I have to agree with Hank. Maybe an 18 gr Berger. Genco has a 19gr VLDHP with a 9 ogive. My buddy is only getting around 3300fps with a 25gr Hornady from a 15in XP100 with a tight chamber.

I do not own a .17 Mach4, but just to give a bit of reference: .17 Remington, 15" barrel, 20 grain Hornady VMAX
max load of Varget yielded 3600f/s
max load of H4895 yielded 3950f/s

I'm trying to get a 17 Mach IV to hit 4000+ fps from a 12" Contender. I've been using 18.7 grains of H335 with a 25 grain Hornady molycoated 25 grain hollow point and a small rifle primer, but I'm maxing out at 3100 fps. Any suggestions for loads using the lighter 15 grain Berger bullets?

Thanks,

Michael J. Green