I've got a buddy's Tippmann A5 that was a bit beat up and needed a bit of TLC. After breaking it down and cleaning 3 years of grime out of everything, replacing some missing pins, all of the orings, and the missing velocity screw, the marker refused to chrono over 225-230. After subsequently replacing the rear spring, the marker's highest reading I could get over the chrono was 240.
The only thing I noticed out of the ordinary was that I did not locktite the velocity screw in and it turned really easily- more easily than I've ever seen on an A5- I'm wondering if this may have something to do with it and it needs a new powertube? I didn't notice any cracks or anything, just the really loose velocity screw.
Try turn the velocity screw in more. Usually turning it out increases velocity, but if you turn it out too much you create a void in the side of the power tube where air can get hung up instead of flowing freely out which will lower velocity.
I had a A5 that no matter what I did it would not fire above 240. I was using a flatline so it wasn't really a big deal but I changed everything in that marker for the better part of 7 years. I finally traded it away. My best guess was the body halves were warped and allowing air to escape, or the internals which were polished were no longer creating a good seal around the bolt. It shot great...just not 280 fps.
^^ i agree. my guess would be a wore out power tube/valve. wouldnt be a bad idea to go ahead and replace the power tube. they arent that expensive. since you are replacing that, a new bolt isnt a bad idea either. you can find them for about $40 for the pair. imo i would upgrade the internals