrel screws into a "barrel extension" which looks like a simple stepped bushing; threaded for the barrel at one end, locking lugs in the other. I assume it's pressed into the upper receiver; I've never had an AR apart.

You could press an AR extension into a simple block and then you'd have a threaded front trunnion with locking lugs, that an ordinary AR bolt would lock into.

The AR bolt carrier wouldn't work at all. You'd have to make an AK-ish carrier from scratch... it might be possible to do the carrier as a weldment, and cut the cam track all the way through with a Dremel instead of an internal track like an AK.

I have no idea about the barrel diameters of an AR, but adapting the RSB and GB wouldn't be a big deal.


I present this as a thought experiment; it would be more work than it's worth as long as AK trunnions, bolts, and carriers are affordable, unless you just wanted to avoid imported AK parts as much as possible.
 
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#2 ·
On the AR the bbl extension is threaded onto the bbl by the manufacturer and is not normally removed after that. It is made that way as a manufacturing expedient, you can machine the locking lugs and such a lot easier witrh access to both ends. The smooth exterior of the extension is a slip fit in the AR upper, there is also a locating pin and a slot in the upper to align the two components. A large nut then threads over the bbl mounting area of the upper, retaining the bbl. Don't have the dimensions handy, but the Ar bbl is so big at the rear I doubt you would have much trunnion left after drilling it out to fit the bbl.

If you really wanted to use the Ar system in an AK, you could cut off both an AK carrier and an AR carrier and weld the two together, then use the stock AR bolt.
 
#4 ·
My two pennies here...
I have turned down AR-15 barrels to 23mm removing the locking lug rear section with a carbide lathe bit. The resulting barrel will fit an AK trunion. The AK RSB will be a bit too large for it without a bushing.

As proposed a simple custom AK trunion machined to the AR .990 inch barrel can retain an AR barrel locking lugs and all. The AK cross pin actually goes through the softer barrel area rather than the hardened locking collar !!! Resulting positioning comes very close to stock AK.

The trick part needed is a clone AR-AK bolt. Perhaps Kernel Krink's clone carrier is a simpler solution.

VD
 
#5 ·
It might be easier to just make an ar15ish style gun out of 1" I.D. tubing. Kinda like the ar15 meets the SGN-9 style. You really would only need an: ar15 barrel; bolt; FCG(a remington 870 fcg would also work and speed up the build); buffer and spring. To keep it simple you could just use the direct inpingement or you could get fancy and put a piston in it. To make the mag catch I'd use the flapper style that is on the galil ar15 mag adaptors.
 
#6 ·
Hello moleman,
A tube gun !!! Gotta do something with that darn gas key. Maybe make it recoil operated with a cocking handle on the side... a .223 Johnson rifle !!! That's back where it all started years ago.

A more radical tube type design could use the Reising system. A prototype of that system was built as a 7.62 NATO rifle by Eugene Reising.

Maybe we need a clean sheet of paper.

VD
 
#7 ·
VD, The first time I looked at a johnshon LMG I thought that stoner had stolen that as the basis for the m16. I haven't messed around with recoil operated guns enough to figure out an easy way to make one. For the gas key/piston on the ar15 tube gun you're right, you'll need to mill a slot in the tube and weld on a channel. A side charger would make it an easier build and a pivoting break in the receiver tube like an ar does(with a gas shield welded to the stock portion to shield your face from the back of the key slot). The ar15 strikes me as a tube gun that just happens not to be round on the outside. The home made ar15ish tube gun build is on my list of builds to do along with a dozen others. Luckily I have extra ar parts gathering dust out in the garage.
 
#9 ·
I mentioned my idea over on homegunsmith, and someone has already built a bolt-action rifle with an AR bolt and barrel extension. They were even talking about it this morning.

That's me, a day late and a dollar short...
 
#10 ·
Hello TRX,
A gunsmith/mfg friend has already experimented with the steel tube AR idea. However he keeps such things secret for business reasons.

Perhaps the gas key solution is to eliminate that darn gas system all together. An op-rod could be used under the receiver tube as was done with the model 50 Reising. A fork like op-rod passed on either side of the magezine to connect with the bolt behind the magwell. Thickwall steel tubing kept the receiver rigid.

VD
 
#11 ·
An op-rod could be used under the receiver tube as was done with the model 50 Reising. A fork like op-rod passed on either side of the magezine to connect with the bolt behind the magwell.
Hm. I couldn't find an exploded view of a Reising anywhere, but descriptions talk about "delayed blowback with a system of levers."
 
#12 ·
Hello TRX,
Check Nelson's Submachineguns of the World (Vol 1) for the Reising drawings. (my darn HP scanner doesn't work).
The Reising model 50 is a delay blowback .45 smg of WW-II fame. The bolt is cylindrical in a thick-wall steel tube receiver. The bolt tilts up at the tail end into a recess area in the receiver tube. The tilting is caused by a small cam edge on the op-rod working on the bottom side of the receiver through a slot.

The op-rod is also the cocking device and is spring loaded. It is not gas activated. It is a simple delay system. The op-rod is a welded-together fork-like steel strap thing. They sometimes broke at the weldment but can be rewelded better with today's processes. The Garand op-rod is also a welded assembly by the way.

The levers are actually the trigger sears that release the op-rod and striker hammer (closed bolt system).

The Reising is a simple and unique design that could have been an assault rifle had there not been such opposition to an AR round by US military brass.

VD
 
#16 ·
My idea for a trunnion would be a simple block to hold the AR barrel extension. It wouldn't necessarily be any longer than the barrel extension, though it might be worthwhile to put a couple of "wings" on it to more-easily attach some rivets.

Press the AR extension in, then pin it in place in normal AK style. The AR barrel would then screw in.

The AR bolt is weird-looking. I may have to just buy one so I can inspect it. Meanwhile, it might be possible to turn/grind it down to AK size on the stem and press on a collar to engage the AK bolt carrier cam track. If not, doing a custom bolt carrier would be the next logical step. Then you'd be independent of the three major parts you still have to source from AK kits.

In the furtherance of taking on more projects than I can handle, I ordered a chunk of 1-3/4 x 1-3/4 1018 steel a while ago.

At the moment AR bolts cost about the same as AK bolts, and a brand-new barrel extension is $20-ish, vs. $50-ish for used AK bits.

The trunnion and extension setup looks like a no-brainer, but the bolt/carrier setup will require some thought.
 
#25 ·
I thought I saw where you were working on making a bolt. If you can do that, then you should be able to make a trunion of sorts. I'd make one with only 3 or 4 lugs. Here's a link to a guy that made a 98 mauser receiver out of pre hardened 4130. Login
 
#26 ·
Ive considered a barrel extension/trunion for a while that would use a bolt action sized lugs and a custom bolt that would fit it. there are bolt heads for various bolts that are sperate and avaliable and just build my own damm Semi auto design of rifle from scratch.
 
#27 ·
There are several sets of "build your own .50 caliber rifle" plans out on the web. The Maadi-Griffin ones are typical. They all use a screwed-on barrel extension and a simple round bolt.

You could make your extension out of 4140 or 8620 round bar and press it into a piece of cheap 1018 to make a trunnion. Two or three lug layouts would be easiest. (the Barrett .50 and Leader Dynamics .223 use three lug bolts)

Milling the bolt lugs is not that big of a deal either.

Heck, broaches to broach your own 8-lug AR extension wouldn't be that expensive.
 
#28 ·
Yes, custom broaches are rather spendy things. The Mauser-type receivers are broached as are many other classic firearms receivers.

There are work-arounds that may solve this challenge such as using a horizontal or vertical shaper with a home made bit to carve the needed internal or external slots and grooves. The bit resembles a lathe bit and is ground for the specific application... by hand if need be.

The uniform spacing of the locking recesses can be established using a dividing head or perhaps a rotary table.

For the machine-tool-challenged fellows I might suggest trying a die-filer machine for some of those inside "grooves".
VD
 
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