Yeah, like others said, an suppressor made for 8.6mm and you are shooting 2.1mm thinner projectile has to be a barrel or mounting problem. I hate to ask, but did you run a rod through the suppressor before starting? I am guessing not, but not judging.
Barrel concentricity on a button cut or broached rifle is quite variable (known as run out), and needs to be checked when cutting the barrel. A typicaly button barrel will have the bore snake aroound more than you think, and the skill, or the luck is cutting the muzzle end where the barrel is concentric, and tossing to scrap when it is not. This is one of the bigger differences in a match grade barrel and a barrel that will shoot okay. For example, Douglas, which makes some of the finest buttoned barrels around, grades the final product as standard or XX (going from memory), and this is all about run-out.
A simple insertation of a bore gauge saves the day. There are a handful of brands out there, but from $40 to $95, you will get a straight guide to insert and use and check before using the suppressor. Dave from Dead Air has a nice how-to video, and others more adept than me might agree with him: he inserts the rod all the way back so that just the end is sticking out of the suppressor. I prefer to use only the last 3" of the barrel, due to the potential snaking of the bore. For this excercise, I do not care about the run-out or variability of the path of the barrel bore, I care about the exit from the bore, and the threads and the squareness of the shoulder of the barrel at the end of the threads, as I am interested in two things:
1) is the angle of the projectile through the suppressor aligned to the suppressor cone and end aperature, and;
2) does the suppressor sit square on the end of the barrel.
