I've been running tomb of elders and contest of elders non-stop, with & without tonics and I keep getting the same weapons as well as the same rolls, more than 3 or 4 times. The weapons which have the least drop rates, at least for me, is noxious vetiever and gridskipper. Also have reset saladin 3 times and even though I've focused all of my engrams on the sidearm and machine gun, have not yet gotten a rimestealer + chill clip on either.