It's bore condition on the first shot. Grease makes it more consistent. You push the junk out on the first shot, what ever that junk is, in a dry? bore. Good old coefficient of friction. If you consider that ~20% of the energy in each shot goes to heating the bbl, you will be surprised to see how hot the bore actually gets. Mosin doesn't do it cause it's an old well used bbl, i.e. more clearance between the bore and jacket.
ironwokerwill: Gee, maybe I am selling my Lee scales short! Have you compared them with the RCBS chargemaster scale? Everytime I weigh something known (like a bullet) on one of my digital scales, they come up exact. I write the weight of the little plastic cup on it, as a check, and it is perfect every time. So I had assumed the digital scales were good to 0.1 grain or better. I think the Lee is probably there also. Is one better than the other?
popper: Wow, I had never thought of that. Putting together what you point out with what hammer previously explained, and I think we are MUCH closer to understanding! I did some math, using Q=mC(delta T), [heat = mass time * specific heat * change in temperature, from high school chemistry ] assuming a 1.5kg barrel of chrome-moly (.456J/g deg C). A 2000 ftlb bullet is 2700 J I found, and taking 20% of this I found that the barrel might gain 1 deg C per each shot. That seems reasonable, in fact it might be more than that, so indeed, 20% -- or more -- of the energy is going into the barrel!! If the coefficient of friction is altered by dried-out carbon residue, then it could be significantly more friction. That agrees with a lower velocity measurement I found on one shot. The grease would reduce the coefficient I would expect.
I checked, and yes, smokeless gunpowder [smokeless means products are almost completely gasseous, whereas black powder products maybe 50% solids] does have hydrogen and oxygen --hence should produce water (vapor). That means any unburned powder or ash (although minute, but present, we know, from patching our barrels) will have some water component -- which could become drier or wetter over time depending on ambient humidity---and thus change the initial friction.
A change of velocity of 50 fps (similar to what I've observed on 1st shot) is on the order of 100 ft lbs, or maybe 150 J. If the barrel is already absorbing 500 J or more, it is very reasonable to think that there could be a change in the coefficient of friction sufficient to change that energy loss to 600 or more J due to dried residue!!
So this is making a lot more sense.