I've been doing the FMJ challenges just to get the mastery titles. Other than that, I'm not actually going to use FMJ or extended mags in regular play.
I've got all of the assault rifle mastery titles now except AK47. I'll probably work on the LMGs ans SMGs during my next prestige. The sniper rifle challenges do seem like they'd be a pain in the ass. I'm a terrible sniper to begin with, so gimping myself with specific attachments would just drag my kill count down further.
A tip with the sniper rifles - glass counts as a penetration kill, but doesnt slow down or weaken your bullet. Its still going to take a long time, but at the start of every match, get your ass to a window and shoot someone through it.