Help! Why did my group explode?

[Kodiakman178]

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?

 

[codyadams]

Scope ring cap screws, ring base screws, base screws, action screws, if you have a muzzle device, check to make sure it is still tight, and I know this sounds ridiculous....but make sure you can't unscrew the barrel by hand. I have now found two guns that I was able to unscrew the barrel by hand.....one was a custom of a builder I will not name, and the other was a factory Begara. It only takes a couple seconds to check. If all this is good, I would look at the optic itself next.

What process did you use to dial in your runout? To me this sounds like more of an issue of the rifle/optic, unless something dramatic happened with the ammo.

 

[kiwikid]

How much runout did your original loads have? Also as codyadams has asked what process did you use, my guess is a Hornady concentricity guage?

 

[Kodiakman178]

Thx for the quick replies, yes re re checked all screws, barrel, compensator, everything I can think of. My glass is a brand new NF ATACR, it's solid and everything was great before I "fixed" the rounds on the gauge. Yes it was a Hornady concentricity tool. Original loads had anywhere from .001- .005 of runout. Prob gonna pull The bullets, resize brass and start over and see if I can get my original results back. You ever hear of a concentricity tool doing this to a grouping before? Thanks again

 

[Joemac6324]

Did you try cleaning it?

 

[kiwikid]

This is just my theory, but if you hold a cartridge by the base at one end and the tip of the bullet at the other and then apply force onto the bullet at the 3 o'clock position where it exits the case neck something has to give. One possibility is the bullet deforms but being a solid copper bullet that is unlikely. Another possibility is the brass neck deforms which may produce an uneven amount of neck tension around the bullet. At the 3 o'clock position the tension increases and it decreases at the 9 o'clock position but stays relatively even at both 6 and 12 o'clock positions.
There are several good threads on this site about concentricity that you could search for. Most of them suggest it is better to eliminate what is causing the runout rather than trying to rectify it once the cartridge is loaded.
It is however hard to accept that what you did in the way of making your rounds more concentric could make a 1/2" load into a 6" load.

 

[Kodiakman178]

I'm gonna send a box of factory loads through it to make sure it's not the gun or glass. If they shoot decent I'll just have to start my reloading process over again. Gun was cleaned after I shot it last, and i gave it a look over before I shot, all clean and clear. I'll get back after I shoot the factory load, thanks again for All the suggestions and help, it just doesn't make sense.

 

[thatguyshm]

Worked on many farms in high school, learned long ago, if it ain't broke, don't fix it. I'd pull them, resize and go back to having a bit of run out. If you can't hit it, doesn't matter how straight your ammo is..

 

[338 dude]

Never bought into the theory of fixing the run out on my rounds if you have a good processes your run out will be minimal and I check my run out but would not try to bend the bullet straight it is what it is at that point if you have bad run out then you need to go back and evaluate your processes

 

[Tiny Tim]

Load a few more without using the concentricity tool and see how they shoot. If it goes back to .5 MOA, you've found your problem. May be an issue with the forces involved with a mono? Anyone else experience this?

 

[Wolf76]

.5 moa turns into 6+ moa = mechanical failure somewhere.

 

[Kmccord]

Shotgun patterns I have witnessed leads back to the scope. I had one that would not hold zero and checked with a different scope, problem solved, my dad had his old 30-06 last year with the same scope for 20 years pulls it out of the safe and had a shotgun pattern, took it off and added another problem solved. On this forum a man with a new Bergara 300 WM had issues with not holding zero he went through the motions of sending it back to Bergara, it came back with no issues, his scope was added back problem still there, he changed to a cheap tasco and his groups came back. His scope was a new Night Force and it is now sent back.

 

[338 dude]

Shotgun patterns I have witnessed leads back to the scope. I had one that would not hold zero and checked with a different scope, problem solved, my dad had his old 30-06 last year with the same scope for 20 years pulls it out of the safe and had a shotgun pattern, took it off and added another problem solved. On this forum a man with a new Bergara 300 WM had issues with not holding zero he went through the motions of sending it back to Bergara, it came back with no issues, his scope was added back problem still there, he changed to a cheap tasco and his groups came back. His scope was a new Night Force and it is now sent back.

I followed that post and was shocked to find out it was the scope I own 2 nightforce scopes never had any problems they have a very good reputation I would've never suspected the scope but of course anything man-made can break hopefully this is not a trend and this gentleman's night force scope is not the problem

 

[Plinker147]

Yes I would load a few more and shoot them before you pull the rest. That way you can be sure if that was the issue. Never heard of that happening from fixing run out?

Make sure the wingnut behind the trigger is screwed in too. Those big boomers take some concentration to shoot, you could have just had a bad day?