AR 15 Pistol need help on Buffer / spring tuning

[markshere2]

I have been burning time looking at the AR15 forum for answers
https://www.ar15.com/forums/AR-15/Th...D-/122-200876/
and i'm crosseyed.

My AR pistol with a 7" barrel shoots fine with Federal M855 ammo (62 Grain FMJ Ball)
....and no other ammo.

My 10" AR pistol and all my rifles will digest every kind of ammo I buy / handload.
Just not this little one.

It is throwing brass only a short distance.
I am certain its because there is very little barrel in front of the gas port to keep the gas flowing backwards to work the action.
I'm not interested in getting a longer barrel.

The Buffer weighs 1,258 grains on my PACT scale, 2.8 oz, same as one of the buffers from my rifles.
The one in the 10" pistol weighs 1309 grains, 4% more.

Swapping that one into the 7" barrel pistol yields no change.

Do I need a lighter buffer?
Do I need a heavier buffer?

Do I need a lighter/ shorter spring?

[USMC3531]

Make sure your gasblock is lined up and there is nothing slowing gas flow. You can chamfer the gas port with a drill bit so it's got more margin for error. Worst case open the gasport until it cycles.

[DucatiRon]

Agreed. Under gassed. Needs aligned or bigger hole.

[AJRB]

I have been burning time looking at the AR15 forum for answers
https://www.ar15.com/forums/AR-15/Th...D-/122-200876/
and i'm crosseyed.

My AR pistol with a 7" barrel shoots fine with Federal M855 ammo (62 Grain FMJ Ball)
....and no other ammo.

My 10" AR pistol and all my rifles will digest every kind of ammo I buy / handload.
Just not this little one.

It is throwing brass only a short distance.
I am certain its because there is very little barrel in front of the gas port to keep the gas flowing backwards to work the action.
I'm not interested in getting a longer barrel.

The Buffer weighs 1,258 grains on my PACT scale, 2.8 oz, same as one of the buffers from my rifles.
The one in the 10" pistol weighs 1309 grains, 4% more.

Swapping that one into the 7" barrel pistol yields no change.

Do I need a lighter buffer?
Do I need a heavier buffer?

Do I need a lighter/ shorter spring?

I have a 7.5 Inch pistol build I run a spikes T2 buffer (4.0 Oz) and a sprinco heavy buffer spring. This pistol will cycle any ammo with zero malfunctions ever. 7-10 inch pistols are more often than not, are OVER gassed, not under.

Make sure your gasblock is lined up and there is nothing slowing gas flow. You can chamfer the gas port with a drill bit so it's got more margin for error. Worst case open the gasport until it cycles.

The chances there are gas block alignment issues are slim. He is using a light buffer with a standard buffer spring (assuming on the spring). The ejection pattern he is describing can also be a symptom of being over gassed.

Agreed. Under gassed. Needs aligned or bigger hole.

disagree, over gassed. It's a lot easier to test out different buffer weights and springs than messing with the alignment of the gas block (or drilling it bigger). I suggest a heavier buffer and stronger spring (not a huge cost), or even a lighter buffer just to troubleshoot the possibility of being under gassed.
If the heavier buffer and spring doesn't fix the issue, then you get better ejection wit ha lighter buffer, then maybe you consider gas block alignment/ drilling.

[:-)]

Check the alignment of the gas block and everything if it's all ok and you tried everything else, shoot what it likes.
Are you using a M16 bolt and a H2 weight buffer?
Is it lubed up good, have you shot it a lot yet?

[Emptymag]

My 7 inch pistol has a "Heavy" buffer.

I don't know the exact weight, but when I bought the KAK Shockwave Blade, the Vendor (Tombstone Tactical) had a deal where you got a free KAK dimpled tube and you could add a "completion kit" consisting of spring, end plate, castle nut, and your choice of buffer - 3 weights to choose from. I already had a spare carbine buffer here, so I ordered it with the "Heavy" buffer so I would have two to try out if the one didn't work.

I just looked to confirm - it has the heavy buffer in it that was purchased with the kit.


Hell, I just looked up buffers on the KAK website and now I don't know WTF buffer I actually have.
It has a red bumper. The only buffers on the website with red bumpers are the 9mm buffers.
Who knows - it may not even be a KAK buffer for all I know. The thing shoots fine though.

[marinville]

I agree with the first two guys. Gas block misaligned or gas port not drilled to proper size. Undergassed. Also check that gas key is tight.

I would research the proper gas port size and check it against the hole in your barrel. I would also consider the possibility of drilling the gas port to a larger size and then using an adjustable gas block to tune it down to just right.

Edit: Forgot to ask, is your rifle locking back on an empty mag?

[Hawk]

What do you mean that it won't run other ammo? If it doesn't run other ammo, how can it be throwing the brass only a short distance?
What other issues are you experieincing?
Failure to feed the next round?
Double feeds?
Failure to eject?

I'll take a stab at it anyways simply based on the most common issue with 7" pistols. The buffer is too light. Install a heavy buffer (4 oz). It will slow down the timing of the BCG.
Also inspect your extractor for damage. Typically on these the BCG tries to extract the round before the case has a chance to contract in the chamber thus putting stress on the extractor. A heavy buffer will correct most issues on a 7" pistol. I've broken a extractor on a 7" pistol before swapping out the stock buffer for a heavy one. It would only run M855. Now it runs everything I feed it (including Wolf Steel Case).

[:-)]

That's what I'm thinking. If you have an H buffer and a mil spec m16 bolt carrier, It should run. If you have a lightweight bolt or buffer or both, you will or may have problems.

[markshere2]

What do you mean that it won't run other ammo? If it doesn't run other ammo, how can it be throwing the brass only a short distance?
What other issues are you experieincing?
Failure to feed the next round?
Double feeds?
Failure to eject?

I'll take a stab at it anyways simply based on the most common issue with 7" pistols. The buffer is too light. Install a heavy buffer (4 oz). It will slow down the timing of the BCG.
Also inspect your extractor for damage. Typically on these the BCG tries to extract the round before the case has a chance to contract in the chamber thus putting stress on the extractor. A heavy buffer will correct most issues on a 7" pistol. I've broken a extractor on a 7" pistol before swapping out the stock buffer for a heavy one. It would only run M855. Now it runs everything I feed it (including Wolf Steel Case).

a.
Sorry for the unintentional obfuscation. The gun feeds and fires fine semi auto with Federal M855 ammo (62 Grain FMJ Ball)
Every other ammo turns it into a single shot that has to be cycled to fire again.
The empty brass falls very close to me.

2) thank you for the experience shared.

Lastly, thanks for the quick response to all yinz! 9 knuckleheads responding in an afternoon - Yinz needs to gets lives!!

C. Them is fun pistols when they run!!