Hard to beat Mosin-Nagants for price as far as rifles go. They are the rifle the USSR used in WW2. They are full powered battle rifles in 7.62 x 54mm. $80 for a rifle and $80 for 400 rounds of military FMJ ammo all over the ‘net now. $350 will get you 2 rifles and 800-900 rounds of ammo.
Yep. I had a character use a Mosin Nagant for “social development” in my first novel. Under $200, scoped, to hit a 10# paper plate target at 600 yards is a damn good bargain.
A battle rifle must be at least a semi-auto, fireing a full power catridge. Mosin-Nagant has that, but it's a bolt action. M-14, FN/FAL, H&K G-3, etc, even the SVT-40, which is a little on the delicate side, but is a semi-auto firing that same round as the Mosin. So do the SVD and the various east block rifles built on an AK like action, such as the Romanian PSL, but those are really sniper rifles. Which of course could be a handy thing to have, but with a just a little clean up and proper ammo selection, the Mosin-Nagant can mange that role too, as it did in WW-II.
I've fired the SVT and the MN, the latter in both "sniper" and standard infantry configurations.