I recently had a gun smith install a muzzle break on my .378 Weatherby Magnum. Since then I have yet to get it to shot tight groups (3/4"@100 yards) like it did before installing the break. The gun smith has a great reputation of doing good work. However, I heard that a rifle will take some time (shooting) before it will settle in with the new break. The rifle has the action bedded and the barrel free-floating. Any suggestion or help. Thanks Scratch

I suggest trying different ammo.

A properly aligned muzzle brake (its a "brake" like a car's brakes. it doesn't "break" anything ) will at the very least change the harmonics of the barrel because it adds weight to the end of it. Sometimes it doesn't change it much, but it can change it a whole lot. Because of the change it may or, like in your case, may not shoot the same ammo well.
You can also try handloading. Handloading will give you a much wider variety of ammo so you can find a load that shoots well in the newly modified gun.

If the break is a thread on type you may want to remove it and check to make sure the the crown is not damaged. If it was modified when he put the brake on it might not be cut square or could have been damaged during the installation. A small nick in the crown can cause some big problems.

Fumbler , I do handload and have tried Barnes, Noster Partitions different powder. The best group was around 1 1/2" with Barnes 270 grain xlc. I thought I'd try Barnes triple shock or Swift-A-Frames next Elkhunter, I like your idea, should I take it back to the gunsmith and have him do it or do it myself. Being I might not beable to detect a small bur or if the crown is out wack. Thanks Scratch

Try playing with your OAL. The brake will add weight to the end of the barrel changing the harmonics as another poster suggested. Altering the AOL with the same load that performed well before should allow you to "retune" your load to your barrel.
The brake shouldn`t need any "shooting in" there are no moveing parts nothing but added weight on the muzzle, although if it`s loose, vibration could be your problem.

Before you do anything, make sure that the brake is not hitting the bullet. There should be somthing like .020/side clearance between the bullet and the brake.

I'm with Tailgunner on this one. There is very little clearance for the bullet on an effective brake. If the barrel is not threaded concentric to the bore, the bullet could clip the I.D. of the brake.

If the brake is removable just try some rounds w/o it and see if your groups change. If your 378 has the stock barrel like mine (I have a 378 w/ a brake) the barrel is pretty light for the bore size and certainly a big brake on the end of the bore could upset accuracy.

sjmci, Your right about the barrel and having a large brake on the it. How does your .378 WM shoot after you installed the muzzle brake. Thanks

Scratch