I purchased four tins of JSB 15.89 gr pellets last month that are 5.55 to 5.57 mm head size. My gauntlet scatters them badly. The previous tins were tack drivers.
Look up Hajimoto videos, he has one on the complete process of setting up the desired pressure on a Ninja regulator, including a complete list of everything you need to do the job. He even includes the parts required for the pressure tester.
Lube the pump! Lube the pump! Lube the pump! Lube the pump! Lube the pump! Shoot the heck out of it and enjoy. I run mine between 2600 down to 2400 PSI. No need for higher pressures in this rifle. The pump will last longer Enjoy your Aspen.