I am wearing a small patch of IV3000 on my left arm, to see if I have an allergic reaction to it. I'm not looking at the arm, but since I don't feel anything, either it fell off or so far so good.
I was doing a shot on Saturday in front of another diabetic and she commented on how short my needles are (5mm). She says that that length is too painful for her, and said that it was maybe because I'm skinnier. I think though, that it has more to do with how thin my skin is (THIN!). If she were injecting, say, her forearm, she'd probably prefer the short needles. This is a graphic I did on the wisdom of picking the right length of needles. To be honest, most people have more fat than this in most spots that they'd inject.
Intradermal injections, which I have accidentally done when I've had too much angle going in, can cause a bump if the volume of the injected liquid is great enough and close enough to the surface. It leaks into the fat slowly, and it hurts for as long as the fluid remains there to irritate the pain receptors in the skin. OUCH!
Intramuscular injections can be as painless as subcutaneous ones, but they're often not. It's pretty easy to irritate a muscle.