I have an odd issue I'm not able to solve so I'm looking for some advice. This is the handload I'm using:

300 AAC Blackout
220 grain TMJ bullet
7.7 grain of Accurate No. 9
~1000 FPS

I'm using a Form 1 7.5" suppressor designed for the blackout and it's very quite on my all of my 300 AAC firearms.

Here's the issue. I can shoot about a 1/2" group at 50 yards with my Ruger American with the suppressor. However, using my 10.5" AR 300 AAC SBR my groups are all over the place at 50 yards. It's like a 6" group! Nothing is keyholing and I'm not getting baffle strikes. When I remove the suppressor from the 10.5" AR 300 AAC upper I can shoot about a 1" group at 50 yards. Is this a barrel issue with the AR or something else? The suppressor works great on the bold action. I don't have a clue what's going on with the AR. Any ideas?

You don't say what the American is chambered in. It matters. Certain cartridges or loads are more sensitive to the aerodynamics inside a can. I generally run my apertures between .045" and .060" over bullet diameter if I'm not too concerned about back pressure, but my Marlin 1894 .25-20 groups looked like buckshot pattern when using my Ninety Four suppressor, a 1.25"x7" .25 cal can I designed specifically for small cartridges in that class. I had the apertures at .305", .048" over bullet diameter. I increased them to .340", and all was well, suppressed groups matching unsuppressed with very little POI shift.

Also, just because your AR isn't keyholing doesn't mean the bullets are properly stabilized, and adding a suppressor can exacerbate an issue with marginal bullet stability. How does it shoot with the can and lighter supersonic loads?