Barrels Types and Cooling: AR-15

[Badger Arms]

I'd like to discuss cooling of barrels. Let's assume a few things. First, we are going to use the AR-15 as a basis for comparison. Assume all things are equal. We are shooting the same loads at the same rate with the same atmospheric conditions, same twist rate and bore type. The Fluted barrel is mid-way in weight between light and heavy barrels. Here's what I understand:

Light taper barrel: Heats up quickly. Bore temperature rises fastest of any round causing greater erosion than any other barrel type. Cools down fastest. Lightest weight. Least accurate. Groups open up quickest with heat. Recoil is greater and slightly more time is required for follow-up shots.

Heavy Barrel: Heats up slower. Bore temperature rises slowly due to large mass acting as a heat-sink. Longer strings will cause least amount of bore erosion. Cools slowest. Most accurate. Groups effected least by heat. Recoil is lighter and follow-up shots are quicker.

Fluted barrels: will handle heat in proportion to their barrel mass. Their primary advantage lies in their stiffness relative to their weight. All things being equal, you'll save weight with a fluted barrel.

Heat dissipation: A fluted barrel doesn't cool significantly faster than the other two barrel types. Here's why: unless air is forced over the surface of a barrel, the barrel creates a zone of air around it. There is a 'boundary layer' of air near the barrel and in the flutes which cools at nearly the same rate as a non-fluted barrel. Yes, it cools faster, but this shouldn't be a primary concern in your purchase decision. The main question should revolve around whether or not you want the lighter weight. Fluting lowers weight between 6 ounces to a pound depending on who does it.

Handguards: 'Dissipator' type barrel/handguard arrangements cool the barrels fastest. The entire barrel is enclosed. A2 style handguards create a 'smokestack, which draws cool air in from the bottom through convection. M4 style handguards cool a little slower, but your handguards aren't going to heat up as quickly due to the double heat shields. A1 style grips cool slower. Free-float tubes without the holes slower. The slowest cooling handguard out there is a graphite-epoxy one like the ones sold by Armalite. They trap heat in and insulate it. They do keep your hands cool enough though.

Rate of Fire: If you want accuracy, don't let your barrel heat up. Barrels won't heat up quickly unless you shoot rapid-fire or full-auto intentionally for more than one magazine. In reality, you probably won't ruin your barrel unless you shoot full-auto and let it get hot enough to nearly glow. Semi-autos are very hard to heat up to this stage without being stupid. This applies to any barrel type. Even if you want to 'rock and roll,' you probably aren't going to do it with your 'race gun' or DCM setup. Given all of these factors, I am under the impression that a light or medium taper barrel is the best for most situations.

 

[DMK]

Is this because I brought up the request for a new production 20" A1 barrel profile in the other thread?

This is good info. Good point about the barrel weight vs. throat erosion trade off. However, I think your statement at the end says a lot: "In reality, you probably won't ruin your barrel unless you shoot full-auto and let it get hot enough to nearly glow. Semi-autos are very hard to heat up to this stage without being stupid."

I can see the benefit of an HBAR for a full auto M16/M4 (or maybe for a tackdriver varmint/DCM/Highpower/match rifle), but for a slow fire, semi-auto, plinker/homeland defense rifle, isn't the full pound or more of added weight for a 20" HBAR unnecessary?

 

[natedog]

This is completly without firsthand experience, but wouldn't the light weight barrel be just as accurate as the heavy barrel for the first couple of shots, before the barrel heats up? IIRC, I also believe that it was said in some military manual that the time to change magazines is enough cool-down time in assault rifles in most situations (IE not the situation were you are trying to melt your handguards).

 

[Black Snowman]

Natedog you are absolutely correct. My taper profile .340 Weatherby Magnum will make 3/4 MOA 5 shot groups every time if you do your job and probably 1/2 MOA or better from a vise. I wouldn't want to see many more than that threw it at a time though. That's a LOT of heat and burning power and the I bet the barrel will start to warp enough to throw the shots off. Never pushed that hard. Rapid firing a .340 Wby is a good way to bruse your shoulder

OTOH I bet I can shoot my CZ 550 Varmit Laminated as fast as I can throw the bolt and maintaing better than 1 MOA due to the cooling slots in the stock and the heavy barrel. Normally it can do 1/2 MOA from a real solid rest and some good shooting.

 

[Badger Arms]

Short and/or fat barrels are ALWAYS stiffer and more accurate than longer and/or thinner barrels. Two barrels of the same weight means the longer barrel will be less accurate. Two barrels of the same length and the thicker barrel will be more accurate. What's at work here is how stiff the barrel is. If you can imagine, a 20 foot long barrel will bend and whip a great deal more than a 2 foot long barrel. All other things being equal, go for short and fat if you want accurate.

Of course, velocity, pressure, and bullet shape also contribute to accuracy, but that's more of an EXTERIOR ballistics issue. That is you need to look at wind drift, time of flight, drop, ballistic coefficient, etc. From a purely mechanical point of view, a stiffer barrel will be more accurate.

So, how does a barrel heat up? It heats up for three reasons:
1) Friction of the bullet going down the barrel
2) Heat of burning powder
3) Pressure of propellant gasses (ever feel a bike pump after use?)

Of these, I think that friction is the primary culprit. A small-caliber bullet simply has less surface area and heats a given weight of barrel less.

What wears a barrel?
1) Friction of the bullet again
2) Chemical reaction with barrel material
3) Intense heat and pressure vaporizing atoms of barrel material
4) Overzealous cleaning

The bore wears more as it heats up. As the BORE heats up, factors one through three become exponentially more effective at wearing the bore down. Chrome plating helps, but only reduces friction slightly.

What other ways can a barrel be ruined? The barrel can heat a barrel to a point where it will heat-treat, anneal, warp, or otherwise damage the barrel structure.

 

[Andrew Wyatt]

Two barrels of the same weight means the longer barrel will be less accurate. Two barrels of the same length and the thicker barrel will be more accurate.

more tolerant of bullet weight and velocity changes outside the sweetspot maybe. the insides matter more than the outsides.

 

[Bartholomew Roberts]

The only thing I would note is that the description of how light and heavy barrels heat up could be a bit confusing to some.

Light barrels and heavy barrels heat and cool at the same rate. The difference is that the heavy barrel has more mass to dissipate the heat, so it takes more rounds for it to reach the same temperature as the
light barrel.

To use an analogy, put a teaspoon of water in the microwave and a coffee cup of water. The teaspoon will reach the boiling point much faster and will cool down to room temperature faster also. The coffee cup will take longer to reach a boil and longer to cool down.

So if I shoot 180rds through a light barrel and 180 rounds through a heavy barrel, the heavy barrel will stay cooler throughout the session and return to ambient temperature faster than the light barrel.

If I shoot enough rounds to heat both barrels to 400F, then the light barrel will cool first - because it will reach 400F faster with less rounds. The heavy barrel will take significantly more rounds to reach 400F so it has more energy to dissipate than the light barrel and takes longer to cool.

Also, it seems the real killer is less bore wear than throat erosion caused by heat checking.

 

[Badger Arms]

With all of the factors, it's the throat that takes the most pounding, yes. That's where the wear will be most notable.