Often times, when you have an integral suppressor, the actual barrel (inside the suppressor) is far shorter than the full length of the suppressor. In other words the rifle may appear to have a heavy 20" barrel. But the actual barrel length is 10" (or whatever).
The point being, that these barrels are designed to not allow a given ammo to go supersonic. I have a M-169 AR15/9mm upper that was designed to shoot common Wal-Mart 115 grain bulk packed ammo at subsonic velocities.
FWIW: my experience with suppressors in general is that the small hole is quieter. I am no suppressor expert and am basing this primarily on two suppressors that I own. One 9mm and one in .45 ACP. This is flawed comparison since the two suppressors are manufactured by different companies, the guns are different............ But, when using sub-sonic ammo, the 9mm can is significantly quieter than the .45 can. They arn't even close.
You know the rest. In the books you have read
How the British Regulars fired and fled,
How the farmers gave them ball for ball,
From behind each fence and farmyard wall,
Chasing the redcoats down the lane,
Then crossing the fields to emerge again
Under the trees at the turn of the road,
And only pausing to fire and load.