Don't worry too much about the comp. tube since they are probably as good or better than anything else out there. There can always be a bad one, but I had one professionally honed and it was very near perfect, from the factory except for the ABT notches. Still only had a 9 fps spread with 20 pellets straight from the tin. Not sure if they are still this good as this was a 20 year old Mark3. Won a couple of state titles and shot many sub .5" groups at 50 m with this gun. The divot on the inside that is just opposite the cocking shoe recess on most tubes isn't a problem because it isn't in the swept area of the seal.
Nearly half a tinful through oday, and still 9.9 _ time for a seal change
have you checked your breech seals ? Not sure how old the gun is, but that can compress and go hard over time...
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.
On reading the thread, I'm not sure of you have an AA or HW piston seal, but with a Mk.2 piston rod and Mk.3 mainspring, 41mm of preload should get you comfortably over 11 ft. lb. with the AA seal. a little more with the HW, unless air is leaking somewhere.
As Jon suggests, I'd look again at breech seals and, if you are sure they're OK, remove the piston and cylinder, then test for air leakage past the cylinder end plug thread by sealing the outlet with your thumb and manually pushing the piston up the cylinder.
Well worth persisting with; TX200s with 85mm strokes and Mk.3 springs took the top three places at the UKAHFT national recoil championship last year.
And, talking about "back in the day", showing how right the HW77 prescription was for sub-12, back in the early 80s. With a tad less stroke, too.
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I am using an AA seal,new from Chambers, but it is still much tighter than it seems should be. I had the same problem problem when I carried out your "simple tune " on my LGV,and you were kind enough, correctly, to point out then that the trouble was likely to be a tight seal. Resizing the seal, very time consuming, completely solved the problem, which is what caused me to start this thread in search of the best way to resize a seal. Nick G is kindly going to send me a run in seal which should confirm that the seal is the problem, or tell me to look elsewhere.