Cooler burning = less energy. That’s what they had to use in Long Bertha in 1918 and still burned the 150+ caliber barrels like candles. There is only so much playing with the propellant grain shape one can do, while maintaining ballistic uniformity.
“The Denel system, which is actually bi-modular, won hands down although bi-modular was less than ideal. The reason for this is a very clever design that puts cooler burning propellant gas against the barrel wall with hotter inside it, in consequence barrel life went thru the roof, for ever in arty terms, so they didn’t need to buy so many barrels.”