Ages back I read a couple articles on suppressors that did some quite impressive sound measuring. The rough rule was that on equal internal volume, similarly designed suppressors, 1 inch of suppressor forward of the muzzle is equivalent to 4 inches behind the muzzle. When you look at suppressor decibel reduction ratings that seems to be reasonably true - overbarrel suppressors tend to have a lot more internal volume than muzzle forward, but they don't reduce the sound as much as the same volume muzzle forward suppressors.
There are obviously trade offs to putting a giant suppressor off the front of a rifle rather than putting some of it back over the barrel, but it's an interesting factor.
As others have said, you can put larger caliber suppressors on smaller guns. It'll be slightly less efficient, but probably not noticeably so. Seems lots of people use .30 cal suppressors on a range of rifles from .22 to .30 just out of convenience.
If your gun isn't factory threaded, check that it was done properly - there are some pretty terrible threading jobs and you can end up with the suppressor threading on wonky and the bullet striking the suppressor on the way out.
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