Grouping issues

[Sabre170]

I’m looking for some pointers and thoughts on an issue I’m having:

Rifle: 5.56 AR platform SBR w/7” barrel.
Brand new. Just finished building it.
Ammo: BlackHills 55gr soft tip.

Rifle shoots acceptable groupings at 25yd with no suppressor.

When I put my 5.56 can (HTG suppressor) on, my grouping goes crazy. Like all over the map.

When I put a buddies 5.56 can on, grouping is just fine.

When I put my can on a different AR platform with 18” barrel, grouping is just fine.

Clearly my can and my 7” barrel alone are fine. There is something specific with the two together that gets odd.

Any thoughts or theories?

My buddy thinks adding a crush washer may help...

[Sabre170]

Hmmm.

My buddy’s can worked just fine on my 7”. I would have thought that harmonics wouldn’t have been so vastly different!?!