Cold shot is always high

[Griz]

This weekend I did my first serious shooting with a new 26" AR upper that I put together for benchrest matches.

It can reliably turn out sub 0.5MOA groups as long as I fire one fouler, then quickly fire a 5 shot group.

The first shot from a cold barrel is always 1.5" high (at 100 yards) and if I pause during a group for as little as 2 minutes, the next shot is always high. (temp was around ~50F when this was observed)

I have put together 2 similar uppers for friends and they both are just as accurate but don't seem to suffer from this cold bore problem.

Does anyone have any theories on this? My best guess is that the bore of the upper that receives the barrel extension needs to be tighter, but that's a wild guess and I don't know how to fix it if that's the problem.

[WS6]

Maybe the barrel was not properly stress-relieved and changes POI when it heats up?

[m700m]

i can understand your disparity w/1.5 POI change, but the cold bore shot being the most important, shouldn't your cold bore shot be your POA/POI zero? then you can take into account the 1.5 POI change. every rifle i have will be different from cold bore to fouling , some more some less. you may try wiping the bore with several clean dry patches,since most Will leave a film of oil in the bore after cleaning. this may help but in my experience only a little. Don..........

[Griz]

i can understand your disparity w/1.5 POI change, but the cold bore shot being the most important, shouldn't your cold bore shot be your POA/POI zero? then you can take into account the 1.5 POI change. every rifle i have will be different from cold bore to fouling , some more some less. you may try wiping the bore with several clean dry patches,since most Will leave a film of oil in the bore after cleaning. this may help but in my experience only a little. Don..........

This is a match rifle, not a combat rifle so the cold bore shot really doesn't matter except for how quickly the POI changes as it cools down... 2 minutes between shots puts me out of the 10 ring.

Clean bore to fouled bore I expect and always see with precision rifles... What I have not seen before and don't understand with this rifle is the dramatic POI shift as the barrel heats up and cools down. I built 2 others nearly identical and they exhibit maybe 1/4 MOA shift as they heat and cool.

I don't know what to change to address this problem, I guess if I don't get any good ideas here I'll just pick something I can change and try it, maybe I'll learn something. :)

[WS6]

This is a match rifle, not a combat rifle so the cold bore shot really doesn't matter except for how quickly the POI changes as it cools down... 2 minutes between shots puts me out of the 10 ring.

Clean bore to fouled bore I expect and always see with precision rifles... What I have not seen before and don't understand with this rifle is the dramatic POI shift as the barrel heats up and cools down. I built 2 others nearly identical and they exhibit maybe 1/4 MOA shift as they heat and cool.

I don't know what to change to address this problem, I guess if I don't get any good ideas here I'll just pick something I can change and try it, maybe I'll learn something. :)


This is a long shot, but shouldn't cost you TOO much, you could have it cryo treated to see if that would help.

[Griz]

This is a long shot, but shouldn't cost you TOO much, you could have it cryo treated to see if that would help.

I'll save that for a last resort.

The easy things I can think of are:
-Bed the gas block with sleeve retaining compound
-Bed the upper/barrel extension fit with sleeve retaining compound
-Swap in a new upper receiver (maybe I can sweet talk a vendor into picking out one with a tight bore for me)

I wonder if I can peen the barrel extension to tighten up the fit with the upper...

[Failure2Stop]

It seems like you have a few possibile problems-

1-Insufficient torque on barrel nut.
2-Receiver flaw, not true at threaded end
3-Bad barrel. (Is the mfg a reputible company?)

I am inclined to think that it is due to insufficient torque or a receiver flaw.

The only way to know is to shoot a different barrel in the upper and the current barrel in another upper.

Of course it is always possible that there is a barrel defect or a defect in the barrel extension.

[markm]

Even after the barrel is fouled, then cooled, the first shot flies 1.5 high?

[Griz]

Even after the barrel is fouled, then cooled, the first shot flies 1.5 high?

Yep, even after shooting a 40 shot match without cleaning.

-Fire one shot, it impacts 1.5" above the X-ring.
-Then fire 5 shots into a ~1/4" group in the X-ring taking no more than 30 seconds between shots.
-Wait 2 minutes, fire one shot and it's about 1" high. If I wait about 10 minutes, it goes all the way back to the cold shot POI of 1.5" high.

At the match last weekend, someone crossfired on my target and I paused to chew him out... then I went and shot an 8 at 12 o'clock high because my barrel cooled and changed it's POI while I was getting my panties unbunched. Talk about rubbing salt into a wound....

The barrel is a 26" Varmint rifle barrel from WOA. I've always had great luck with WOA's barrels before.

The upper is an Alexander Arms billet upper and the face of it's bore for the barrel extension appears to be square with the bore. The bore is supposed to be undersized, but I was able to slide the barrel into it without any trouble, so it's not the interference fit I would prefer.

I don't remember how much torque I put on the barrel nut.. I usually go way overboard though. When I reassemble it I'll make sure it's OK.

[Griz]

I disassembled the upper in question, peened the barrel extension so that I had to drive the barrel extension into the upper, and also used sleeve retaining compound on the barrel extension and gas block....

The cold first shot problem was gone when I shot at this month's benchrest match yesterday.

The only issue is I changed 3 things at once and the ambient temp was drastically different so I don't know exactly what solved the problem... I guess what I did learn for sure is that it is indeed possible to tighten up the barrel to upper fit by peening the barrel extension.

[Mereel]

whats the round count on the barrel? Was the barrel broken in correctly? Is the powder of the rounds your using to hot for the metal of the barrel? "the rounds loaded hot, extreme primers? From experience barrel break in has been the biggest cause of having cold barrel change.....

[jmart]

I know this is probably not the suggestion you want to hear, but if the behavior is always consistent, and you always shoot at the same known distances, just aim low for the 1st shot. And keep track of your shooting pace to account for warm barrel groupings.

Another option could be to moly coat your bullets and bore. I read of one SR competitior who didn't clean his barrel for a whole season, and all he shot was moly coated ammunition. he dry patched the barrel, but that was it. he did keep the chamber and throat clean, but the bore never saw a brush or solvent, and he maintained his rifle shot more consistently.

[Griz]

Late posters might want to read the post a few up from this one... The problem has been solved and in an effort to expand the pool of knowledge here I posted the steps I took which solved it.

[jmart]

Reading comprehension and paying attention are wonderful things.:o

Glad you got it sorted out.