Six inches at 10 meters seems excessive. And this started happening suddenly? Are you using open sights or a scope (could be a scope issue). I would think this amount of wear would be something you would have noticed increase gradually. I have an HW35 circa 1978 and there is only a tiny amount of slop in the latch

Can you detect any movement in the barrel if you try to wiggle it when it's closed? Is your barrel pivot screw and nut tight enough to hold the barrel in any position when it's broken open? If those check out then it almost sounds like your barrel is coming partially unlatched during the shot cycle when the gun is recoiling rearward (maybe due to a worn receiver) and then latching up again when the receiver suddenly comes to a halt from the piston reaching the front of the receiver......maybe? 

Try placing your left hand at the front of the forearm and pull on the latch with your index finger or thumb so it stays put when you shoot the gun. Are your shots still stringing? I'm no air gun smith but those are things I'd look at. Hopefully the new latch and spring fix the problem. If it is a worn receiver and a new latch does not fit tightly maybe someone can make you a new one.

Let us know what you find out.
 
Harves, The barrel latch has a tapered grind. As it wears, the spring would push it rearwards to compensate. and still maintain a solid lockup. Unless someone has monkeyed with it, I can not see it causing that much of an accuracy issue. If you have vertical movement in the barrel it could be a worn barrel pivot bolt. 

What year is your rifle? I have several dating from the 50's to the 80's. All are great shooters with a little TLC

I would start with tightening up the pivot bolt and locking nut and replace the breech seal if older rifle. 

Shooting over a chronograph to make sure velocity is consistent and up to spec. would be ideal. Could have a busted mainspring.

If it has a scope, remove it and try shooting with open sights or try a known good scope.

Keep us posted with your findings
 
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Harves, The barrel latch has a tapered grind. As it wears, the spring would push it rearwards to compensate. and still maintain a solid lockup. Unless someone has monkeyed with it, I can not see it causing that much of an accuracy issue. If you have vertical movement in the barrel it could be a worn barrel pivot bolt. 

What year is your rifle? I have several dating from the 50's to the 80's. All are great shooters with a little TLC

I would start with tightening up the pivot bolt and locking nut and replace the breech seal if older rifle. 

Shooting over a chronograph to make sure velocity is consistent and up to spec. would be ideal. Could have a busted mainspring.

If it has a scope, remove it and try shooting with open sights or try a known good scope.

Keep us posted with your findings

All good ideas to isolate the problem.
 
...if you rest it on a shooting bag and shoot from bench, try setting it down on the bag a little harder to see if it truly is the barrel lock up area. i alway look at the reflection of light going down the side of the reciever / tube as compared to the reflection on the barrel. if its one consistent line of reflection, or eve parallel reflection after plopping it down to shoot, then it may be another issue. i first thought about the leather seals those older 35 have. impressive for leather, but air leaks arond and thru them as they get older and i replace that leather with a modern plastic seal and the adaptor you have to use (from air rifle headquarters). just get a mainspring, those piston seal parts, and a breech seal from macarri ....but you should be able to notice any barrel pivot issue just by gently slapping the barrel after cocking and loading - it'l move downward a tiny bit. i have to space up the old spring with washers sometimes on these turkish .20 webley vmx rifles quite a bit. the first .177 walther talon mag i ever had was sent back due to this same problem. and when i buy a new airgun at any store nowadays, i unbox it at the gun bar and hold that gun up against the edge of that glass encasement for pistols - i make sure there no barrel droop ..... i use a piece string pulled tight against the the whole metal area of the rifle when i'm at the shop. it seems to be a problem with turkish guns (kral, hatsan, and i forget the other brand - but they made the accuforce 1100). - paul.
 
Problem solved! I replace barrel latch & spring with parts ordered from the U.K. Air Venturi serviced the gun last month but did not have parts to replace barrel latch. Receiver does show wear but it was difficult for me to believe this 1982 vintage rifle would need a new receiver given the quality of the HW guns. Thanks for the help provided.