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45 acp AR that isn't a blow-back

363 Views 1 Reply 2 Participants Last post by  jmj1060
So I got an itch for a PCC in 45 acp, since I have a couple of carry pistols in that caliber.
I didn't want a blow-back.
So, about 18 months ago I paid way to much for a Macon Armory .45 DI upper.
It ran well enough, but it was less than what I wanted, too many problems.
Proprietary mags where harder than hell to load.
Odd-Ball free float hand guard moved around too much to have a front sight mounted on it.
The bolt carrier was way too loose in the receiver.
And since the gas port was just a 1/4" off the case mouth, it put a huge amount of gas in my face.

I addressed the hand guard issue first. It was a clamp-on type, with TINNY clamp screws, and an adapter that went on over a standard barrel nut. No stability at all. So I made a front bushing that fit the ID tightly, and was retained by screws, and the bore just slid over the barrel.

That took care of the front sight constantly moving around, but it still was a pain if you wanted to add a short piece of picatinny rail, because it used slide in steel inserts to attach the rail sections, and the hand guard had to be removed to hold the threaded insert in place, and then start the screws to hold the rail in place.

I finally decided to change it out to an ADE Pro hand guard, as they are 7075 aluminum, made in the USA, and use 6 button head cap screws to hold it to their barrel nut.

Went to take the barrel nut off the receiver, but when it was installed at Macon, not only did he not use a moly based grease, just cheap lithium, it was WAY over torqued. Guess he didn't want to use a shim under the barrel nut to get a notch to line up with the gas tube, and just torqued the snot out of it to get the gas tube to go in.
Every other AR that I've had, the barrel nuts came off fairly easily, but not this one!
Couldn't get it free with a 1/2" breaker bar, had to add a cheater pipe to it, and when it finally come free, the receiver cracked...

AT that point, I said screw this CaCa, and since I was going with a new receiver, figured I would take care of all the issues at once.
Picked up a smooth upper from Total AR15 (made in Minnesota), a lower that took M3 Grease Gun mags from CNC, and a nice, barely used Adams piston kit with the light weight bolt off Gun Broker.

Called up Adams arms for any suggestions on how to use their piston kit on a 45 acp, and they said it probably wouldn't work, and why.
A .556 develops around 60,000 psi chamber pressure, but the 45 only develops 20,000 psi.
I thanked them, then thought about it.
Having a lot of industrial design, and building experience under my belt, how would I approach a similar problem on a piece of machinery?
Say there is a pneumatic lift, and it can't lift the load needed, with the air pressure available?
The easiest solution would be to put a larger bore cylinder on the lift.

So that is what I did.
The Adams kit used a 5/16" piston, so I made a 1/2" piston, but it wasn't enough. The bolt would pull the spent case out a little, but not all the way out of the chamber. Next was to cut the buffer spring back.
After several attempts, the case would leave the chamber, but not clear the lugs on the barrel adapter.

So I made even a larger piston, and took a new Strike Ind. flat wire spring, and instead of shortening it, I made a mandrel to hold it compressed in a lathe, and a tool post grinder to reduce the diameter by nearly 1mm.
This gave me the same closed bolt pressure as the cut spring, but the full travel pressure went down by 50%.

Am running a pistol length gas port, no need to have it right at the case mouth.

At the range, Success! It runs good on a wide variety of ammo.
The grease gun mags load easily, and hold 10 more rounds than the Macon proprietary mags.

I ended up using a Magpul MOE-SL hand guard because of the piston size, and even that had to have a little material removed to clear the piston, but I really like the shape & feel of it.
The front sight is a YHM hooded flip-up, mounted at the very front for increased sight radius, and the rear is a MI flip up.
Being a piston driven action, I made my own anti-tilt buffer, that is Teflon hard coated.
, and A5 buffer tube.
And just last week added a Smiths side charge handle. It is sweat! Way better than the Devil Dog one I had.

Standard Adams piston kit

Oversize piston & op rod made of titanium

Old Op rod vs. new op rod & piston

Smiths side charge w/ folding handle
Automotive tire Bicycle tire Crankset Bicycle chain Wheel

And my anti-tilt buffer. The titanium nose fits inside the Adams bolt carrier, and because it's tapered, the weapon can be snapped open, and closed without pulling both pins.
Wheel Tire Automotive tire Bicycle part Rim
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Wow. That's some grade A thinking and engineering you did there. Kudos to you sir!
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