I use the 12.7 mm (1/2 inch) 30 gauge insulin syringe. Dr. Bernstein explains that this depth will only just reach the muscle and the needle must penetrate at 90 degrees. Bersnstein also says it will not work if you have “fat” arms. More usual IM needle lengths are 1 - 1.5 inches long. I do observe faster insulin action when I do this so I assume I’m hitting muscle.
Here’s a Bernstein video talking about and demonstrating an IM shot.