i can barely squeeze primo through a 23G much less a slin pin.. i don't even think you could load it.
tren E with 200mg per ml that i have takes forever to load even with a 25G, would be hell trying to get it through a slin pin. but i know some guys do it or claim they do it. who knows. i guess you could hit the forearms or traps with a slin pin.. or even the biceps.. ouch! i used to get allergy shots in my triceps, not sure what size needle they used though.