found an air rifle under my shed. ..

Be aware any volatile oil in the compression chamber, or barrel, will ignite under pressure, as when the gun is fired and the spring forces the piston forward.

There are several non-dieseling lubes/greases available for spring compressed air guns.

Your biggest rebuild challenge will be compressing the spring after you have everything taken apart and cleaned up. It takes a special tool to do this. Ask around the forum, as it is likely someone can help you out with that.

That's a classic high quality air gun...worth the time and effort to restore it.

If it's been under a shed for a few years then don't fire it again - certainly don't dry fire again - until you've given it a good seeing-to.

Careful with the spring, it is powerful and a b*gg*r to get recompressed and back in if you release it when disassembing..

Careful with lubes and where they go and don't go, as you may end up with a dieseling air rifle (as above - avoid this).

I'm not a spring rifle expert, but you may find some local spring rifle expert hobbyists via one of the UK forums.

I suggest airgunbbs or airgunforum. Someone in your county could look at it and guide you to a list of things that need doing.

Final thought, unlikely, but possible that someone left it there as it is an over 12lbs (UK FAC) rifle and they had "plausible deniability" while it was there.

This is very unlikely if the rifle is a .177. slightly more likely if it is a .22 or a .25.

But now you possess it, you are responsible for ensuring it is under 12fpe with nasty penalties if it is found >12fpe.

Cleaning the barrel and getting a local to chronograph it with pellets of known weight is probably wise. Get a local to help you with this.

I'm in Staffs.

Mark.

Cool find!
As far as the Ft/Lbs restrictions in the UK, I cannot speak to them (living in the US), but it is definitely possible that a .177 could produce 12+ ft/lbs. My .177 Hatsan 155 puts out a low functional range of 23.32. That being said, you may need to call in an expert to reduce the ft/lbs to an acceptable level for your locality, either by modifying the air outlet or by installing a weaker spring. If it purports 12 ft/lbs and is .177, it's likely a model designed for the US market.
As for the cleaning pellets, I would not recommend them for in-depth bore cleaning. They work OK for routine maintenance (I run 3 through the barrel every hundred shots or so), but they don't have enough abrasive force to remove rust and caked-on fouling. Better to go with a bore brush and some non-imbedding cleaning paste.
Keep us updated as the project proceeds!

Tell her that you'll give her the polished treasure from the shed and you'll suffer through a newer, less valuable gun, such as an FX Impact, or Verminator, just so you can be together and enjoy each other's company... ;-)

What about when the groundhog comes looking for his rifle in the night? Its not going to be pretty since the ones I've seen that have a rifle also have a knife! I would be out there stacking up rocks around that shed to block him in.

Idea...Air Arms TX200.

I think your UK buddies are correct. Looks just like the SMK Super Grade XS20

http://www.newatlanticonline.co.uk/products/shooting-and-country-sports/air-riflespistols-and-bb-guns/spring-powered-air-guns-by-smk/smk-xs20-22-air-rifle-open-sights.html

https://www.wonderlandmodels.com/products/sportmarketing-supergrade-custom-xs20/