I believe this is a terrible system, I'd prefer having to unlock the weapon first with my level, and then with squad points (called coins here).
Squad points are very easy to earn - just play the game! Sometimes you'll get them for getting killed a couple of times in a row, but most of the time you get them naturally over time.
As for being "stuck" with a gun - thats not a problem, actually. Yes, you'll have to play 3-4 games with this weapon, or one of the standard-issue guns, but they're all pretty good, so you shouldn't feel limited in any way because of it. And after a few games you should have enough squad points to buy the next gun or attachment.
I know, I constantly buy more weapons and attachments and have 250 unused squad points. I had around 200 when I prestiged. Now I almost reached 2nd prestige.