I dont remember the specifics, but I recall reading somewhere about a particular type of solvent that recently became popular that greatly increases the mg/ml possibilities, but that many are allergic to and have reactions to.
does that ring a bell for anyone?
also, as for the site pain part of the problem,
a) try heating the shot in hot water or in front of a space heater immediately b4 injecting.
b) I took 1-2 gr of beta-alanine about a half hour b4 shots and this greatly reduced pain. imo this works because just as beta-alanine provides a buffer vs lactic acid and helps to clear it from worked muscles, so too with the BA and solvents that cause IM shot pain. this is only an opinion but seemed to work for me