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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?

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5 Answers 5

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These are problems modern weapns already have to solve,

  1. 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.

  2. 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.

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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.

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  • $\begingroup$ These are relevant points but do sabots actually reduce friction, or just permit more kinetic energy in the round? The sabot is engaging the barrel the same way a soft round would. $\endgroup$
    – Vogon Poet
    Mar 14 at 0:54
  • $\begingroup$ @VogonPoet because sabots can be made of much much softer material than a round they can greatly reduce wear, how much friction they reduce is arguable but it has to be a decent amount for the same round to have much less wear. $\endgroup$
    – John
    Mar 14 at 20:38
  • $\begingroup$ I’m into speculation now but trying to stay with the question, less barrel wear would happen with a sabot for the same reason as soft rounds. Reducing heat? Not sure. Increased ballistic coefficients? It seems it may. $\endgroup$
    – Vogon Poet
    Mar 15 at 1:19
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    $\begingroup$ @VogonPoet but a sabot can be MUCH softer than a effective round can be. $\endgroup$
    – John
    Mar 15 at 21:25
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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.

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    $\begingroup$ actually many machine guns have internal coating on the barrels to reduce wear. Reduced friction does not harm the performance, since the round is still spun, there is just less friction on the round. Stellite being a famous example $\endgroup$
    – John
    Mar 13 at 18:29
  • $\begingroup$ @John I think it's a matter of degrees. A friction-less muzzle would not spin the round at all. It's a balancing act between competing priorities, so reducing one parameter will have both good and bad effects. $\endgroup$
    – Vogon Poet
    Mar 14 at 14:24
  • $\begingroup$ You left off "change the barrel" — most machine guns are issued with additional barrels for this explicit purpose. I don't remember the specifics but it's along the lines of "full cyclic for 1 minute, change the barrel, let it cool". $\endgroup$
    – coll
    Mar 14 at 15:27
  • $\begingroup$ Also, bullets come in many different flavors — some are coated in a copper jacket with a steel core: en.wikipedia.org/wiki/Armor-piercing_bullet $\endgroup$
    – coll
    Mar 14 at 15:29
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    $\begingroup$ Friction has nothing to do with how rifling applies spin, it’s rigidity of the outermost material the projectile is made of that matters. Traditional rifling literally carves into that jacket, and modern polygonal rifling (as found for example in Glock handguns) deforms it. Both approaches work perfectly fine even if there is exactly zero friction between the barrel and the projectile, provided the jacket has sufficient rigidity. $\endgroup$ Mar 14 at 15:30
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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.

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  • $\begingroup$ To clarify compressed gas isn't generally all that heavy, but containers that are strong enough to hold compressed gas are very heavy. If you scuba dive you'll know that your air tank is just as heavy when you get out as it was when you went in (to within a very small margin). Also if we are going to use a solution like this then is there a reason to use compressed gas instead of something like liquid nitrogen? $\endgroup$
    – Kevin
    Mar 14 at 18:44
  • $\begingroup$ @Kevin - you could absolutely tote a dewar of liquid nitrogen and get even more cooling out of that than your compressed gas. I figured compressed gas tanks could be repressurized in the field. $\endgroup$
    – Willk
    Mar 15 at 1:21
  • $\begingroup$ The trouble with repressurizing in the field is that then you need a source of power, either gas or electric, which means you have to carry even more gear with you, at which point I thought it would just be easier to carry extra liquid nitrogen $\endgroup$
    – Kevin
    Mar 15 at 16:19
  • $\begingroup$ @Kevin Resupply generally is an issue with projectile firearms. You use up ammo and you gotta get more. So additional resupply either with compressed gas or liquid N2 does not seem implausible. $\endgroup$
    – Willk
    Mar 15 at 16:26
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We are allready there :-)

https://en.wikipedia.org/wiki/Muzzle_velocity

... 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, ...

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  • $\begingroup$ This is more a comment than an answer, a good one though. $\endgroup$
    – John
    Mar 14 at 20:42
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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.
My 2¢

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  • $\begingroup$ Lubrication of the bullet in one form or another has been the solution to this since patched round ball was replaced by unpatched conical bullets. Grease, moly disulfide, jackets, powder coating, all same result: harder than the lead to reduce stripping, and lower friction than fully hard metal (jackets are gilding metal that slides easier than pure copper or harder brass. $\endgroup$
    – Zeiss Ikon
    Mar 14 at 16:17

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