Can a muzzle device change POI? ...by a lot?

**Eurodriver** · 11-22-14, 15:59

I had my BCM 11.5" LW w/ T1 zeroed at 200 yards. It had an A2 FH on it.

I put a BattleComp 51.0 on it and shot it at 100 yards today and I wasn't even on paper. I eventually figured out I was shooting over a foot low. The device was installed using a geiselle RR. I might have over tightened a bit, and some google searching says that is a very bad thing. It appears to be grouping alright, but not as good nor where it once was.

ETA: I just broke it loose and there's no way it was overtorqued. I just reinstalled to 20ft lbs, which is about identical to where it was installed previously.

That's the only thing I've changed on the weapon since I last shot it at 200 yards. From what I recall, I believe a 200 yard zero in an AR15 will result in an AR15 shooting high at 100 yards which makes it even more unbelievable I was shooting so low at 100 yards.

Are there other factors at play here, or totally normal due to weight and barrel harmonics changes? I eventually got the zero back up, but I was about 15 MOA low and 5 MOA right. It has me a little concerned the zero shifted that much.

Also, if I did over torque the BC on the barrel last time and now that I've removed the BC and reinstalled to torque spec will the barrel "return" back to normal or is it permanently toast? I didn't go crazy with it the firs time, 30-40ft lbs max.

**JimT** · 11-22-14, 16:08

From my personal experience with two different uppers, a change in muzzle device did alter their POIs.

Upper 1(16" midlength, Aimpoint T1, DD Omega 9.0): Removing a PWS FSC556, and installing a BCM Mod0 compensator altered the POI by 1" left, and 1.5" up at 100 yards. Zeroed for 100 yards.

Upper 2(16" midlength, Aimpoint T1, DD Omega 9.0): Removing an A2 birdcage, and installing a BCM Mod0 compensator altered the POI by 6" down at 100 yards. Zeroed for 100 yards.

Updated to add a third upper.

Upper 3(16" carbine gas, Aimpoint ML2, DD Omega 7.0): Removing a PWS FSC556, and installing an A2 birdcage altered the POI by 2" high, and 2" left at 100 yards. This upper is zeroed for 100 yards.

**Clint** · 11-22-14, 18:28

Did you install it with a crush washer?

**Eurodriver** · 11-22-14, 19:33

Originally Posted by Clint

Did you install it with a crush washer?

No, I used flat washers until it needed about 3/16 rotation to be aligned. In hindsight, this was probably a little much. Should have had it until it was 1/8 rotation and just snugged it up.

Also, when suppressed, I get about a 5 MOA change of POI to the 8:00 from unsuppressed. Something seems off.

**vicious_cb** · 11-22-14, 19:35

Originally Posted by Clint

Did you install it with a crush washer?

This. I would just use a shim set to time it then rocksett in place. Its alot of extra worth but its worth it.

**Eurodriver** · 11-22-14, 19:43

Originally Posted by vicious_cb

This. I would just use a shim set to time it then rocksett in place. Its alot of extra worth but its worth it.

I used a shim set to time it and then rocksett it in place.

**vicious_cb** · 11-22-14, 20:50

Originally Posted by Eurodriver

I used a shim set to time it and then rocksett it in place.

Then Im not sure how you manage to put 20 ft/lbs of torque on it. Hand tight and rocksett should be enough.

**Eurodriver** · 11-22-14, 20:54

Originally Posted by vicious_cb

Then Im not sure how you manage to put 50 ft/lbs of torque on it. Using shims and rocksett it shouldnt be torqued to even close that amount. A little over hand tight should be enough.

It wasn't. I rechecked the muzzle device and updated the OP.