Tuning Artemis PP750 15 ft.lb possible?

Not sure on the pp750 but you may need to increase the transfer port size and probably plenum as well. 

Your biggest issue will be shot count - at 15 ft/lb it's going to be an air hog. 

Honestly I have the pp700w and getting 12 fpe was a pain. You can do it if you drill out the transfer port and get a bigger spring etc. One way is to take the gun apart and remove metal from the valve with a dremel to increase the plenum. If you take out too much you could have an accident tho so not advisable. What I did was tune to 11 fpe and accept the fact that I won't be killing critters past 20 or 30 yards.11 fpe in 22 actually knocks the piss out of squirrel and rabbit. Got to be precise and head shot them. It's possible that a heart lung shot would also work but I have never tried and it could just wound the animal. I just head shot every time and they die instantly. 

If you try to go up in power you may risk poppet damage then you have a very good leaker. No problem of you have a machine shop or at least a lathe to build your own replacement parts though...

https://www.gatewaytoairguns.org/GTA/index.php?topic=170095.0

A short answer directed to OP question - YES, it is possible and it is not hard to get there if you have parts in your drawers.

I'm assuming you have a .177 as my .22 will get 15-16FPE in stock form.

Do you have the full power version of the UK/Canadian version?

Hello --

I just had reason to look up the last chrony for my PP750 and am getting 16FPE at the muzzle with JSB 15.89gr diabolos. I was achieving velocities near 700fps, until the gun threw a stem through the brass poppet. Backed off the reg a bit and the hammer spring quite a bit, for velocities around 675fps. 

Mark

It sounds like you need to enlarge the valve port and transfer port to 4mm. 

A longer barrel will help as well.