@andrew1220 From John Beard:
"WWII-era mainsprings were manufactured individually on a spring-winding machine and have formed ends. They have either 29 or 35 coils, nominal.
Pre-WWII mainsprings were cut from a large spool of coiled wire and have 33 coils, nominal. They do not have formed ends."
I also read that NM guns had 21 coils but not sure if that's correct.
"WWII-era mainsprings were manufactured individually on a spring-winding machine and have formed ends. They have either 29 or 35 coils, nominal.
Pre-WWII mainsprings were cut from a large spool of coiled wire and have 33 coils, nominal. They do not have formed ends."
I also read that NM guns had 21 coils but not sure if that's correct.