remington 878

[U.S.M.C. Clay]

hey ya'll i came across a remington 878, it was in decent condition with a shortened barrel, but the auto part aint working, how exactly does this weapon operate?? i do not see and o-rings, so how does the piston work and how exactly could you break one??? i picked it up for free (from my pop). but would like to use it as home defense gun, so i would like to get it fully operational.
thanks
clay