I have about 70 shots on it so far, no change in POI or accuracy that I can tell, at least at the distances I can shoot in by backyard which is 20 yards or so.At 77 degrees I was getting 650-660 fps, now it's at 690-700 fps. (with 14.3g CPHP)With two 12 gram carts it gets 25 shots above 650 fps, at 30 shots it's down to 580 fps which is about where I stop and put in new carts.
the original qbs were about 520 to 530 fps guns at 75 degrees. best thing to do is taking breach off, making a new seal for the co2 port (cents in home depot), take the valve out and remove the felt inside of if (free), replace the 3 orings on the barrel and breach end (i think they are metric... i bought a bag online for 6 buck... 100s of them), place a couple of washers behind the hammer spring (less than .25$ at ace hardware), polishedall moving parts so they dont drag (free), take a dremel and expand the hole in the probe (free) and voila! reassemble and live in mid 600s fps with 14.3 grainers. rifle will be louder and get maybe 30 shots per 2 powerlets but it will be truly transformed. after that polishing the bore and cleaning the crown may be beneficial. i have had several qbs and 160 and all have gotten the treatment so the result is repeatable. i know have only 1 160 fully restored, and a qb78 that runs on either hpa or co2 (some part swap may be involved) with 3 barrels, a .22, a .177 and a shortened carbine .177 barrel. that QB was a present my wife presented me in 2008... its being the base of all my qb tuning. as it sits that kit is priceless to me... i love my QB.PS: by the way, the HPA kit still uses the original air tube. its a low pressure mod (1400 PSI) and only yields about 8 to 9 shots, but with the appropiate spring it pushes 14.3 at 910 fps... howitzer by qb standards:)
I know Dave... but that is what I have done and i take that responsibility for whatever "hazardous" condition it may or may not arise...lol. Those tubes by most engineering standards should be tested to at least 2000 psi. we all know that C02 can get to 1500 psi on a really hot day, so they gotta be able to withstand that at least. So making a low pressure system that runs as low as i have it is just to explore the power potential on a limited air supply. I ( and that is MY qb and my qb only) have run this setup for over a decade, no leaks or catastrophic failures at all. not even a mild deformation or stretching of the threads. I for one im a believer that the platform (and the xs60) can sustain that load safely. It my opinion and please do not take that as Gospel... this in no shape or form is an endorsement of that idea, and i acknowledge that there has been issues. But I will continue to use mine as such. there has been several low pressure pcp rifles made (the spitfire from BSA is an example), older Falcons, and more recently the good old disco.
Here's a link to the bolt probe mod.I think that pic was mine Jeff https://www.gatewaytoairguns.org/GTA/index.php?topic=188973.msg156177638#msg156177638