rraisley::: You are not missing a thing. You are correct in your calculations for real world effect on a firearm with a rebounding fireing pin, such as the 15-22......If you are having any light FP strikes, it is most likly caused by the fact you have lightened the Hammer spring, by cutting, bending, or replacement; done to no doubt lighten the trigger pull......If you are satisfied with the lightened trigger pull, and not to debate the merits of this.......To correct the light strickes, from the rebounding FP, with a lightened hammer spring, you will most likly need to cut the length of the Fireing Pin Spring period .....Here is the real "formula" that works: Remove the FP and spring from the bolt. With the spring still installed on the FP. Look at the end of the FP that fires the shells, and then Cut the FP spring, two FULL coils LONGER than the length of the FP, past the end of the FP. I believe this to be about optium length FP spring for a lightened hammer spring setup. This is what works for me.... However if you just can't take my word for it, then make several longer cuts and try them....best regards Plum