another short stroking AR

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Tommygunwinter

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hi all,

I know this is not a new topic and have read and applied what I found on other threads...but still have an issue.

I have a 16 inch Ruger 556 MPR that runs fine with both commercial 223 and 556 ammo as well as my reloads, which are 62 grain behind 22 grains of H335. Those rounds also run fine on my other AR, a combination of Aero Precision parts and 16-inch Faxon barrel.

However...

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

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

The rifle operates correctly using higher pressure commercial .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

thanks for any help


target 223.jpg
 
If you installed a new gas block you surely verified port to block alignment. I would oversize the port and install an adjustable gas block.
 
There really isn't much you can do if you don't want to oversize the gas port. Definitely sounds like the rifle is under gassed.

I suppose you could mess with the buffer spring.
 
You can use a lighter BCG, or lighter buffer springs. JP Enterprise has an adjustable weight BCG and Silent Capture Buffer where you can change the spring and buffer weight. The silent capture recoil buffer system will work good in your application.
 
Don't take this the wrong way, but I don't see any evidence of a terribly accurate load or rifle. Sure, that's a pretty impressive 100yd offhand group, but my cheapest AR will hold 2MOA with reasonably competent ammo from the bench.

I would not consider parts-swapping or modifying a functional AR to suite a slightly underpowered load, unless it was consistently 1MOA or better, from the bench. Maybe your slightly underpowered load is exceptionally accurate, but you didn't indicate that.
 
Tommygunwinter said:
hi all,

I know this is not a new topic and have read and applied what I found on other threads...but still have an issue.

I have a 16 inch Ruger 556 MPR that runs fine with both commercial 223 and 556 ammo as well as my reloads, which are 62 grain behind 22 grains of H335. Those rounds also run fine on my other AR, a combination of Aero Precision parts and 16-inch Faxon barrel.

However...

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

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

The rifle operates correctly using higher pressure commercial .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

thanks for any help


View attachment 1065495
Click to expand...

Accuracy of a particular rifle should be done from the bench only. You want to eliminate every possible variable, and the trigger pressing mechanism is the most variable of all when shooting off hand.
 
well, that's one kind of accuracy for sure. But for offhand competition you also have to factor in rifle balance, weight, recoil, the shooters strength and probably some other factors I'm not thinking off. So I'm looking for the load that, for me, works best when I'm standing up with iron sights....
 
Well you're under gassed. You can either use higher pressure rounds, ream the gas port, lighten the bcg or lighten the buffer and/or the buffer spring. Reaming the gas port likely won't affect the accuracy enough to notice with the type of shooting you are doing. A heavier bullet might help. The Lee factory crimp die will help the cartridge build pressure as well. It tightened the groups on my bolt gun just a tad.
Is the bcg riding free and clear in the receiver? Lots of lube?
 
I propose a time out. A few questions first: is this your first assembly? You have another rifle but we don’t know the origins so yes, gas block alignment is important and some are spaced to index off the barrel, some with a Handguard cap installed.

Next, BCG: is it new? Have you performed your function checks for the gas rings and tested gas key for leaks? Even new parts can be subject to defect.

Check those things first, along with leakage around the gb (soot) and if you own pin gauges, check you gas port. I would not recommend drilling the barrel either, certainly not at this point.
 
It sounds like you need to work up to another accuracy node that's a little higher pressure and you need to bench that rifle to know if you actually have the optimal charge weight. What's your load data?
 
I had a 20" barrel that had a tab in the gas port that didn't allow enough gas, after removing it the gun ran fine with factory and reloads, something to look at, also check the gas tube for leaks like was stated earlier I've seen that happening on a couple of factory rifles.
 
In addition to what has been mentioned, check the gas tube fit in the gas block. If the fit isn't tight, a wrap of shim stock around the gas tube will eliminate gas leakage. There could also be a burr in the gas port of the barrel or gas block that is causing a hindrance. Also, make sure the gas key is tight.
 
Posted about a (sort of) similar problem; 16" bbl with a rifle gas system, which has had a rep for problems.
It runs fine with factory 55gr (it's 1:12') and a Colt full circle or SP1 BCG, but not on another new full circle BCG.
Quit fighting with it and just used the part circle carrier. Why ask why.
Moon
 
Absent any other modifications, you can change to a lighter bolt carrier or less buffer mass to make a rifle that is not fully cycling, cycle, with a given ammo.
 
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