What is your go to cheap ar ammo?
for random test and verification after i service a weapon i'll usually go through a 20 round magazine of Federal XM193 or equivalent (S&B, IMI, MKE, PMC, Fiocchi, Winchester, CBC, e tc all make an M193 equivalent) and a final 20 round magazine on full auto or burst (if applicable) of the same. if the customer requests a particular ammunition type or it's an H&K roller delay, i will ask for a sample of the ammo they want to use or the SKU so i can order the same stuff, otherwise it too will be tested with M193. i will usually also test 10 rounds of Mk 262 (5 semi, 5 auto, open tip match or if it's a hunting rifle then with HSM GameKing's or TRU GameKings, whichever is in stock), M855, and M196 (tracer, long bullet). i bought 2 barrels of Q3340 that i'll use if i run out of other stuff or if it's something that doesn't play nice with anything else, since it's technically a .223 cartridge by external dimensions but has an M193 equivalent powder charge.
if i have had to rebarrel i'll use two rounds of M197, a high pressure test load after the rebarrel and then inspect for defects. recently i've had to stupidly test with M855A1 by request since some agencies want that done despite probably never ever going to authorize it for use.
i go through a little under 1000 rounds a month for rifles typically. pistol testing has some slightly different ammunition choices but they're the equivalents pretty much, and i go through more of that than anything else.