I can't help but think that it might actually be slightly quieter with a SHORTER barrel.
Basically, the gas escapes from the gap because the bullet is blocking the barrel. As soon as the bullet leaves, pressure drops rapidly by way of the muzzle. If the bullet spends less time in the barrel (shorter barrel), it may blow less gas out the gap.
Whether or not the fractions of milliseconds difference produces any discernible real world difference is another matter.
On the ther hand, the shorter barrel means that the supressor has to deal with more pressure too.
Might well be 6 of one 1/2 dozen of the other.
