Huh. I had immunoglobulin shots ("It's just like maple syrup!" the nurse told me) in my butt back in college, and I don't remember it hurting at all. It was just an odd sensation of so much heavy fluid being injected so slowly.

Mostly, I remember it being funny every time I went back and presented my bare butt to the nurse.

I got bit on my finger tip by a feral cat and had to get the first dose injection in the fingertip. It was not pleasant. The rest of them were in the butt which was not great but nothing compared to the fingertip. The cat did die and my parents were able to send it in and luckily the tests for the cat came back negative after a couple of days and I did not have to get all the doses.

Weird. I had the immunoglobulin shots, two in the legs, two in buttocks. Despite the syringes being larger than ones I usually see for vaccine it was nothing too memorable. The intramuscular shots (J&J for covid and Tdap honestly hurt more). The ~$17k bill the hospital sent me was the most painful part of the rabies post-exposure process to deal with, not any of the shots.

I had immunoglobulin + rabies vax in my arms and butt and it was just a standard vaccination kind of discomfort. I don't remember thinking "omg pain" at all.