I use 1" 23g or 25g. Depends on where i'm shooting.
3ml barrels are perfect for me, as each time i shoot one it better be full.
I've met a few guys who had Slin pins break off in their muscle. One was easy to remove and the other had to have surgery to remove the needle. Those Slin needles are very weak. I have broken two of them and bent at least ten doing GH shots in my delts.
Good luck!