Bolt Carrier change sheds 1/2 MOA.

[a1712]

I purchased a new Bolt Carrier group for my factory DPMS Varmint. I just wanted a coated carrier group to ease cleaning. The rifle will hold MOA to 400 yards, longest I've shot it, and is absolutely consistent. Well I changed the Bolt Carrier group out, checked headspace and headed for the range yesterday. It was warm out but high switching winds. I used no flags and just tried to shoot in the letups/lulls. My first 5 shots at 200 were just over an inch, my next 5 were just under an inch, the next 2- 5 shot groups were right at an inch. This rifle has never shot this well and I figured my setup on the bench and biorhythms must have been just right. It must have been the same today because I shot 4-5 shot groups under and inch this morning at 200. I have no idea why a bolt carrier group would make such a profound difference in group size, but I'm not complaining. Hopefully these winds will lay down in the next week or 2 and I'll run it at 400. Has anyone ever experienced this phenomenon? Brian.

 

[jepp2]

I have no idea why a bolt carrier group would make such a profound difference in group size

If the new carrier is heavier (i.e. an M-16 carrier like the one on the right in the picture) it can slow down the opening and thus change the barrel vibration.

 

[Delfuego]

Heavier can improve lock-time and if may also have less carrier-tilt too.

 

[Rsadams]

It's hard to tell of course... But dpms is known to be over gassed as are many ARs anymore to keep from getting phone calls on why won't my gun cycle cheap ammo in a 5.56 chamber... MAYBE , the heavier BCG slowed down the unlock time... If it's throwing the brass forwards at say 2 o'clock and your have a rifle length gas system you might try a heavier buffer spring.. If you have a carbine or mid length gas system you can try a heavier buffer and or spring...

If you want to talk to somebody about different springs I would suggest SPRINCO springs , they are more expensive 20 bucks or so but they will last as long as the barrel... They can guide you to what you need without just guessing...

 

[a1712]

Both BCG's are at 11.7 oz.. So I don't know if I'm getting better/more consistent lock up, better ignition, etc.??? I'm liking it either way. Brian.

 

[searcher]

Changing out those springs does make a profound effect. I carry an assortment of springs (I always have a Wolff light and heavy as part of the assortment) to the range when tuning a load to a new gun. I always start out with the Wolff light spring with varmint loads and, once the best load is found for the accuracy and velocity I want, I start changing springs. I have changed bolt carriers, upgrading to chrome, etc. and have experienced the same (and opposite) experience. Doing anything to change the pulse in any way will affect the vibes at the muzzle. If it is negative, a change in charge of 4/10 grain +/- usually puts thing back in order.

 

[boltman223]

Is it coated inside where the gas rings of the bolt slide?

 

[a1712]

Is it coated inside where the gas rings of the bolt slide?

Yes it is. Brian.

 

[Rsadams]

I was suggesting changing the spring to take a little bit of beating off the gun if it's overgassing ... Not really a problem in rifle length gas systems but it can really help in short gas systems... No sense in letting the gun beat itself if it doesn't have to...

 

[velocette]

The post said that he changed the BCGm, which usually means the bolt and carrier together.
It is entirely possible that his new bolt has given him a tighter headspace which can and will improve accuracy.

 

[a1712]

I was suggesting changing the spring to take a little bit of beating off the gun if it's overgassing ... Not really a problem in rifle length gas systems but it can really help in short gas systems... No sense in letting the gun beat itself if it doesn't have to...

Running a Tubb flat spring w/a 5.4oz. buffer. Looking for a clamp on adj. gas block. Brian.

 

[a1712]

The post said that he changed the BCGm, which usually means the bolt and carrier together.
It is entirely possible that his new bolt has given him a tighter headspace which can and will improve accuracy.

Actually increased my base to shoulder datum by .004. Brian.

 

[Rsadams]

Running a Tubb flat spring w/a 5.4oz. buffer. Looking for a clamp on adj. gas block. Brian.

Nice.... If it's still locking back on an empty mag your good....There is really no sense letting the rifle beat on itself...

 

[Beau]

Maybe the original bolt had a carbine extractor spring and the new one has a rifle extractor spring.

 

[1lnbrdg]

Reminds me of M1A shooting, swapping gas pistons until you found the best for the current rifle/barrel combination.

 

[D-mon]

Hi,
You could try fitting the old carier back and check the accuracy once more and you will definitely be fixed on what improved the groups: the new bolt carrier or the barrel/ mechanic cinetics braking in .