Crosman Challenger 2009 (PCP). It maxed out at 530fps from factory- so I decided to pull it apart and expand the transfer port sleeve a bit. Seems I left the transfer port ('detuning' sleeve from a Crosman 13xx/22xx) at the office, so I had to drill another out. My drillbit collection seems to have diminished (seems someone borrowed a few and didn't return them yet)- so I ended up drilling the port out to 7/64ths.

So far so good. I test the rifle- shooting 680-ish with 14.3gr pellets. I take about 10 shots to ensure it's consistent. I note that by screwing the hammerspring adjuster in, I actually lose power, so the adjuster comes all the way out and things stabilize at 680 pretty much even.

Then after shooting for awhile, when the pressure drops from it's initial 2000psi to 1500psi-ish, the bolt starts being blown back on firing (was not occurring previously). This goes on until I reach 1000psi and decide to re-fill the reservoir and shoot some more before determining what to do about the bolt issue. Rifle at 2000psi now shooting 580fps, and then when pressure drops to 1500psi, bolt starts moving again and power goes up to 725fps. Hammerspring adjuster has little effect, but when screwed fully 'in' (i.e. hammer spring as tight as possible) velocity actually drops.

Without having more time to fiddle with it just yet- I am guessing I have a couple issues- hammer bounce and possibly not enough hammer travel to open the valve properly at higher pressures.

Any ideas?