How I ruined a good gun--is their hope?

[crypto666]

A year and a half ago I bought a DPMS LR-30 .300 RSAUM and mounted on it an IOR scope. I was very impressed with its ability. It shot everything pretty well, factory loads grouped 1.25-1.5 @ 100yds, hand loads grouped such as 178 A-max/varget and 200 GK/R22 grouped .5-.75 inch @ 100 yds and sub MOA @ 300 yds. I tried lots of different loads and found the previously mentioned ones to be the best; nothing under 178 gr shot well, probably due to the 1-10 twist, not everything above that shot well either, but wasn't horrible. After spending a couple months grouping on paper, I moved to just shooting 1000yds and had great success. On good days with little wind I could typically keep it on a 12" gong 7/10 shots. I played around with different loads, upping powder and trying some Berger 190 VLDs and didn't find much improvement. After some time (~500 rounds) it seemed that my accuracy was slipping (I remember coming across a forum where someone mentioned taking this same gun to a competition and found that the gun shot great the first half of the day and horrible the last half, this could be any number of factors however.) Rather than putting it on paper again to confirm changes in accuracy, I just decided to get it barrel frozen (easier to clean, better groups, longer life?) and after it came back I decided to have a brake installed to improve the time needed for follow up shots, though the recoil wasn't bad to begin with (like a 12 gauge).
The first tests after getting it back from the gun smith were in the moonlight, and I did have some ok groups and was on target, but was taking it for granted because of the light conditions. The brake works wonderfully, however it did destroy the speaker on my M2 chrono. I have put about a hundred rounds through it since getting the work done and it won't group my old favorites like it used to; 178 A-max group @ 1-1.3", 200 gk group @ 1-2". Before, I would get a vertical dispersion of groups (the actual group did not disperse vertically) between different loads, all remaining centerline of target, or close to, now some loads group high right, others low left, etc. I have had best groups with 185 Lapua scenars, some sub-MOA, but they are high and to the right of the 178's. The 200 g are also high right with H4831 and high left R22. There also seems to be no correlation between group size and velocity S.D. Before having the work done I never had to touch my knobs, now I find myself playing with them trying to get everything somewhat centered.
Obviously the harmonics of my barrel have changed considerably for the worse. Before shooting it, the gunsmith told me his experience with cryo solved the problem of flyers, but opened up the overall group. I don't know what the major causal factor here is; cryo or brake. I am going to continue to try and tune some loads, and once I find a one that is acceptable I will try turning the brake out ½ turn at a time to see if it tightens things up. This is getting to be a really expensive waste of time and bullets.
The moral of the story—if it ain't broke, don't fix it.
If anyone has any suggestions, please advise.

 

[msalm]

I had very close to the same experience on the first rifle I ever barreled. I started out with a new Shilen select match unturned blank in 7mm. I made up a 280 AI and turned down the barrel contour on the lathe as I wanted to do 'eveything' for my own education. Well as you might imagine I induced a bit of stress in that barrel and although it shot pretty well (3/4 MOA) it would start to walk a bit after several rounds. I had it cryo'd and that solved the problem of a drifting impact but the 140 and 150 gr bullet I shot out of it no longer grouped better than 1 1/2 MOA. Funny thing is it would take a 110gr (I think that was the one) Sierra and plug them into 1/2 MOA all day long. I even remember getting a little upset at that rifle at the range one day and thought OK, if you like these so well lets see how many of them you can take. I shot 12 or 13 rounds as fast as I could load and get a good shot. That barrel was smoking after that little display of stupidity but the rounds STILL all clustered into less than a 1" ball on target. Haven't shot that rifle in about 5 years, and thinking of rebarreling it to some other caliber.

With your thought of the muzzlebrake, if you plan on loosening the brake, I hope you're planning on using a spacer to butt it up tight on the tenon before shooting it.

I don't think you ruined your rifle, but you did change it's characteristics. You have to just go to a different bullet and play with more loads to see what it likes again.

 

[crypto666]

Msalm, thanks for letting me know I am not alone. Education can be expensive. I am going to try some lighter bullets, though I like the heavies for elk and their long range performance. I have some 208 A-max on the way (those things are backordered everywhere) hopefully they will do something good. I think the ogive on the A-max lends itself to well to anything using a magazine and limited bullet length. The gun has always liked the A-max.
Thanks for making me realize the need to back up the brake. Anymore problems and I mught have to convert it to full-auto.

 

[DougH9]

My AR10 shot inconsistantly (sometimes great, sometimes OK, somtimes terrible) until I bedded the upper & lower together. Now it always shoots great.

You can experiment by tying the upper & lower together with several large tie-wraps (pull 'em real tight with pliers) and try it out. If that helps-bed it.

 

[Augustus]

.dpms

This may not be your problem but its easy to check. Check your gas block, the set screws have a nasty habit loosening off. Mine has done this a couple of times and it will screw with the groups. When you tighten it make sure everything is lined up right.

 

[crypto666]

things are looking better

I just got back from shooting and I am feeling better. Apparently the gun no longer likes any of it's old loads. It hates the 178 A-max and 200 GK with any powder combo, but I have been able to get down to ¾" with 185 scenars and 190 MK. It absolutely will not shoot anything with H4350, still loves varget and seems ok with 4831. I like the varget and have been getting single digit S.D. on velocity with most loads.

But I think the key is what DougH9 suggested. It totally made sense after putting more thought into it. The gun seemed to be losing accuracy before the work was done, but I didn't confirm it on paper. I had installed a little rubber thingy that I got from DPMS to tighten up the upper and lower assemblies since it loosened up over time. However, after I thought about it for a minute I realize that this thing could be causing some serious problems. It pushes the upper and lower apart in order to remove slack, but during firing this thingy is likely going to compress and could cause problems. light bulb Combine that with the fact it doesn't like the old loads anymore and I was in panic mode. So I chucked it, and replaced it with pieces of feeler gauge between the upper receiver just above the rear tack down pin. Tacked them in place with a little (very little) silicon. No more slop.

I think everything is going to be ok and the world is a better place now.

So thanks Doug, I think that little tidbit is what did the most good.

 

[Catfish]

This might be a silly question, but did you try seating your bullets out alittle??? .015 or .020 out might help too.

 

[Wild_Bill]

Things to try

Ok the first thing to do is take of the break and shoot it. if the groups are large have a gunsmith look at the crown. When threading the barrel was it recrowned after the machining if not their might be a mark from the live centre that would have been supporting the barrel inside the muzzle. the reason for removing the break is in case it is not perfectly centred and the projectiles are nicking it on exit.

If after removing it it shoots well have a diferent smith check the clearences. if it does not shoot haveva diferent smith check the crown and recrown it then refit the break checking the clearence.

Don't shoot it with the break loose you wil just create an accedent and might wreak the rifle.

The cryo treatment should not make the groups larger this is the first time I've heard that. Did the gunsmith do it himself or was it sent to a specialist cryo company?

Make sure every thing is correctly tensioned and try shooting itbwithout the break as above and it will tell you where to look. if you know a realyngood smith that can lap your barrel try that\as a last resort or even some lapping bullets it is more likley if the barrel was lapped it would have helped the groups more than cryoing tha barrel will ever do.

Cheers Bill
Australia

 

[DougH9]

My pleasure to help Crypto. Some say that reciever play in the AR's does not matter, but I have found this not to be true (it could be how I shoot; I never shoot from a bench, but instead prone with the rifle tensioned between my left hand & right shoulder). I burned a lot of ammo before I figured it out, but my AR10 now holds it's own with my bolt guns.

Have you shot the rifle with the play removed to comfirm the fix yet?

ETA: I would advise staying away from the 4350 & 4831 class of powders (excessive port pressure). The rifle was designed for powders like Varget, Re15, 4895, 4064. My favorite load is 44 grs. of Re15 with a Sierra 175 MK.

 

[crypto666]

Thanks for the tips Bill. However, I am particular about who I take my gun to and I like to keep it local and visit the shop so I can see what kind of operation it is. In this case I had Match Grade Machine in St. George, Utah thread and recrown my barrel. He did an excellent job and building barrels is all they do, though he doesn't normally do ARs.
I have got the gun shooting what it did when it was new, so I am happy. It just took a lot more load development than I had anticipated, and I quickly found myself in a panic.
I think that slop in my upper and lower was playing a part in things too. Which would explain why I and others have experienced a degradation in accuracy after putting a number of rounds through these guns. I am going to try and remove the shims to confirm this as Doug suggested and to resolve an argument with my brother.
One thing to note, once I got it shooting like I wanted a severe wind came up and blew over my chrono and, well, took my gun off the bench where it fell about five feet to concrete (adjacent shooting platform is lower than the one I was on, which is where it landed). I hate the wind and this didn't help. I picked the gun up and it fired it, it was still on zero. I love my valdada scope.