FMJ Roundnose was not permitted, due to the high percentage of ricochets. JHP, JSP, FMJ-HP, SWC and lead RN were allowed. Lead RN just seemed to "bite" on the pins better than FMJ RN, with fewer ricochets, but JHP & SWC slugs were the preferred ones.
Pin guns were built with compensators, hi-vis sights, and oversize levers for speed. All the other shooting games of the time also came to benefit, and as pin shooting declined, "pin guns" became "race guns", and other mods became more common, winding up with what we have today.
I saw another guy get nailed in the testicles and he dropped like a bag of rocks.