Chrono expectations in a semi vs bolt gun?

[ToddM]

Question for the guys that have way more experience than I do in this.

I know of course that raw velocity #'s will be lower in a semi-auto of similar barrel length with the same ammo compared to a bolt gun cause of course you are losing some pressure to cycle the action.

However, do you also expect worse spread and standard deviation numbers or should they run pretty similar to the same ammo in a bolt gun?

 

[dms416]

Everything's going to be on a case-by-case basis. I've been doing stupid precision rifle stuff with about 2 dozen different 14.5" AR15 (other types of function will be different) 5.56's in the past decade and
- Make
- Twist
- rifling type
- chamber
- material

ALL make a difference. I'm willing to bet that you would probably see MORE variance from my fastest to slowest in an AR than you would if you had an "as close as you can get" 1:1 between a semi auto and bolt in a reasonable barrel length.

 

[oldrifleman]

Yeah I agree. My 14.5 JP upper is slower than my 16 Lmt which is slower than my 22 Craddock bull upper. Compared to my 26” heavy bolt gun the 22 runs about 275 for slower. Accuracy is based on the barrel and build quality. My Jp is about as good as a 14.5 Lmt with a converted Craddock RtR 14.5” barrel. My bartlein barreled ar are a bit behind my CRB barreled bolt gun but I would expect some drop in accuracy based in the stability/ recoil management difference.

 

[ToddM]

Yeah, in terms of raw velocity yes there's going to be lots of variation platform to platform. However do you also see much higher swings of velocity spread and standard deviation?

I expect the average velocity to vary quite a bit from gun to gun to with the same ammo, but it seems logical that the spread and standard deviation should be similar. Unless something else in the AR platform is introducing 2x as much shot to shot velocity variation as a bolt gun running the exact same ammo.

As an example, let's say you had an AR shooting the same lot ammo, same barrel length compared to a bolt, but the AR has twice the high/low spread and twice the standard deviation. I would expect based on that alone groups out of the AR would be much worse just due to 2x high/low velocity difference shot to shot.

Example bolt gun runs ammo A for 50 shots with velocity from 1100-1130 (30fps) spread, AR runs same lot ammo but with a 90 fps spread 1025-1115 (90fps) spread. There's no way the AR can group as well at distance just due to the velocity delta. However the more interesting question is what would cause this. Lower velocity sure, you have to cycle the action, but why 2-3x the variation in speeds? It can't be the ammo because it gets 1/3 of the velocity change in a bolt gun, so in my brain it has to be something that's AR platform specific, or a specific issue with that gun, that's causing 3x the pressure/velocity variation shot to shot. Poor ignition/light strikes, poor bullet sealing, inconsistent unlocking etc.

 

[oldrifleman]

Example bolt gun runs ammo A for 50 shots with velocity from 1100-1130 (30fps) spread, AR runs same lot ammo but with a 90 fps spread 1025-1115 (90fps) spread. There's no way the AR can group as well at distance just due to the velocity delta. However the more interesting question is what would cause this. Lower velocity sure, you have to cycle the action, but why 2-3x the variation in speeds? It can't be the ammo because it gets 1/3 of the velocity change in a bolt gun, so in my brain it has to be something that's AR platform specific, or a specific issue with that gun, that's causing 3x the pressure/velocity variation shot to shot. Poor ignition/light strikes, poor bullet sealing, inconsistent unlocking etc.

I actually do shoot the same lot ammo in both my 22” Bartlein AR And my 26” CRB comp/impact/foundation bolt gun (federal 77gr smk fgmm) velocity drops a bit and yes SDs grow ( goes up about 5 fps). Is accuracy the same, no, but fairly close. My bolt does ,55-.67 average. My 22” heavy ( .936) Bartlein JP upper does .75-.85”. Is this due to inherent quirks with the AR platform maybe some but mostly me not shooting the AR often enough to remember how to ride the recoil better.

 

[ToddM]

Thanks! So it would seem like then perhaps something could be amiss then when you double or triple those bolt gun SD values, not just small increases. Small ones I could see there's a lot more going on in a gas gun that might induce a bit more velocity variation, even the shooter, I think we've seen old posts/videos from Frank showing that chrono numbers change by position or say prone -vs- a bench etc.

 

[lonestar1845]

It's all in the ignition. Look at all the steps an AR has to go through to hit the primer compared to a bolt gun.

Since the firing pin floats, It could be in any position when the hammer hits it. All the way forward, at the rear, or somewhere in the middle. Now add carbon build up and oil/lube onto the pin, it will change even more.

I have an extreme example of just trigger variance that happened recently. I rebuilt an upper once, then rebuilt again with a new barrel after not achieving acceptable accuracy. Still shot like ass, so just for grins I threw a different lower on the gun that was set up exactly the same. Buffer, spring, A5 and geissele SDE trigger. Instantly went from 3-4" groups to 1" groups. SD/ES and average velocity changed too. I then swapped triggers in the lowers and the problem followed the one trigger, which happens to be a cosmetic blem i got on sale from geissele. But just from comparing triggers, its pretty hard to tell a difference.

I think if someone were to make a striker fired AR, the accuracy would get much better. Still not bolt gun good, but would bridge the gap.