If you just want something else to shoot M43-ish ammo in and have $300 and you already have an AR15, consider a 7.62x39 Upper. PSA is currently offering Carbine format 16+ inch barreled uppers with bolt Carrier Group and Charging handle for $289 as of this afternoon. Bear Creek also occasionally has them for less than that.
You do need a special magazine and either a rear sight or some sort of optic to go with that. I will say the C Products Defense 20 rounder seems to work and hear good things about the 28, 30 and 10 round magazines as well. I chose the 20 because I felt the 28 &30 were to long to allow me to get in a good prone....just as are 30 round AR .223 mags, which is why I use 20 rounders there as well. The M43 ammo mags are longer than AR 20s but seem a bit shorter than AR30 rounders while the 28 &30 rounders seem a bit longer than the .223 30 rounder.
I seriously considered getting several 10 rounders for the M43 ammo upper....I mean an SKS is a 10 shot and a Garand is an eight shot and I liked both of those in the way back, granted both are clip loaders (SKS stripper and Garand enbloc) but I hope not to be in anymore military battles at my age but do like keeping myself as low to the ground as possible for my prone shooting and as close to the table as I can bench shooting.
-kBob
