Gunsmithing Anyone have much experience with pistol-cal ARs?

[Rancid Coolaid]

I know, I know, "why run a pistol caliber in a rifle", let us forgo that conversation and focus on the actual implementation of the thing.

I have a 40-cal 11" AR upper up and running, but notice that parts that have never failed before are failing now. Last time out, my bolt stop got sheered off. I fully appreciate that parts fail, but blow-back ARs seem to deal with forces not common on DI or piston guns (at least not ones in standard AR15 calibers.)

Be aware.

 

[2156SMK]

Re: Anyone have much experience with pistol-cal ARs?

Bolt stop breakage is caused by having excessive bolt carrier travel.
With a rotary locking bolt, you have extra carrier travel before the bolt actually opens the breach.
With a blowback setup, this length changes by about 5/8".
You can either install a 5/8" long spacer behind the buffer, or use a buffer that's 5/8" longer.

You only want 1/4" more travel than is required for the bolt to get behind the bolt catch.
The bolt and buffer bounce off the inside end of the buffer tube in normal operation.

The reason for the 1/4" gap is that if you have much less than that, there's not enough time for the bolt catch to pop up while the bolt is behind it.

If you have more than 1/4" travel, the bolt will get a longer running start before the bolt catch tries to stop the bolt when the magazine is empty.

Here is a spacer that I have made to solve this problem:

 

[Rancid Coolaid]

Re: Anyone have much experience with pistol-cal ARs?

Good to know, thank you.

 

[TN-MadDog]

Re: Anyone have much experience with pistol-cal ARs?

I've got a 45acp upper with a 10" ported/integrally suppressed barrel. I initially used it with a standard lower with modified Uzi mags. Later got an AR45 lower and use unmodified Grease Gun mags. Both lowers used standard A2 stocks and buffers. 2000+ rounds without any problems.

Here's a couple pics just for the hell of it.

AR45 lower

Standard lower

 

[dill84]

Re: Anyone have much experience with pistol-cal ARs?

Spikes tactical makes a spacer just for this if I recall.

Dill

 

[at4rxj]

Re: Anyone have much experience with pistol-cal ARs?

Posts here are spot on. I installed one of the Spike's travel limiters on a 9mm build with great results.

 

[Rancid Coolaid]

Re: Anyone have much experience with pistol-cal ARs?

Awaiting a response from Spikes on whether they have a plug for anything but 9mm. They have the 9mm plug but say it is 9mm only, will let you know what they say.

 

[hemigtx]

Re: Anyone have much experience with pistol-cal ARs?

Years ago I bought an Olympic Arms carbine that fired .45acp from modified UZI magazines (10 rd) because I wanted a long gun that fired the same ammunition as my 1911. I eventually sold it because while the concept is fantastic, the execution was lacking:
-The bolt carrier (blowback) in that carbine was HEAVY and it gave the gun a long, pronounced, and heavy recoil impulse.
-The ten rd mags were VERY difficult to load and capacity was ludicrously low for the length of the magazine. Mags were also very expensive and nearly impossible to obtain.
-The blowback operating system was filthy beyond belief; one mag fouled the rifle worse than running two cases of ammo through a standard AR.
-Was not impressed with Olympic Arms quality. The dang thing began running full auto on me because the weight of the bolt carrier slamming into the breechface had enough force to knock the hammer off the sear after the sear (which got worn out very quickly).

Fast forward 15 years and I bought a CNC AR-45 (see post above) and a Ron Williams DI upper. Couldn't be happier. High capacity (30 rd) readily available mags that are easy to load, gas operated system that keeps the gun clean and would allow me to work up a hot load if I were so inclined (and I might be soon), runs like a sewing machine with virtually no recoil. The only problem is that all those 230 grain bullets crashing into your target stand in a short period of time means you will be spending more time at the Home Depot buying cheap plywood..

 

[Rancid Coolaid]

Re: Anyone have much experience with pistol-cal ARs?

Spikes only has a 9mm spacer, no 40 or 45.

 

[2156SMK]

Re: Anyone have much experience with pistol-cal ARs?

All blowback bolts should be the same length regardless of caliber.
That would include 9mm, 7.62 tokarev, 40, 45 and 5.7x28.

That means that the spacer needed for any of them would be the same length as well.

 

[Rancid Coolaid]

Re: Anyone have much experience with pistol-cal ARs?

I wondered about that, but figured something must cause the "9mm only" designation on the Spikes buffer.


We'll see!

 

[Rancid Coolaid]

Re: Anyone have much experience with pistol-cal ARs?

So, yes the Spike's spacer prevents bolt stop breakage, mostly beacuse it shortens the bolt travel so much that the bolt stop is never engaged.

Does it prevent a 9mm upper from locking on an epmty mag?

It works, just not without an adverse effect.

I could accomplish the same by never letting the mag run dry.