Gunsmithing Crowning issue

[newbieshuter]

For the life of me i cannot figure out what is going on. I built a 9LB 338 lapua a few weeks ago and have taken it out and shot it, proof fired and started just shooting a few rounds through it. Was not bad in the sled and got it dialed in. Took an off hand shot a few days later at a coyote and thought hell i better muzzle break it. Turned it down 5/8-24 and put a break on it. Re did the same crown on the barrel and everything seemed fine. Went out shooting with it and although the recoil is easily tolerable now the groups have went from 1/2 moa to 1 1/2 with the break on. Took the break off and shot a few rounds on the sled to see what was going on. Same thing 1 1/2 moa gun. Before it was threaded it was a 1/2 moa rifle with an 11 degree crown, now its a 1 1/2 moa rifle with same 11 degree but threaded. Anyone ever experience this?

 

[buffalowinter]

Might not be the brake or crown...you're assuming. Go back and check all the obvious things...action screws, bedding, scope, rings, and ammo.

 

[Blowby]

You changed the structural characteristics of the barrel. Maybe a little change in your load to accommodate the only change you say you made.

 

[SPIN1963]

Sounds like the end of the barrel has been deformed slightly ... I just went through something similar on a supposedly "High end" muzzle brake. I would stick it back in the lathe and indicate inside and out to compare. Just a hunch. Also try a gauge pin of proper size and see how it feels going into the muzzle end of the barrel.

 

[ptm121]

Yup, 5/8-24 is pretty small for that bore. It probably had some residual stress in it, and belled out at the muzzle when you threaded it. Chop it and thread it to the largest diameter that you can (at least 0.750”), and check the bore before/after with gauge pins. It’s worth owning + and - plug gauges for common bore diameters to do these checks, so you don’t have to slug and mic, and so you can get land measurements.