Thanks for all the effort you put into the mods and according game stats. But in case of the MP5 it looks weird to me to have a barrel extension with those stats.

E.g. the MP5 has about a 9 Inch barrel designed to use most of the power of the 9x19 caliber. Even if you double that to 18 Inches (longer than an AK-47 barrel) you only get about 5% velocity increase. The increase in accuracy might be even less. So if you want to use barrel modifications maybe a heavier barrel to reduce recoil or something else might be more viable?

On shorter SMGs and Pistols it's a different matter though.

Just a suggestion to think about, love your work. :-)