Buying multiple "Case Box, 5.56mm" in one transaction from a vendor will cause the game to crash. I have repeated this several times to make sure it wasn't a one off, each try resulted in a system crash. Buying them one at a time will not cause a crash.

I have only tried reproducing this bug using Gun Runners, as, at the time, I could not find another vendor stocking the item. I am running the latest Patch and I have both Honest Hearts and Dead Money installed. I am not running any pre-order DLCs.