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Nathan asks:

I have a standard basic Bushmaster M4 carbine. I’ve noticed lately that it’s accuracy decreases very significantly when the gun heats up a bit. Small fist sized groups at 25 yards become double the size after a magazine or two through the gun over the course of a few minutes. This seems to occur when shooting while standing, or when using the indoor range’s bench to support the vertical foregrip (I know, not the best for accuracy). […] So the question is, would adding a free-floated handguard to my rifle be likely to increase it’s accuracy once the barrel heats up? If so, any guess by how much?

It sounds to me like you’ve properly identified the problem, but you’ve got the wrong solution. Let me see if I can steer you down the proper path…

Heat is the #1 enemy of accurate shooting. Well, #2 if you count a crappy shooter. As barrels heat up they have a tendency to change the point of impact for your gun, and even a fraction of an inch of movement in the barrel could be enough to throw your shot completely off paper downrange. There are some things you can do to fix that, but first I want to make sure you understand what’s going on and why it happens.

If you were paying attention in physics you should remember that when things heat up they have a tendency to expand. The molecules become excited and require more space so they start moving away from each other and cause the object they compose to expand. Expansion in the barrel causes the bore to expand slightly, meaning the rifling doesn’t grip the bullet quite as well as it used to and leads to that increased spread you’re talking about.

Once the barrel starts to expand it starts to move, as certain parts of the barrel have a tendency to expand before others (moving the shots generally in one direction or the other). For your average civilian shooter that will be the extent of the inconvenience (rounds off target) but if you’re shooting fully automatic fire you may eventually get to a point where your barrel will literally begin to melt off your gun. Remember, steel can and will melt — how do you think they made it in the first place (happy Patrick?) ?

For the most part, once you figure out how your barrel moves when it heats up you can compensate for it. Precision shooters know that their point of impact will differ between their “cold shot” (the first shot of the day) and when the gun is warm, and that change in PoI stays rather constant. Bench rest shooters and hunters typically sight in their guns based on that “cold shot” because their guns will never heat up enough to have the point of impact move when they’re in use. Cold shooting is typically more accurate, but if you have a need for speed then heat will begin to rear its ugly head.

The way to minimize that barrel shift (and make it repeatable and predictable) is to have a thicker “barrel profile.” Barrel profile refers to the thickness and shape of your barrel — has a handy reference chart for the various profiles available for the AR-15 platform. The general idea is that the more material you have on your barrel (the thicker it is) the less it will be affected by heat. The excess material will deter the barrel from warping, keeping it straight and tight for a longer period of time. The ArmaLite AR-50 in .50 BMG that I reviewed earlier in the year (and miss so very, very much) had a very thick barrel for that specific purpose — combating the effects of heat.

Barrel thickness does come at a cost: weight. Adding more material means there’s more weight on the end of your gun, and as you add more weight it becomes less portable. Bench rest and long range shooters don’t mind the extra weight as they don’t move around very much, but if you’re running and gunning in a 3-gun competition a bull barrel might slow you down. WAY down.

For the 3-gun competitor, a better option than adding material to the barrel might be the addition of a “heat sink” or “dissipator,” such as the blue monstrosity tacked to the barrel (under the handguards) of the rifle in the middle of this picture. The idea is that, much like the heatsink on the CPU of your computer, it wicks away the heat and dissipates it over the many “blades” that it has to increase the surface area and speed the cooling process. In theory, the barrel should never heat up enough to move and can still remain lightweight. In theory. A very expensive theory.

In short, the best way to keep your groups small when firing fast is to use a thicker barrel. Thicker barrels take longer to heat up and move around so your groups will remain relatively small. Free floating handguards solve a different issue, one which we’ll talk about Monday. Probably. if I get off my ass and write the article.

If you have a topic you want to see covered in a future “Ask Foghorn” segment, email [email protected].

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  1. “…the best way to keep your groups small when firing fast is to use a thicker barrel.”

    I’ve always wondered how much of the improvement a thicker barrel gives is due to heat, and how much is due to harmonics.

  2. Nick has it exactly right: thin barrels heat more quickly and warp more noticeably than thick barrels. My testing and experience shows that an HK-93 or SCAR-L will open up its group size quickly under sustained firing, but a Colt HBAR or an ArmaLite with a full-profile barrel will largely shrug off the effects of a hot barrel.
    When the pencil-thin SCAR barrel got hot, it gave groups that were between 1/2 inch and 1.0 inches larger than its cold-barrel groups. The ArmaLite M-15, with a 16-inch bull barrel, only opened up its groups by 0.2 inches when it got hot. And it got hot much more slowly than the SCAR.
    Thin steel barrels with carbon-fiber sleeves are an excellent combination of light weight and thermal stability, but they can be prohibitively expensive and they only work best with blowback and recoil-operated guns. Gas-operated guns need not apply.

    • “they only work best with blowback and recoil-operated guns. Gas-operated guns need not apply.”

      Why is that?

      • The carbon fiber sleeve is manufactured separately and the thin barrel liner is pressed and epoxied into it. Where the gas port or tappet comes off the barrel, the carbon fiber needs to be precisely cut away and shielded from the heat. Carbon fiber doesn’t melt like plastic, but the resins in it can’t hold up to the 3,000 degree heat of burning propellant gasses.

        Making a carbon-sleeve barrel for a gas-operated or piston-operated gun is technically possible, but it’s so much work that it’s prohibitively expensive for civilian uses, and even most military ones. They’re most commonly encountered in bolt-action rifles and blowback .22s.

    • Damn… Are you bustin out on my HK 93 again? Ok, I admit that the POI shift is very noticeable between round 1 and round 50, shot at 1/2 to 1 second intervals. Maybe an 1&1/2 or so – up and to the left.

    • To increase the surface area (and hence cooling) while reducing weight. There is a debate about the whether the stiffness is reduced or not.

      • If 2 barrels have the same length and mass, the fluted one will be more rigid. Fluting an unfluted, stock barrel will reduce relative stiffness.

  3. I had to deal with this recently. I had a heavy RRA barrel on my AR-15. It shot great nailing targets at 200 yards. But it weighed a ton and the balance was all the way forward. Sitting on the bench at the range my AR was great, but I never wanted to use it any where else like camping and hiking.
    So I ditched that heavy SOB and installed a much lighter profile barrel. Yeah my groups opened up a little bit but now I actually take my rifle with me.

    • That’s why the AR-15 platform is so versitile. Just get a 2nd upper with a shorter barrel. There’s no need to adjust optics, and you have 2 guns in 1. In other words, don’t change the barrel when you can just get another upper.

      P.S., I would love one of those massive RRA barrels for target/varmint shooting. Gonna try to pick up a varminter in 2012.

      • I lack an excess of money, it took me quite a while to build my AR. Which if your interested in a heavy profile 16″ midlength chrome lined RRA barrel with about 200 rounds through it, let me know.

  4. “Remember, steel is nothing more than a supercooled liquid and to liquid it shall return”

    Um…. what? Steel (iron-carbon mixture) can exist as all three phases. With enough heat and vacuum, I’m pretty certain that it can exist as a plasma as well. A quick glance at the phase diagram shows this pretty clearly.

    Also, “supercooled” implies that there is a substance existing as a liquid while at a temperature below the substances freezing point. Cumulus clouds are an example of this. They exist as liquid droplets suspended in the atmosphere at temperatures below freezing because there are no available condensation nuclei.

    This shouldn’t be confused with amorphous solids (sometimes known as amorphous liquids). Historically, glass was considered amorphous because it lacked the long-range crystalline order of other solids. However, it is now understood that glass is “special”. The term amorphous solids is now used to describe extremely thin films, particularly those used in the semiconductor industry. Tantalum oxide is commonly used as an ARC layer and is deposited in very thin layers (30-100 Angstroms). These layers are so thin that the molecules cannot align in the traditional crystalline lattice and are technically “liquids”.

      • Haha, yeah I dreaded most of my chemistry courses too… I actually learned more working as a chemist than I ever learned in class 🙂

    • I’ve read some gushing testimonials about various Mini-14 accurizing accessories, but I’ve never tested them myself and I don’t own a Mini-14 to try them with.

      I borrowed one last year to test a Leupold scout scope and a laser boresighter, but the gun was so inaccurate that I gave up using it as a test bed for anything. 2 to 3 inches at 50 yards was the best I could do, and I can do almost that well with an iron-sighted slug gun.

      Without the benefit of very much data, my initial hypotheses would be that older Mini-14s have generous (read: sloppy) chamber dimensions, and their thin barrels have too slow a twist rate to adequately stabilize even 55-grain bullets. They’re fun and reliable, but accuracy? Not so much.

      • Thanks for response.
        Mine will shoot 2-3 inches at 100 yards until it heats up and then starts stringing.
        Not a target rifle, definitely short-range defense
        Some of those products claim 50% improvement by stiffening barrel and heat sync.
        Don’t want to spend $100 +/- to improve on minute of milk jug accuracy without some reviews.

      • The newer Minis have a heavier barrel profile, both beyond the gas block and down near the receiver. They are much improved in this regard. Just a shame it took Ruger so long to do it.

        Nick, can you please comment on fluted barrels? Are they worth it? They cut down on weight, increase the surface area and, hence, cooling. But do they retain their dimensions and stiffness longer per shot?

      • +1 – I have heard, both in person and online, from many people that their Mini-14’s are accurate enough for prairie dog hunting. Some even go as far as to mention MOA characteristics. In the half-dozen or so Mini’s that I’ve shot in my life, not a single one could keep 3 rounds inside a 4″ target @ 100 yards. The last one was a modern synthetic/stainless model with a 3-9x scope owned by a coworker. I thought I was losing my touch, barely able to hit 2 “Tide” containers consecutively. I have heard (“heard”) that some years are better than others, but I’m not looking to purchase one and never looked into it.

        • I have 2 sub MOA Minis, one is a 6.5 Grendel with a 24″ barrel, and the other is a Mini Target with the stock barrel cut and crowned down to 18″. Not worth the money they cost tho. However, just bedding the action and tuning the gas system will tighten up most Minis considerably for less than $50. Alot of folks swear by the Accu-Strut, but I’ve never tried one.

  5. I liked most of the article but i have to comment on one part.

    I’m no metallurgist so I could be wrong, but doesn’t metal expand in all directions from the center when heated? Meaning a heated barrel would indeed have a larger outside diameter but ALS a SMALLER inside diameter. Since the barrel can be thought of as a sheet of metal rolled into a circle, and a sheet of metal, when heated, would be thicker on both sides from the middle.

    Am I making sense?

    • Your question make sense but your reasoning is flawed. When the barrel expands, everything gets bigger, including the bore. If a barrel hoop is heated, it’s circumference increases much more than it’s thickness (negligible).

      • Forgive me for bringing up a non-gun topic but a good example of this are the large flywheels such as found on rock crushers. They are about six feet in diameter and have spokes. If you heat the hub you will not get the thing off as the spokes prevent expansion. Instead you heat the outside ring of the flywheel. The spokes then pull the hub outwards and the inside axle hole becomes larger. Be careful where you stand as you hold the torch. When three tons of cast iron flywheel pops loose from the end of the shaft things can get interesting very fast.Instead of using torches, some prefer to turn the flywheel over a bed of coals to accomplish the task. This adds to the excitement when the magic moment arrives and the turning flywheel pops off the axle. You can assume my friends sometimes have laundry problems.

  6. To keep the gun accurate as more rounds are shot and it heats up.
    Objective is to keep the barrel from getting hot. Heavy barrel, materials, weaker cartridges, bullet selection, quick change barrel.
    Objective for the barrel to resist bending due to heat. Heavy barrel, flutes, wrapped materials.
    Objective is to dump the heat. Heat sink, water cooling, fins, flutes, heat sheild, etc.
    Objective is to let the barrel move as it expands. Free floating, stock design, bedding, etc.

  7. When I was an undergraduate, the physics lab had a cute demonstration. There was an iron rod (wooden handle) with a ring on the end, and a second iron rod (again wooden handle) with a ball on the end. As stored on the shelf, the ball had been pushed through the ring. Cold, you couldn’t pull the ball back through the ring. It wouldn’t fit. The trick was to heat the ring over a Bunsen burner. That expanded it, and you could pull the ball through the expanded ring. Very neat demonstration of how metal expands when heated. For the same reason, when a barrel gets hot, it expands outward. The bore will become slightly larger, which may in itself contribute to reduced accuracy.

  8. This is why I created the Chamber Chiller rifle barrel cooler. The concept is simple: cool the chamber, where heat often sinks into the loaded cartridge which then causes higher pressure, and deliver cool air down even the longest rifle barrel with a high-output fan. Chamber Chiller is arguably the best rifle barrel cooler available, so get your prototype sample before they’re all gone!

  9. I did want to say that Handguards may not “solve” the heat problem, but they can make it better by allowing for more air to get to the barrel. It really depends on what handguards you had on first though. 😉

  10. The first answer was tmi and like many others didn’t get to the root of the problem. The M4 barrel profile gets fatter after the gas block and then has a very skinny constriction where the grenade launcher mounts. It doesn’t take a rocket scientist to figure out that is where most of your warpage occurs. A pencil profile would be better than the M4 barrel and lighter too. Or you could go to a straight profile barrel, not add any weight at all but get less warpage.

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