The point of different ones is to
1. smooth out the pump stroke
2. lighten the pump stroke
3. To be able to "quick strip" the marker for cleaning.
4. (anyone else have any?)
Also I know there was a bolt made for sheridans that converted it to a centerfire, but I don't know if they made it for the PGP. I believe you can see one in the 'Aftermarket parts" section of this sight.