12.7mm is only 1/2 an inch. It’s really not that long for many adults.
Try this:
Tighten your thigh muscle as hard as you can. Now try to pinch it. The part that is easy to pinch in your fingers is your skin and fat layer. The hard part underneath that does not move is your muscle. This can give you an idea of how long the needle needs to be to get into the muscle. Different areas of your body need different lengths to reach muscle.
I use the 12.7mm’s when I want to make absolutely sure.