Cheat code for hitting glutes -it's pretty simple:
Hold the syringe with your thumb and pointer finger, with the needle facing the opposite direction that you'd hold it for, say, pinning quads (If you're holding your right arm out straight, palm down with pin in hand, the needle would be facing the left).
Hope that makes sense. Anyways, if you hold it like this, you can easily dart in into your glute no matter how big/inflexible you arm. If you can wipe your butt, you can pin like this.