I shoot my quads, sometimes its sore for a few days, other times its not at all.
I found using a 25g needle is good for two purposes, one it is painless and the second is it helps control how fast I inject (I find it difficult to inject slow when I don't need to).
I run the rediject under warm water for about a minute and also massage the area that I'm going to use. Then I inject, followed by another massage, and then use a heat pad. I try to work that muscle (legs) either that day, or in another couple days.