It all depends. Were you aiming at your middle finger? Or were you giving it the middle finger? LOL
I don't see how it is hitting it, unless your grip is loose. Are you shooting with the stock grips? I could see your finger getting in between the trigger guard and the grip. Have someone look at your hand placement when you shoot. You might be putting your hand too far around in front and that is letting the recoil push back against only your thumb. OUCH.
Standard factory laminate grips. Working on turnibg my habd to where only the pad on the index finger is on the trigger. Middle finger is behind the square cut trigger gaurd. Need an experienced single action shooter to watch me shoot.
I've been shooting single actions for so long I had to think about it.
Are you holding it in a "death grip" trying to control muzzle rise? I let mine roll up and back under recoil. The curve of the grip helps in this.
I shoot Winchester 240 grain JSP's. Are you perhaps shooting a heavier load? Like Buffalo Bore Heavy's or 285 grain hunting rounds?
I just went and gripped mine to see where my finger hit the trigger - it's on the pad, but not centered, closer to the first joint. There is maybe 3/16" (I used my Stanley tape) between the rear of the trigger guard and the middle knuckle of my middle finger.
Shooting winchester 240's also. Definately the grip. Need to work on it. I did not count discharges, shooting til I had empty chamber, not flinching, or pushing. The last single action I had was in the early 80's, so I don't have experience for sure.
My favorite way to shoot a SBH or any single action is one-handed and let it roll up in your hand. Never had my knuckle rapped that way. Same thing using two hands, take a firm grip but don't fight the recoil. Let it rise. Oh yeah, and don't lock your elbows.
I seem to remember the same thing happening to me a few times. But with my long skinny hands, I went with a custom grip (Hogue) and now that gap is covered up. Not all that expensive and it is easier to get my hand set for a better trigger finger set.