A pal of mind came up with a great idea why not have a certain number of attachment points spent unlock the gun on the opposite side?
Example: As level 32 I want both the Guns on my Engineer, in order to unlock it on the other side, I need to spend a certain amount on attachment credits. That why you need to level up to get the weapons you want and you have to spend a certain amount of credits to unlock them. It gives access to ALL guns on both sides, and once more guns are added, everything (gunwise) will be balanced.
Example: As level 32 I want both the Guns on my Engineer, in order to unlock it on the other side, I need to spend a certain amount on attachment credits. That why you need to level up to get the weapons you want and you have to spend a certain amount of credits to unlock them. It gives access to ALL guns on both sides, and once more guns are added, everything (gunwise) will be balanced.