The max xp bonus can be completely random it seems. As far as the value of the bonus, it really does matter how rare the gun is(green and blue get a low level bonus, where as purple and orange get a significant bonus).
Everyone needs to get over the 'having to change weapons to level' constant gripe that I keep hearing. Get over it. The gun levels are additional bonuses that are OPTIONAL. Nothing is forcing you to do it. It's entirely up to the player. If you want to stay with your safe gun, than do so. If you want all the leveled up stats, you have to work for it. Sorry. I know that stings.
Defiance Primary: Victim Platform: Xbox 360 - NA Current EGO: Gigantic
I -believe- a currently "capped" weapon isn't actually supposed to be capped, but instead level 2. If you noticed, it simply says "weapon level up" on the screen and the bonus isn't very big, which would make me inclined to think it could get bigger/better with more levels.
Yeah, the +Mag bonuses for weapons that have locked magazine sizes (Sawed Offs and some of the Launchers & Detonaters) is pretty silly.