I wouldn't, if you dont pap, you just can go get ammo off the wall where the gun is. If you pap it you'll have to wait for a max ammo if you run out.
Nah You can still buy ammo of the wall if you PaP but it'll cost a lot more. Which wouldn't really matter in higher rounds.
OT: the ray gun is ok, the thunder gun is the way to go, with that you can easily get to higher rounds with no probs, I use the M16 pap and it also has an under barrel granade laucnher, it's useful tbh since you can buy the ammo right of the wall in the theatre. Also the AUG and the spas 12, RPK would work good as well PAP they are my personal fav, get mule kick, that way you can 3 weapons and it's quite handy.