Diana scope 3-9x 40 AO - I need a parts diagram and how to find spare parts to it. In a 'just in case mode' (Magnum airgun recoil) what spare parts should I have to this scope?

Air guns have recoil both ways - front and rear - and this presents a challenge to mounting scopes on them. You need an airgun rated scope, and if it looks like the mount was cemented on, it probably was. There are a number of companies here in the US that work on them. I would check with Pyramid Air - they sell a lot of airguns, and they would be the first place to check. They will know who to contact for mounting the rail back on the gun, and then you will be on the way as long as you have an airgun rated scope. Country of Manufacture is a dicey proposition at best.- Benjamin was always a premium US airgun manufacturer, but today is anyone's guess.

Red Laser Sight 650nm Weaver rail mount - Gamo USA

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650nm Red Laser Sight with Weaver style rail mount.

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Air Rifle Scope Mounts/Rails. Air Rifle Scope Mounts/Rails. Air Rifle Sights. Replaceable sights for air rifles. Air Rifle Spare Parts. Replaceable parts for air rifles.

I found this video that may be able to help you. Good luck.
www.youtube.com/watch?v=SSnQTdMh80k

clean it, if that doesn't work they have adjuster between the power selector and the tube of the scope

If it fits the rail, it'll work, just make sure to adjust the scope to absorb the recoil, by target practice at about 60-80 ft, and adjusting the scope up or down until it absorbs the motion of the recoil. Hope that helps!

can you increase the power of this air gun

http://www.crosman.com/airguns/service/manuals/crosman-rifles

Look up the correct model (it depends on when your gun was made) and download the manual. Once you've identified the parts you need, just call or email crosman with your credit card in hand and they'll get you squared away.

Well, it Barska says it's .50 cal proofed, whatever that means. One would presume it could take 30 magnum recoil then.

On the other hand, Barska is a low end quality manufacturer. You cannot expect durability in a low end scope. Of course, you could get lucky.

Unless you're target shooting at an immovable object, 10x - 40x is much more power than you'll ever be able to use. You're much, much better off considering a 4x - 16, for instance, in a higher end scope line than this scope.

Consider that at 40x the exit pupil would only be 1.25 which would make even the highest quality scopes quite dim. Barska won't have be best glass in the world and so would probably be unusable beyond some level like 18x unless in very favorable conditions anyway.

The problem with heavy recoilers and scopes is that the heavier the scope the more likely it is to take damage due to recoil. It's because it hangs out there on those high scope rings, and is heavy.