did you do anything to compensate the gas system for the extra presssure?
I use a Schuster Adjustable Gas Plug to deal with the increased gas pressure. I adjust it till it works suppressed and it does fine (unsuppressed as well). Theres still a moderate amount of gas thats pushed back down the barrel but it's tolerable.
Does this adapter thread on or just clamp?
With respect to attaching the Delta P adapter, it is splined to fit the splines on the barrel (just tap it on and tighten the castle nut and cinch down the set screw that keeps the castle nut from backing off).
I am confused on the squaring up your muzzle comment. How could it be unsquare?
Mass produced barrels (especially barrels for military rifles whose muzzles are designed for flash hiders/compensators that will cover the crown) can have muzzle faces that arent square. In fact, it's fairly common in mass produced "factory" barrels.
How an unsquare muzzle effects suppressing the M1A, is that once the castle nut on the adapter is tightened the adapter indexes on the muzzle face (when the castle nut is tightened it pulls the adapter tight against the muzzle face).
If it's not square the adapter will be canted and you may get a massive POI shift or worse baffle strikes with your suppressor attached.
The muzzle face on my Scout was not square and my POI shift with the suppressor attached was 2 feet at 100 yards.
I had a gunsmith square it and it was good to go after that.
I can not stress how important it is to ensure that the muzzle face is square on your M1A prior to installing this adapter.
I was lucky that my suppressor wasn't damaged.
With regards to suppressing the M1A, you need to manage your expectations with respect to suppression on this specific platform.
It will be a louder platform than a bolt gun due to the gas system and the action noise (not to mention the sonic crack of the bullet).
I'd rate mine as being about as loud as an unsuppressed 22 magnum.
Definitely quieter than without the suppressor but still (depending on where I'm shooting it) ear ringing loud.