Where to look for lost accuracy

[jambau]

Guys,

I have been working with a new 6.5 Grendel upper with a 24 inch ER Shaw barrel. It took a lot but I finally got a load that would shoot just over 1/2 inch at 100yds. I know this is not a top-shelf unit but I was happy with this performance since it is my first long range AR. After finding a good load I worked up a batch of 100rds. The first couple groups were on par. Now, however, this same load won't even hold 1 inch. I gave it a thorough cleaning and let it foul up a bit as this is when the best groups came. All the brass and primers were from the same lot. Same canister of powder etc. What else should I be looking at?

Tikka

 

[JFComfort]

Re: Where to look for lost accuracy

Give it another go, maybe you were having a bad shooting day.

 

[RTS]

Re: Where to look for lost accuracy

Is the weather much hotter? Getting any signs of pressure? Any nicks on the crown?

 

[skeetlee]

Re: Where to look for lost accuracy

What type of barrel does your rifle have? Chrome lined? Maybe you have a little carbon issue or perhaps the other two fellas are correct. Could be a change in the weather. It does matter some. it might be you just not being on your game as well. Very possable. Good luck brother! Lee

 

[Greg Langelius *]

Re: Where to look for lost accuracy

Accuracy loss can be frustrating to diagnose.

The first thing I do is review the things that have happened since the last time things were doing well; because something has changed. Identifying that change is the first step to renewing prior accuracy.

The rifle: Crown, optical mounts.

The ammo: Heat exposure (car trunks get hot!)

The scope: impact/shock, objective lens adjustment changed

Objectives can be adjusted for focus, and this can affect parallax. Parallax compensation error is probably the most frequently misdiagnosed accuracy problem.

Bob/wag the head to move the eye around relative to the eyepiece lens/optical axis. If the motion causes the image to shift around behind the reticle, the parallax is miscompensated. Adjust the objective lens focus until this relative motion goes away. Many (especially cheaper) scopes don't have the focus/parallax adjustments properly synchronized, so you may have to choose between a crisp focus and proper parallax compensation.

If the distance has changed significantly, the parallax compensation most likely needs to be readjusted.

AR's can be difficult to establish a reliable cheekweld with scopes, and problems with this can combine with parallax compensation error to create a frustrating diagnostic problem.

Greg

 

[Tangodown911]

Re: Where to look for lost accuracy

Very sharp Greg. Parallax is something often overlooked and forgotten about when it comes to hole in hole accuracy. Good point.

Also may want to make sure the neck and chamber is clean. Carbon build up can cause excessive neck tension. Something else to consider and often overlooked as well.

 

[head2h2o]

Re: Where to look for lost accuracy

If you are shooting a ball powder, those are the ones that tend to be most affected by heat in my experience. I would also heed the words that Greg Langelius said. Also, check all your screws and pins to make sure they are not loose or out of place.

 

[jambau]

Re: Where to look for lost accuracy

Thanks for the responses guys,

Since beginning work with this gun we have gone through a seasonal change. Currently it is in the high eighties here in PA. Work began in the fall so there is a substantial change in temp. The "golden load" was found during the winter. I was afraid to give the gun a 100% cleaning since it achieved the best accuracy after about 12 rounds. I'll examine the throat more closely. I have noticed that the case necks get fairly heavy scratches and dents on them while being ejected. I have also discarded several cases that have seperated (cracked) along those cracks. Is this inconsistency typical of AR's and gas guns in general? I was toying with the idea of getting a Les Baer or other upper end rig. but if they are going to display the same degree of inconsistency why bother. Also, I am a relative newbee to long range gas guns so I'm wondering: do long range shooters develop different loads for different temps or they rely on the consistency of their equipment.

Thanks again,

Tikka

 

[Greg Langelius *]

Re: Where to look for lost accuracy

If your load works good in Winter, but less in Summer; redevelop a new load for Summer. Your load should be tested under conditions that most closely resemble the conditions under which you can most likely expect it to be used. For example, match loads should be tested with a warm/hot/fouled barrel, and climactic conditions similar to match conditions.

The energy included in a powder charge also includes the latent heat present from its pre-ignition temperature. When it's warmer, the same powder charge is more energetic; so when developing Summer loads from Winter loads, begun by stepping the charge <span style="font-style: italic">down</span> in small increments. Logically, a cold Winter load would have a slightly heavier powder charge, and may benefit from a magnum primer since intense cold can retard ignition.

Likewise, hot chamber cartridge retention time should be checked for its effects on accuracy. If match conditions require I retain a cartridge in a hot chamber for unusually long, I will extract it for cooling, and replace it with a cool one. Under switchy conditions I will not chamber the cartridge fully until I have decided the conditions favor a shot soon; and this also allows more air to circulate in the chamber, assisting the cooling process.

When I develop a load, I use standard, not match, primers. Once development is complete, I substitute match primers, to see if they help enough to justify their cost. If I did my development job right, quite often they won't be needed.

My service rifle and hunting loads are deliberately developed not to need match components.

I had a stubborn neck scratch/neck split issue that turned out to be a resizing die that had hard foreign particle imbedded in the neck. This stuff can be tough as nails to polish out, and can also occur in chamber necks. It laughs at conventional cleaning methods.

Greg

 

[TonyAngel]

Re: Where to look for lost accuracy

Is this a chrome lined barrel? If it isn't, have you cleaned it with a copper solvent? The bores and throats on less expensive barrels aren't exactly mirror smooth. I've seen rifles that needed to have copper fouling removed after as few as 40 rounds.

Other wise, try using a powder that isn't so temperature sensitive. You'd probably be best served with some sort of stick powder. I live down in south Louisiana and the temps range from in the 30s (and colder) to approaching 100 and I've had good luck with Reloader 15. What shoots good in the cold, also shoots good in the hot.

 

[jambau]

Re: Where to look for lost accuracy

TonyAngel,

The powder I was using is Benchmark so I guess it's too sensitive to temps. I have Relosder 15 on the bench but have no load information for it in the Grendel. What's your pet load?

Tikka

 

[wfjames22]

Re: Where to look for lost accuracy

Benchmark should be less temperature sensitive than RL15.

 

[boltstop]

Re: Where to look for lost accuracy

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Tango__Down</div><div class="ubbcode-body">Very sharp Greg. Parallax is something often overlooked and forgotten about when it comes to hole in hole accuracy. Good point.

Also may want to make sure the neck and chamber is clean. Carbon build up can cause excessive neck tension. Something else to consider and often overlooked as well. </div></div>

You guys know how I know Greg is RIGHT ON with what he's saying?

Because when I fire my .22 LR Rem 597 semi-auto, recoil does not disturb my cheek weld. I can watch the "hole in hole" shooting as it occurs. I think with high-power rifles, you lose that cheek weld to some extent and have to re-establish with each trigger pull.

 

[jambau]

Re: Where to look for lost accuracy

The lower is a bushmaster with an upper from Model 1 Sales. It has an ER Shaw chrome moly barrel 24 inches with 1 in 9 twist.