No reliable way I know. In Division 1 we used to joke about it being "*insert gun or gear piece here* hour", because you'd pick up 3 or 4 of the same thing in a row often times. Division 2 seems to be the same way. If I loot a shotgun, I'm sure to loot 2 more right after. Once an LMG drops for you, run to your nearest Control Points and see if you can loot a couple more and keep the best one.