AR short strokes

[midatlantic]

I have a Ruger 556MPR that runs fine with both commercial 223 and 556 ammo as well as my reloads, which are 62 grain behind 22 grains of H335. 

 

For a service rifle competition I built a new upper with a heavier ballistic advantage 18 inch barrel, the Ruger was a 16 inch barrel. I used an Aero Precision upper receiver, handguard and low profile gas block. 

 

That combination is very accurate, and with a half pound of lead added to the stock balances very well. However... It tends to short stroke.

 

The rifle only operates correctly using higher pressure .556 ammo, but with any other commercial ammo, as well as my reloads, which I want to use cause I find them very accurate,  the bolt will not lock back, and will not chamber a fresh round. If I manually operate the bolt it's fine. 

 

I've taken it apart, checked everything, even replaced the gas block, cleaned the bolt, etc. etc. I've ordered a lighter recoil buffer spring but I wonder if there is something I'm overlooking.

 

target below is off hand iron sights at 100 yards

 

[Don_B]

You might want to compare the barrel port diameters. 

[TonytheTiger]

How centered was the carbon ring over the gas port when you took the gas block off? Loose gas key screws? Good gas ring seal against the carrier bore? No gas obstructions at all? 

If all that checks out you could go lighter on the buffer, spring, carrier or a combination of all of those. Or you could do none of that and enlarge your gas port instead if it's just too small to allow lower powered ammo to function.

[verla]

I would start by measuring the diameter of the gas port- agree with Tony- do not think you are getting enough gas to operate the bolt. Then ck the other suggested items above, especially for a gas obstruction. You may not be getting enough gas!

 

Several years ago after many rounds through the gun my AR-15 suddenly started to not cycle with an aluminum carrier. Sent the carrier to JP who measured various dimensions and was told that I had worn it out- gas was escaping resulting in the gun not cycling. They offered me a replacement or a discount on a SS one.

Edited March 14, 2022 by verla
addl infro

[Runnin2live]

I’m in agreement on the gas side of things. I had a reliable functioning 300 BO, did a barrel swap and started having  cycling issues that seems to pop up every so often. It ended up being the gas block was slightly off and causing some blockage

[Braxton1]

I once had a gas key screw break.  The head of the screw, being staked in, stayed in-place and gave no clue to the issue.  After chasing ever other possible cause known to man, I decided to remove the gas key to check for cracks in it.  The screw popped right out and upon removal, I could see a carbon trail where gas was leaking.

[TonytheTiger]

19 hours ago, Braxton1 said:

I once had a gas key screw break.  The head of the screw, being staked in, stayed in-place and gave no clue to the issue.  After chasing ever other possible cause known to man, I decided to remove the gas key to check for cracks in it.  The screw popped right out and upon removal, I could see a carbon trail where gas was leaking.

Had the same thing happen also. It was a relatively new AIM Surplus BCG that was 99% reliable and I was checking to make sure everything was tight when the screw head just kept spinning.