Zero shift after changing BCG?

[DreadPirateMoyer]

Sorry if a similar topic has been posted before; I tried searching here and on Google and couldn't find a concrete answer.

I plan on changing the BCG in my Colt to an LMT Enhanced BCG at some point in the future (when BCGs are available again). However, since it may be a long time until that option is available to me, I was hoping to shoot/function test/zero my rifle this weekend.

My question is: if I zero my rifle's sights this weekend and then swap the BCG in a few months, will my zero possibly shift? If so, what range of shifts are typical?

And if so, what if I only swap the bolt carrier, but keep the stock Colt bolt? Will that possibly change my POI?

Thanks for your help, everyone.

[markm]

I've never noticed a POI shift with a bolt swap. I suppose it's possible.

[wahoo95]

Can't imagine how or why zero would shift from changing a BCG.

"If you're gonna be dumb you gotta be tough"

[markm]

Can't imagine how or why zero would shift from changing a BCG.


A worn or different bolt could conceiveably alter chamber pressure or the way the cartridge sits in battery... These are kinda far flung things to worry about, I'd guess.

[patrick sweeney]

Possible, but highly unlikely.

Plus, if the shift is smaller than the average group size, you'll never notice it. As in; if your rifle shoots 1 MOA, and the POI shifts 0.25" at 100 yards, how could you tell?

Don't sweat it.

[Onyx Z]

i bet the headspace is slightly different. i would think this could cause a change in accuracy... but if the accuracy remained the same and only the zero moved, who knows... don't listen to me, I might as well be talking out of my ass, just throwing ideas out there.

[Blak1508]

I dont see how this could happen, even if it did it would be minimal.

[Col_Crocs]

Just to add to the above, just get the enhanced carrier and use your current bolt in it.

[Krusty783]

If you were unfortunate/uneducated enough to be running a barrel that was crap or well beyond the end of it's useful life, I suppose it's possible that the different tolerance stack-up of a new bolt could induce a different cant to the round once it's chambered (we're probably talking <.001"). This coupled with a worn out barrel could conceivably induce enough wiggle room to allow the projectile to attain a slight AoA in the barrel/upon exiting the barrel, which would manifest in a change of POI.

However, unless you tried this with one of the Luckygunner carbines before they tore them apart for inspection, or you've been in the sandbox for the last 10 years using the same rifle, I doubt you could test my hypothesis.

In reality, I don't see this ever being an issue with mil-spec parts. If, however, you swap a BCG for one with a different number of gas vents (or whatever they're called; the holes in the middle of the swarf under the dust cover), this would change the backpressure of the gas system and might induce a slight POI shift, though I suspect the affect would be less than that of switching modes on an adjustable gas block.

[Caveat; I have no evidence here; I'm just an aerospace engineer waxing hypothetical].

[Hmac]

i bet the headspace is slightly different. i would think this could cause a change in accuracy...

Interesting idea. A change in chamber pressure could cause a change in muzzle velocity. I wonder if it would be enough to make a difference, or if it would be anything substantial compared to just normal variation between commercial rounds.

[Failure2Stop]

I would submit that round to round variations would be greater than the differences between bolts that pass headspace gauging.

Typos brought to you via Tapatalk and autocorrect.

[xtrememachines]

Lug just sheared off my bolt, replaced the bolt only and my groups were off 3" left at 100yds. Shot 2 10rd groups to confirm then Adjusted zero back to center.

[Iraqgunz]

Had you waited until March, it would have been 5 years. You're still in the Top Ten for Necroposting though.


Lug just sheared off my bolt, replaced the bolt only and my groups were off 3" left at 100yds. Shot 2 10rd groups to confirm then Adjusted zero back to center.