Vertical stringing when suppressed

[Ratbat]

Just shot some American Eagle 220 grain subsonic 300 Blackout. Horizontally they all stay within 1/2 inch but they string vertically by about 2 inches. All stay within 1/4 inch of each other. Checked and rechecked the mount and all was tight. Shooting a YHM Phantom on a Riggio Arsenal lower with an unknown upper. Without the suppressor, all shots cluster in an acceptable group.
Any thoughts and recommendations? Thanks

[curtistactical]

Just shot some American Eagle 220 grain subsonic 300 Blackout. Horizontally they all stay within 1/2 inch but they string vertically by about 2 inches. All stay within 1/4 inch of each other. Checked and rechecked the mount and all was tight. Shooting a YHM Phantom on a Riggio Arsenal lower with an unknown upper. Without the suppressor, all shots cluster in an acceptable group.
Any thoughts and recommendations? Thanks

My thoughts would be inconsistent loads, you would need to chrono the shots to know for sure but this would be my first guess.

[CanOfWhooppass]

How was the rifle supported? Any chance a sandbag was touching the barrel or suppressor?

[Ratbat]

Ammo all came from the same box and the rifle was supported on a rest with just the free floating handguard touching. I would load 10 in the magazine, fire five with the suppressor on, fire five with the suppressor off, on side by side targets. Same result each time.

[thecameraman79]

My guess would be what curtistactical said. You didn't mention the range you were shooting at but I'm going to assume 100 yards. An ever so slight variance in powder (.1 grain or even less) behind a heavy projectile traveling at subsonic speeds can really make a difference at 100. 50 FPS variance with rounds traveling 3,000FPS vs 50 FPS variance traveling 1,000FPS means a much larger difference at 100 with the heavy subs.

[T-Rex]

Would you be able to post pics of the targets?
What is the actual group size, suppressed and unsuppressed?
Do you remember which way the suppressed hits traveled? Meaning, did they start low and climb, as you continued to fire?

[tf2addict]

If the unsuppressed shots grouped better it's unlikely it's an ammo variation issue unless that is just a bizarre coincidence that it just happened to be in the five rounds tested.

[CanOfWhooppass]

I'm guessing that with the weight of the suppressor some barrel mechanics is in play. If it's off a sandbag and the barrel gets partialy supported from the sandbag touching it, that could cause the barrel to shoot from different levels of flex.

[Tony M.]

It could be from the heat if you're firing very fast. The suppressor adds enough weight that barrel heating would be more pronounced with the can on.