Problem
Regardless of pressure right up to 300 the cylinder should be an easy fit right up until the last little twist, which is where it opens the valve into the regulator, which is where the first time it has pressure in the action. On yours it is clearly acting against the pressure most of the way in, hence its stiffer with more pressure, so this is an issue/problem/fault either with the valve in the cylinder or the action actuator.
If you have any one you know with another cylinder LP300 or 400, just try theirs and see if it goes in easily, if it does its a cylinder issue on yours, if its the same as yours, its the pistol action. Either way its easy to resolve, but best by some one who is familiar with them, I'd suggest contacting Wonky Donky on here, he'll sort out either the cylinder or regulator.
Have Fun
Robin
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?