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 — AR15barrels.com 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.
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