Firearm modifications are one size fits all - which makes sense in a gameplay standpoint.
However, the way the descriptions are written currently it makes some of them that are particularly length inclined to look ridiculous on certain classes of firearms, especially when the modification is twice to three times the size of the gun itself.
A way to easily fix this would be to either make the descriptions when installed purposefully vague - changing a specific numerical length of something to something closer to "extending past the end of the barrel" or similar.
Another way to fix it would be to make each class of firearm skill (submachine guns, rifles, pistols) to have a different description when a mod is installed so the sizes don't look insane when installation occurs.
It wouldn't affect the game at all besides some slight hilarity when it comes to descriptions, but it has been a thing I've noticed multiple times over the years while playing characters that encounter the various mods and this is just one of many times I've been bothered by a pistol with a mod that makes it nearly three feet long.