I have an RWS Diana MOD 45 .177cal. that was built in 2001. Nearly all the shooting I have done with it was at a 3 pound coffee can filled with foam floating in water at about 65 yards. I have a Tasco pistol scope mounted on the “barrel block?” where the original rear sight was.
My father recently purchased a Gamo Expo 26 and I noticed I hit the can much more often with it, so I decided to do some accuracy test on my MOD 45. Very disappointing, it s grouping about 1 ½ inches at 50 feet from a sandbag. I know this rifle should do much better than that!
I mounted another scope and got the same performance. Then I oiled the piston with a high quality synthetic gun oil and cleaned the bore with Hopes #9 and several Q-tips until they came out clean. There was no change in accuracy.
This is my only air rifle and I am not familiar with all the terms but I am reasonably mechanically inclined. If any one has ideas on what I can do to get this rifle shooting the way it should please help me out. Just explain what you mean in baby steps so it does not go over my head.
It's kinda like working on a car...start with the simplest thing, and you'll usually fix the problem before you get to the difficult possibilities! Check out these things:
1 The previous poster is probably correct. If the scope is not specifically designed for spring-piston guns, you have probably shaken the reticle or lenses loose. Airgun scopes also typically have much closer (or adjustable) parallax settings than firearm glass.
2 Make sure the stock screws are tight. I like to drop the action out of the stock, sand any rough spots in the inletting, de-grease all three stock screws and their seats, and re-assemble with the lightest grade Loctite (light blue). It can also help to put additional washers and/or lockwashers under the smaller front screws.
3 Try many different pellets. Just like firearms, different airguns like different ammo, with no apparent rhyme or reason.
4 Be sure the breech seal is working correctly. Inspect it and hold your hand over the closed breech while firing, to see if you feel a puff of air.
Lastly, do not use firearm oils or cleaners in a spring airgun. Use only silicon oil or better, moly lubes in the chamber, other oils can detonate under the compression heat the piston generates. Hoppe's is total overkill since of course there is no powder fowling, and its solvents will harm the gun's plastic seals. Use citrus cleaners and/or gun oil to clean the bore.