You can have problems if you don't go deep enough to get it into the muscle. I once shot my calf subcutaneously by accident. It got better, but it felt really fucking weird for a while.
5/8 seems pretty damn small for ass shots. What's your bf?
Depending on your BF%, you may have to bury the 5/8 needle to get it in there. If it breaks off you are out of luck because you had to bury it so deep to get your shot. I use 1.5 for glute and I am lean. 1" would probably be ok. 5/8 too short IMO
Interesting, thanks guys. I have tons of 1.5" and usaully go in around 1". I'd rather bury a needle and keep it there, rather than attempting to hold it in place as I push it in.
You are basically running the risk of injecting into fatty tissue, which will slow the absobtion rate down dramatically as blood does not travel nearly as effectively through fat as it does lean tissue. So you may get residual releases for some time after injection (perhaps months even).
I don't know about potential problems but 5/8 are all I use and never had a problem. In fact I get less trauma from the needle so recovery is much quicker if any is even required. Of course I don't inject my glutes and my body fat is relatively low. I just bury the needle and even press the base into the skin then usually surf the web as I inject.