Range Report Problems with custom build accuracy.

[ken226]

I built an AR15 using a 24 inch 416 stainless 1:8 free floating heavy (1 inch) barrel, RRA match trigger and PRS stock. I've been to the range with it several times and am having some interesting problems with the accuracy.

Back when I first got it a few months, it was a nice 80-90 degrees out and I could shoot .6 inch groups all day. Cold shot was clustered nice and tight with the rest, in a little half inch circle, I felt like i was cheating shooting next to the deer hunters trying to sight in thier rifles.

Now that the temp is in the mid 30s things aren't going so great. The first shot seems to be consistanly about an inch below POA, then they start making a weird 45 degree angled line of bullet holes up and to the right as I shoot 5 round strings. If I wait awhile between shots, or just shoot 3 round groups I usually get 1 shot low, then two touching each other right at the POA.

I'm thinking that as the barrel starts heating up during strings, the POI starts to shift up and to the right.

Anyone tried cryo accurizing, i've heard that it helps minimize the differences in POI between a cold and hot barrel?


Forgot to add, the diagonal strings were with Speer 64 grn Powerpoint factory new ammo, every other problem was with Federal 62 grain PSP and Winchester Ranger 69 grn match.

 

[Pogo57]

Re: Problems with custom build accuracy.

My guess is that it has more to do with the ammo being effected by the temp than the AR. It's well known that temp changes will effect powder burn characteristics.

One thing to consider is that the rounds heat up as the go into a hotter and hotter chamber. Competition shooter will often eject a round if they have to pause in the middle of a string and the round has sat in the chamber and warmed up.

You don't mention how big the new groups are. I've seen loads that grew by more than an inch.

 

[ken226]

Re: Problems with custom build accuracy.

Biggest is about a 1 1/2 inch diagonal string with the speer. With the 62 grn and 69 grn the first one goes an inch low and left of my POI, the other two into a .5 group dead center.

Or sometimes the first round will go about an inch high, then one low, then a couple dead center. It't not always consistant. On a warm day, 80+ degrees, I can shoot half inch groups without much effort, never drops a round.

I'm just guessing its the temps, unless I wore the barrel out since Oct, about 200 rounds ago. The only thing thats different now is that my shooting day starts out about 35 F and quickly warms up to the high 70s by midday.

I guess thats whacha get in the desert with 0 percent humidity.

 

[The Mechanic]

Re: Problems with custom build accuracy.

As stated some powders can be very temp sensitive. You may try to warm the rounds before a shooting.

 

[Goldbeachman]

Re: Problems with custom build accuracy.

I had an upper with a similar but more exaggerated vertical stringing problem. The order of problems to rule out was scope mount and rings, gas block, barrel/receiver connection. Once all those were ruled out, it was ultimately diagnosed as stresses in the barrel causing movement when heating up. Cryo was suggested but I just ended selling the upper with full disclosure.

 

[MitchAlsup]

Re: Problems with custom build accuracy.

Need information:

What torque was applied at the barel nut?
What is the weight of the trigger?
Those 0.6" groups were shot with what ammo?
How many rounds through the gun?
Has the gun been thouroughly cleaned?
Did the accuracy go down hill at one fell swoop or slowly over time?
Have you checked the free-floating-ness of the grip again?
Does the ammo feed well?
Does the gun have M4 feed ranmp, or polished feed ramps or std?
Was the bolt headspaced to the barrel?
Has the BCG been cleaned internally?
What lubrication regimine is used?

 

[ken226]

Re: Problems with custom build accuracy.

I don't know what the barrel nut was torqued to, i came pre-assembled.

trigger is 2.5 lbs

.6 inch groups were shot with all three brands, Speer 64grn PP, Federal 62grn PSP and Winchester 69grn match.

about 200 rounds total through the gun

Its thoroughly cleaned after every trip to the range, 50 rounds average.

The accuracy seem to go downhill all in one fell swoop. I don't go to the range frequently, maybe once a month average. I noticed the goups weren't so great in December, the first time I had been to the range in cold weather.

Everything looks good, barrel still free floating, nothing touching it.

Ammo feeds good, never malfunctioned

its a standard flattop upper, no M4 feed ramps, everything is standard nothing polished.

Bolt came with the upper, i'm just assuming it was headspaced properly from the factory.

BCG is thoroughly cleaned every time the weapon is cleaned.

Internals are thoroughly cleaned with breakfree CLP, wiped clean and lighly lubed with the same.

I took it apart this morning and shipped the barrel to be stress relieved and cryo-treated. I'm not sure if that will do anything or is just snake oil, but it was only 35.00 so it wont break the bank.

 

[i_Adam_i18]

Re: Problems with custom build accuracy.

Sounds like ammo. Try keeping some ammo in your pocket to keep it warm and see if it still does it. What are you shooting from? a diagonal pattern sounds like it may be the shooter.

 

[MitchAlsup]

Re: Problems with custom build accuracy.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ken226</div><div class="ubbcode-body">The accuracy seem to go downhill all in one fell swoop. I don't go to the range frequently, maybe once a month average. I noticed the goups weren't so great in December, the first time I had been to the range in cold weather. </div></div>

If the shooter was "on his game" that day and not shivering in the cold, afraid to produce a proper cheek-weld, and felt like he was shooting fine, then I would suspect the ammo.

Thanks for answering the other questions.

 

[Criver600]

Re: Problems with custom build accuracy.

I'd look at your scope.......