Hum, I did not think about the water jugs, as I do not drink anything in gallon jugs any more. But I did think about trying wet newspaper to see what actually happens, as to penetration and expansion. Sounds like another reason to shoot more.
Regards the .267 bullets, that is the actual measurement I get, I do not alter them in any way and seating them to 1.794 is required to get functional cambering in my gun due to the bore diameter being .264 The bullets actually will not fit in the throat past the round nose. I have shot oversize bullets before in experiments and can tell you they look quite normal if within .003 over the bore diameter. Actually you are talking about a .0015 per side reduction and moving something 1.5 thousands is pretty negligible for what we are doing and at the pressures we are using. But it does give the burn in the case a millisecond delay as the bullet is swagged. Note here: if the nose is at/in the bore and the base is in the case, the case neck keeps the bullet centered as it feeds into the bore under pressure. This creates an acceptable accuracy for short distance but, I would not want to try to hit anything much past 100. I am a firm believer in base of bullet to base of neck for best accuracy.
Little changes make a lot of difference in the Grendel and the pressure curve's timing is critical for function in my opinion. That is why my barrel length and tube type is so important. TX could not get my load to function his gun, at least with the .264 bullets, as he is using a longer barrel. I suspect the function has everything to do with the pressure pulse at the gas port and the pressure pulse point changes with different powders.
The only reason TX and I were talking about a duplex load was to possibly use the slow powder as a case filler to try to reduce the ES in these loads. The SD at the current level is OK, but the ES is higher than I would like as I would really want to have the entire string be closer to a stable 1,100 fps. However as TX noted, if the slow powder does not burn, as has been his experience, it is the same as using a buffer that is not combustible. In such a case with the gas gun, it would not take too many shots before you are out of business with the entire gas system plugged up.