The coating is getting the bullet to move quicker out of the chamber but is lacking the obturation at the base of the bullet to seal the gases. So they need to up their charge weight to get to what they got without the coating.
No, no. I'm not suggesting the low friction coatings affect obturation. I'm saying that there's a common conception that higher friction between the barrel and bullet will decrease overall velocity, and that's not necessarily true.
Smokeless powder burns faster the more pressure it's under, so as the powder burns, pressure goes up, which causes the powder to burn faster, which causes pressure to go up, which causes the powder to burn faster, and so on, and so on. So anything we do to increase pressure early results in a geometrically greater pressure overall. It's a positive feedback loop. When the primer goes off, powder starts burning, pressure starts rising, and the bullet begins to move into and down the barrel. The thing is, while the ever faster burning powder and additional gaseous nitrogen works to increase pressure, the bullet moving into and down the barrel has the effect of expanding the combustion chamber, which reduces pressure. Through the first part of the burn, the expanding gases have a greater positive effect on pressure than the expanding chamber has on reducing pressure, but as the fuel source dwindles and the chamber continues to expand, the pressure starts falling. That's why the pressure curve rises then falls.
So, while increased friction certainly has the effect of decreasing acceleration, that decrease in acceleration has the affect of slowing down the expansion of the combustion chamber, which has a positive effect on pressure. What I've read of the bench rest shooters needing to increase powder charges after coating bullets with low-friction coatings to achieve the same velocities they had before would seem to indicate that the decrease in acceleration you get from higher friction is outweighed by the increase in pressure you get from the more slowly expanding combustion chamber. I suppose that which is the greater effect
could change with a given bullet weight, shank length, actual diameter relative to barrel, standard pressure levels, the particular barrel, powder, etc., etc., but it should not be assumed that higher friction between bullet and barrel results in lower velocity because in some cases the opposite is demonstrably true.
NOW, the reason all that mattered for the previous point is that for the two bullets I referenced -- the 125gr Blue RN and the 124gr JHP -- it would seem that given the same charge weight, the 124gr should have the advantage with velocity. It's lighter (by a hair) but it's seated much more deeply into the case. But there is some other factor that is giving the 125gr Blue Rn the velocity advantage at a given charge weight. The 125gr Blue RN coated lead bullet gets an extra 35 feet per sec at charge weight I referenced, and the JHP does in fact need an extra .2 grains to achieve the same velocity, AND the deeper seating depth should have been worth an extra .2 grains worth of powder, so it would seem the .001 larger, better obturating bullet is worth about .4 grains of powder there, which is significant. I know it won't play out exactly like that with every bullet comparison of the same or almost the same weight, but I would guess the better obturating bullet in that situation is always going to show the higher pressure. The reason it mattered to point out that a higher friction coefficient doesn't translate to slower velocity is to preemptively remove the idea that the reason the 124gr JHP needed extra powder was to overcome the greater friction. I was basically anticipating and addressing an argument before it was made.
edited to correct three type-os
