New features in latest build: 01.02.15

It’s only natural that if a soldier uses the same weapon alot, they will be familiar with it and how it works. This is now represented in the game by a small accumulative bonus that the soldier gets, the more they use a weapon. This bonus will also slowly degrade over time.

Source:

http://www.moddb.com/news/new-features-in-latest-build-0102