Those 170 grain FMJ bullets at 1300 fps will be made with Accurate n°9 powder. Max load is 15 grains for the 170 grain bullet and I plan to work up from 10% less like the manual indicates, so starting at 13,5 grains.
I already tried AA9 powder with my 180 grain plated bullets and it gives acceptable performance with the hotter loads (all rounds in the black of a standard pistol target), but for really good accuracy I need those FMJ bullets. Max load with AA9 and a 180 grain bullet is 13,5 grains and at 13 grains I got no pressure signs, cases did fly about 15 feet though. I don't expect pressure signs with AA9 as this is a very slow burning powder. Maybe if you hotrod it with above max loads, but when you stay within the data given in the manual you should not expect Glock smilies or bulged cases with a stock G20 as far as I see it.
Thanks for the tip of the aftermarket recoil rod, but for now I'm sticking with the stock one. It works well, and I will mostly shoot 180 grain plated bullets at +/- 1100 fps and 170 grain FMJ at +/- 1250 fps.