Let's say you kill me and notice I have a snazzy SR-25 and you're also a Recon.
You pick it up and try it out. You then see a nice Mk16 SV next to me. You can't pick that up because you're not an Assault. Wert, wert.
Would being able to pick up same-class weapons be a good feature to allow players to demo weapons? They'd obviously lose it when they die, but it gives people a chance to try out of weapons if the firing range idea isn't taken. The kill credits can work as normal, so if I'm level 1 Assault and I kill 10 people with some Lv 30 Assault's F2000, I get 10 XP for leveling up the crit rate on that. This would also encourage cross-leveling all gun crit rates so that you get the same benefit from picking up a random gun as a high level if you're low on ammo, so if I kill a Level 1 with an M27 I can pick it up and go to town with my pre-leveled crit rate.