I think I did okay with this air rifle? May need a new spring? Any recommendations as where to get one?
IIRC, this is the recoiling version of the FWB Mod150 sledge action spring rifle.
It is ostensibly a 6 Ft.lbs rifle, however, they can deliver a bit more poke with a slightly stronger spring, although I would advise against it with such a rare model given that spares (other than seals) will be extremely difficult to source.
Whilst it was intended for primarily 10m shooting the Mod150 and Mod300 / 300S rifles are surprisingly good at greater distance as well.
I would look on this a being a user not just a collectable although treat it with respect.
It should deliver between 575fps and 600fps if in good order, I have found they vary between these limits with Mod150/300's I have had and currently own.
I would suggest that you get a new piston buffer and breech seal as a matter of urgency for this rifle before shooting it any more.
These tend to degrade badly and disintegrate over time and a knackered piston buffer will cause damage if the rifle is fired too much before it is replaced.
The consistency suggests that the metal piston ring itself is OK - they rarely wear out on these guns.
The 'wearing parts' kits would fit the bill.
Note that one of the kits on the link features an 'export' spring that would give higher velocities, but just be cautious with this as it would be adding higher stress to older parts with possible unavailability of spares should something break.