@sockyjimYou can get all attachments on weapons and vehicles but kills will stop counting once you reach Mastery 12.
So if you at that point want to progress to T1 you have to play MP for the kills to count.
I appreciate all of the answers. So my understanding is that when you first use a weapon, you have zero kills. You have to kill a certain amount (5-10 in most cases) of enemies to reach Mastery 1. The upgrades that require kills of less than Mastery 1 are accessible by playing solo. However, killing a bot does not count as a kill.. so there is no progress made on your Mastery 1 of that weapon. I noticed most every upgrade, but the first sight required at least a Mastery 1. Meaning a person can only (in most cases) open one of the sight upgrades by playing solo or multi-player against the bots. If it had to be just one upgrade.. that would be my pick. My iron sight game is weak. :/
