Short version, get  head space go and no go gauges to see which bolt is correct in it recess depth back from the start of the lugs.

Hence what we do know, is that from the start of chamber shoulder, to the bolt face at lock up, should be in the 1.121" range.
What is not given since this can come down to the wildcat type manufacture of the barrel, is what bolt recess dimensions they are using for the round, and this would determine the start of shoulder from the front of the barrel extension lugs itself.  The fact that the round is based off the 7.62X 39 case, one would think that the bolt recess depth is also the same as the X39, but the Grendel got a change instead.  The Grendel has a .136" recess, while the 7.62x39s made now have a .125" recess instead.

Simply, if the bolt in question is head spacing on the tighter/no go side, it going to cause problems with the working pressure of the round being much higher.  On normal head spacing with full sized case, you have the normal blow by as the case is firing forming to the chamber before it seals off the chamber, so there is an initial lose if pressure that does lower the over all working pressure of the round.  Now take a chamber that is head spacing too tight, that the round is being compressed into the chamber during the load, and since this will reduce the amount of blow by during fire forming, it will increase the working pressure of the round.  To add here, if you where to compare the spent primers from the two different cases, will see that the case that is being chamber wedge during the load,  has it primer much more flattened out.

So first determine which bolt is the correct recess back to the bolt face from the lugs, to give the correct head spacing of the barrel with it cut chamber.

Next, make sure that the cases are being full sized correctly for that head space, and when bullet crimping, that you are not using too much bullet crimp pressure to bulge the case shoulder to cause wedging problem at loading (read ammo test gauge comes in handy here to make sure that the case is still full sized after the round has been loaded).

Lastly, as you are working up loads, keep in mind that you are loading for a gas gun, that the burn rate of the powder will differ the pressure at the gas port, even though all the powders that you are using may all produce the same over all working pressures them selves.  Being that the slower the burn rate of the powder, the higher the gas port pressure will be, and again, with all the different powders loaded to the same over all working pressure.