Beeman 1783 questions

Well greetings from Nebraska everyone. I just moved back home after 10 years being abroad, and do a great deal of gardening. The problems are as follows: my family as a whole is having serious issues with rabbits and squirells in town, and seeing as how we all rely on trading I thought I would spend the winter blasting the vermin and filling my freezer. Everyone's happy. But I grew up with my trusty .22 rimfire in the country, so the pellet gun is a bit foreign to me. I have this Beeman 1783 that sat in my uncles garage for 14 years. I test fired it into a tree the other day and it seems to be in good working order so I bought a Barska 3-9 power by 32mm obj scope for it as the old one is just unsatisfactory for me. Does anyone know what i should do to ensure the internals are in good working order? I'd also like to know what range you guys would zero at. 
 
Its a 500 fps easy to cock gun. Zero could probably be around 15 to 20 yards. Probably need to replace the breach seal and if its not brittle you can try flip it around might help or just find an O Ring the same size from a local hardware store. Store it with the barrel in the popped slightly open position so it doesn't get squashed.

https://youtu.be/BSj33Q39RBM


 
I just got a 1783 that needs this seal. I'm thinking this should be the "breech seal" on the dual caliber model? Can't find any other reference to it.

BSB1783B.jpg


BSB1783A.jpg