Help! Why did my group explode?

[Mikecr]

I had a NF NXS that caused double grouping. THE ONLY way to figure this out was to swap scopes back & forth.
Sent it to NF, they found a glass bedding issue, fixed it, returned it with 3 days turnaround!
Good company

I don't like neck benders, and would throw the Hornady into a river.. Good idea.
But 6" is too much for this alone.

 

[J E Custom]

Got my new gun, Weatherby MK V Accumark Pro in 338 Lapua. Dialed in a load that gave me half inch moa. Once fired Lapua brass, neck sized, Fed 215 primers, 91 g Retumbo ( ya I found some), 250g Barnes LRX. I loaded 50, shot 15 and was pleased w half inch group. Then I wanted to be fancy, got a concentricity tool and brought the remaining 35 rounds all to within .002 of runout. I went and shot today and my target looked like pepperoni on a large pizza, I was all over. High, low, left, and right. After wasting 15 rounds and feeling like a joke at the range I left with a 6+" group. What did I do wrong? Double checked the rounds and all still within .002 of runout, COAL all the same. Do I pull my bullets and start all over, throw my concentricity Gauge in the river? Double and triple checked all screws on my scope? I'm upset and confused, please help, any ideas?

Many things can cause the group to change, But 12 times larger, has to be some mechanical in my opinion. Straitening loaded ammo should not have that much effect on accuracy unless you over did it. First, if you are loading ammo that has .002+ runout your procedure is poor/bad and needs improvement. Two much straitening can impact the accuracy some but not that much. First loading's shoot the best. Straitening ammo normally helps, but it never shoots as good as non straitened ammo with the same runout.

It sounds like a glass problem or a mounting problem to me. Ammo Improvement should not hurt/change the accuracy that much. There is one other thing that could have been the problem, You may have been having a really bad day and started flinching or punching the trigger. (We all have them).

Your idea of trying some factory ammo Is a good one, because you will have a comparison between two Different ammo's. If it is slight, it is an out side problem. If the difference is extreme, then the ammo may be the problem.

Hang in there.

J E CUSTOM

 

[P7M13]

Good idea, factory ammo as a sanity check.
After three rounds with no consistency, I'm surprised you weren't checking your hardware already. I've had scope mounts fail >3x over the years. Had a ring *break* just two weeks ago.

You may have been having a really bad day and started flinching or punching the trigger. (We all have them).

Time of day too. Hungry? Too much coffee? Alcohol consumption the night before? Medicine / diabetic?
Shouldn't be 6moa bad, but does affect.
There are some days my mind is so cluttered with work and other drama that I cant control my breathing or heart rate or trigger pull. I pack up and leave or go bust clays.

 

[blvette75]

I agree with J E Custom most of the times when I have seen groups open to an extreme loss of accuracy with a new rifle, new scope/mount (typically the mount of the scope). Two other occasions had a stored rifle (confirming zero) that were previously dialed and suddenly lost accuracy, my stock on my BAR got loose, on another rifle installed a new sling eyelet in my forearm and the new screw interfered with my free float of the barrel.

 

[ndking1126]

Got my new gun, Weatherby MK V Accumark Pro in 338 Lapua. Dialed in a load that gave me half inch moa. Once fired Lapua brass, neck sized, Fed 215 primers, 91 g Retumbo ( ya I found some), 250g Barnes LRX. I loaded 50, shot 15 and was pleased w half inch group. Then I wanted to be fancy, got a concentricity tool and brought the remaining 35 rounds all to within .002 of runout. I went and shot today and my target looked like pepperoni on a large pizza, I was all over. High, low, left, and right. After wasting 15 rounds and feeling like a joke at the range I left with a 6+" group. What did I do wrong? Double checked the rounds and all still within .002 of runout, COAL all the same. Do I pull my bullets and start all over, throw my concentricity Gauge in the river? Double and triple checked all screws on my scope? I'm upset and confused, please help, any ideas?

Sounds like an equipment failure to me.. loose scope or broken scope as others has mentioned.

 

[Wv shooter]

You said it was a 338 Lapua I agree with all on most reasons for accuracy problem Have you checked the break

 

[Scott E Ames]

Light rifle with a relatively big recoil round like the 338 Lapua may have shaken something loose in the scope. It might not be a bad idea to remove the action from the stock and double check the recoil lug/ bedding areas out. Proper action screw torque may have been mentioned and I missed it as possible culprit but re-check them when putting action back. Good luck, I know how good it feels to go from shotgun groups to 1/2 MOA. Must be equally bad going the other way.
Was a Lead Sled involved in this?

 

[JimFisher]

A couple years ago I bought a new Wby 270 with composite stalk. First time out, it grouped 0.5 MOA with factory ammo. Decided to clean it thoroughly afterwards, and took it all apart. Next time out it wouldn't group better than 5 or 6 inches, same problem as you describe. After all the same troubleshooting steps, I finally called the Weatherby shop. Turns out, I hadn't properly torqued the trigger guard action screws for that type of stalk. 35 inch lbs rear, then same front screw, then up to 55 in/lbs rear, then 55 front. I did that and the rifle returned to 0.5 MOA with the same ammo. If you took the rifle apart to clean it, as stated, you might try re-torquing the action screws, as Scott also suggested. Good Luck!

 

[Blackhawk]

Many things can cause the group to change, But 12 times larger, has to be some mechanical in my opinion. Straitening loaded ammo should not have that much effect on accuracy unless you over did it. First, if you are loading ammo that has .002+ runout your procedure is poor/bad and needs improvement. Two much straitening can impact the accuracy some but not that much. First loading's shoot the best. Straitening ammo normally helps, but it never shoots as good as non straitened ammo with the same runout.

It sounds like a glass problem or a mounting problem to me. Ammo Improvement should not hurt/change the accuracy that much. There is one other thing that could have been the problem, You may have been having a really bad day and started flinching or punching the trigger. (We all have them).

Your idea of trying some factory ammo Is a good one, because you will have a comparison between two Different ammo's. If it is slight, it is an out side problem. If the difference is extreme, then the ammo may be the problem.

Hang in there.

J E CUSTOM

I myself would remove all hardware and start from scratch
1) Remove the scope from the rifle
2) Remove the all mounting hardware , base , rings etc.
3) Wipe down the receiver screw holes and the very screws themselves with alcohol, acetone etc.
4) Wipe down the scope removing any residual oils.
5) Clean all ring and base screws and screw holes.
6) Remove the action screws and separate the stock from the rifles action.
7) Examine and wipe down everything.
8 )Reassembled rifle stock and rifles action. ( use correct torque settings)
9) Reassemble scope rail or bases to rifles action.
10) Carefully remount and center scope up.
111)Use blue lock tite ,or thread locker if you choose and retighten all scope hardware but not completely.
12) Check scope foe levelness and proper eye relief., and adjust if needed.
14) Tighten all screws to proper torque settings .
15) Take to the range !
Best Of Luck !
Side Note : their is no step #13
Addendum :
After reading other posts please just check one more thing. Have you checked your action screws and tightened them to the correct torque settings?

 

[TheBoctor]

Is the gap between the barrel and the stock looking any different? Can you slap the forend without it knocking the barrel? The barreled action could've walked around in the stock and now you're making contact. I've had rifles that would only have stock/barrel contact under recoil with a loaded bipod and the groups would go crazy.

 

[Skimbleshanks]

If your .001-.005" rounds shot well, then the ones you "fixed" shot worse, I would stop fixing them. I've read about runout in ammo. Some say it matters a lot. Some say it doesn't. Skilled shooters and reloaders on both sides. Let the results dictate what you do.

 

[Savage 12BVSS]

Notice OP has stopped posting, perhaps found the problem or was overwhelmed by responses. Something mechanical with rifle or scope not runout. You need to refine your loading method's until you get runout below .002 from your loaded rounds, the excessive tweaking involved to straighten causes more problems than it solves. Before you throw it in any river send it to me I'll pay shipping and with a few simple mod's the hornady tool works quite well for its intended purpose. Look at other possible problems mentioned in post's. My goal is to assemble cartridges with about .001 runout and they don't really need bending and yawing on them to correct.

 

[Laelkhunter]

Remove the brake from the barrel, and take a bullet and see if it drops through the brake without getting stuck. I have heard that sometimes the brake gets a burr inside the brake, and it causes the bullet to deflect a little. I believe it is caused by the gasses cutting the vents in the brake and causes some rough edges. If it does seem like the bullet is hitting the interior of the brake in a spot or two, use a chainsaw file (round rattail file) and remove just the burr until the bullet slides through.

 

[Don Garlow]

Got my new gun, Weatherby MK V Accumark Pro in 338 Lapua. Dialed in a load that gave me half inch moa. Once fired Lapua brass, neck sized, Fed 215 primers, 91 g Retumbo ( ya I found some), 250g Barnes LRX. I loaded 50, shot 15 and was pleased w half inch group. Then I wanted to be fancy, got a concentricity tool and brought the remaining 35 rounds all to within .002 of runout. I went and shot today and my target looked like pepperoni on a large pizza, I was all over. High, low, left, and right. After wasting 15 rounds and feeling like a joke at the range I left with a 6+" group. What did I do wrong? Double checked the rounds and all still within .002 of runout, COAL all the same. Do I pull my bullets and start all over, throw my concentricity Gauge in the river? Double and triple checked all screws on my scope? I'm upset and confused, please help, any ideas?

One possibility that I have personally experienced is this: I some how must have accidentally bumped the crown of the muzzle on something hard and burred the crown. The rifle shot sub half minute and then, suddenly out of no where my groups would not stay on 8 1/2 x 11 sheet of paper. I tried everything that I and others could think of. 300+ rounds later, I had the the muzzle re-crowned. Problem solved!