FWB300S experts - repair

[Nikkormat]

My 300S Junior is in a somewhat battered state. It looks as if it's been a hard-used club gun from the day it left the factory in 1982 to when I bought it earlier this year. It remains accurate though, and from an inspection through the transfer port, the piston buffer looks to be fine. It generally shoots well, but occasionally I get the impression that the sledge action is a little loose, a little too free. Also, if I hold the rifle vertically and slap the butt, the action unlocks and slides back.

Does this suggest a worn part in need of replacement? Or is there a way of tightening it up?

I'd send it to a professional, but over here I think it's pretty much a case of DIY.

Thanks,

Matt