Crosman 1300KT has lost accuracy

Having the same situation today with my Diana 34. It's 7 or 8 years old, installed a 12 fpe spring last Fall. Been accurate - one inch groups to 30 yds till today. Tightened all the screws as you did,, and yea all tight. Scope and mount the same. Still patterning all over a 3" circle.

It's been hot here already, 105 today, 112 over the weekend. There's a race track 5 miles from here and their internet weather readout gave density altitude of over 8100 ft. I'm at 4300 feet. So I'm thinkin' there isn't as much air there to be had. Springers and pumpers, are down on performance. I didn't set up the chrono for numbers, but will be testing some lighter pellets for accuracy tomorrow. Hope this helps.

The track's weather is presented by Kestrel. I'll take their word on that...

Buck n Willcox.

I highly doubt you shot the rifling out of it. I would look at the barrel band pump plug thing. If yours is plastic, maybe from all the pumping it has developed some play. Every time you pump it up, your barrel is either moving around or the pressure that is exerted on it is different.

Just replace it or at least see if your barrel is snug. I am at work now so I cant look at my 1300kt to give you more guidance.

Steel or plastic breach? If plastic the barrel could be moving around. 

Inspect your crown and the rifling just inside the muzzle. Fliers are usually a last inch of the barrel issue. Crosman has been known to chew up the rifling just in from the muzzle. When your gun was really dirty, it may have masked the issue.

No, you were cleaning it properly. Maybe just got it too clean.

If you have slop in this area were the pivot pin goes through, or the barrel isn’t pinched tight, that could be an issue.

Inspect the rifling inside the muzzle and look for chatter marks before you do anything.

Did you switch to a hollow pellet probe looking for more velocity? They are accuracy wreckers in .177

Take the barrel off and push a pellet through it. Nice and slow and then inspect it. See if it has nice little rifling marks on the skirt. Or maybe it will show signs on the skirt that it is snagging on the transfer port.

I’ve found Crosman barrels to be a crap-shoot. Had a 1322 with ‘ghost rifling’, never grouped tight ‘till I replaced the barrel.

Wouldn’t break the bank to try

As Vet suggested, slugging the bore can often reveal problems. Sometimes you’ll feel rough spots or a burr at the crown.