Let’s say a new high energy propellant is now in use for infantry rifles. With this new propellant, bullet velocities are increasing into the range of 1200m/s with a 110 grain projectile out of a 20’ barrel. The problem I am foreseeing is that with these increases in projectile velocity there will also be overheating issues on rifled barrels due to friction. Could a wax or polymer coating be applied to the projectile to engage rifling without shredding the barrel? If not what are some things that could be done?
These are problems modern weapns already have to solve,
is coating the barrel with exceptionally wear resistant coatings, this is done to machine gun barrels since they will see a lot of rounds in rapid succession, wear and heat is a problem. look up stellite which was put on M60 barrels.
use a sabot, this is much like what you propose but solid. A softer solid material surrounding the round which engages the rifling instead of the round. the sabot can be a much much softer material than a round and thus reduce wear dramatically. you can have integral sabots or discarded sabots that fall away from the round after it leaves the barrel. this is the solution railguns use. You can read more about sabot here. You likely want to use both solutions.
Heat dissipation is a question of active or passive cooling, again look at machine guns for inspiration since they have the same heat and wear problems, in their case due to volume less than velocity.
We already account for the friction
The way we do this is by making the projectile out of a softer metal than the rifle. The round suffers a slight ablative loss traveling through the rifled scoring. Bullets are lead or copper, partly for this reason (among other benefits). Have you ever seen a steel bullet from a rifled barrel? It doesn’t bode well for the barrel.
Removing friction removes the benefit
If your round doesn’t bite into the rifled scores in the barrrel, how will it possibly gain a spin? The friction is what causes the rifled barrel to “bite in” to the bullet, giving it the stabilizing spin. A perfectly frictionless exit will have a round leave your barrel and fall into a tumble. Your accuracy will suffer greatly.
Heat dissipation is the only solution
Gun design has to account for this heat of friction as a necessary part of the design. A lot of engineering goes into a complete gun design, that limits the rounds per minute and operating environments and lubrication. Friction is not the bad guy here, it serves a purpose. You just need to mitigate the heat damage operationally.
Compressed gas cooler.
I was thinking about radiators for the gun barrel but those are apparently done.
Better: a tank of pressurized air! On releasing the pressure the expanding gas absorbs heat from the environment which is why those compressed air boat horns get so cold.
Your future soldiers have a big tank of compressed air. It runs thru the barrel when the gun is in use and absorbs heat then comes out the front.
"But wah!" the comments say. "Compressed gas is heavy!". The freaking gun is heavy as well as all the ammunition. One more heavy thing along for the ride will not be a deal killer.
Also when the barrel smokes after use, the smoke will go down because it is cold.
We are allready there :-)
... Firearm muzzle velocities range from approximately 120 m/s (390 ft/s) to 370 m/s (1,200 ft/s) in black powder muskets, to more than 1,200 m/s (3,900 ft/s) in modern rifles with high-velocity cartridges such as the .220 Swift and .204 Ruger, ...
Coat the bullets (the projectile itself) with molybdenum disulfide before they're inserted into the casing. This is used in some applications to increase duty cycle in automatics and such. It's not an extremely durable coating though; if you have a bunch of rounds loose in a bag, the coating will get scratched off.