I agree the weight of a muzzle break can affect the harmonic "trim" of a barrel.
That's likely the problem.
But consider this...something to keep in mind. With a typical AR brake,the muzzle threads are 1/2 - 20. The minor dia,or the thread relief will be about 7/16,or around .437 Now,if you figure the cross section area of a .437 round with a .224 hole in it,you hve about 1/10th sq in of steel cross section.
Just to relate to ft lbs of torque,lets figure a 12 in lever arm. Run that through one rev,it travels 12 time 3,14...someplace around 37.7 in travel to get .050 travel out of the thread.(OOPS!!. The 12 in lever arm is radius, Circumference is Pi D. So the travel is about 75 in to get .050 in Seems like that gives about a (753? No),1500 to one mechanical advantage.
Now,I don't claim high proficiency in physics.I might be looking at it wrong.
But it seems as if I put 20 lbs of force into 1500 to one mechanical advantage,,minus whatever friction and inefficiency...the gross number is about 30,000 lbs applied to 1/10th sq in of steel.
I know that doesn't sound right. It probably isn't.
My point is don't use three white knuckles worth of torque on your pickle to get it to clock "just a little more" because you just might be stretching barrel steel for 5 minutes on a clock face of rotation..
That's a bad place to distort if you want accuracy.
It seems a lot of folks think of a barrel as a rock solid,rigid,near indestructible piece of engineering structure.
Maybe its closer to being the neck on a violin
Actually,a match grade,cut rifled barrel I have ...made by one of the old school respected barrelmakers is a little disappointing in the accuracy dept.
I suspect I may have overtorqued the pickle and caused it. Its an 18 in fluted SDMR weight bbl that shoots just over 1 MOA at 100 yds.
I'm not going to blame the barrelmaker.
I think I'm going to cut the 1/2-20 threads off,and go with either no pickle,just a crown,or stick it in the lathe and thread it 5/8 24 or whatever the 30 cal standard is. I put my flash hiders in a collet and bore them to about minor dia of the 1/2 in thread anyway...roughly .430
The upper I used was one I was given,and it had been through three builds.
Maybe a fresh upper, lapped square,cutneww muzzle threads and maybe glue the barrel extension into the upper I might find out my barrel maker did me right.
I think a new cornea in my right eye will help,too.