Big guns: taming the beasts

Guns Magazine, July, 2009 by Glen Zediker

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Once there was one. Then another. And now they're all over the daggone place. I'm talking about what I think are best called "big-chassis" rifles, the AR-10, SR-25 platform guns. They have become, without a doubt, more popular than ever and, as suggested, availability and configuration options are a leading reason why. With popularity or competition (related), prices have also lowered on many and that's a huge incentive for some to try a big gun. This next little bit will give a few ideas to help the proud owner of a big gun grin a little wider.

The whole idea sounds just right. If the AR-15 gets hacked on because of its diminutive cartridges, well, we can fix that in a hurry with anything from the .308 Winchester family. Want more power? Easy. A bigger bolt face and more room in the receiver makes it possible. There's the rub. The bigger bolt face and more room in the receiver makes for a honking lot of reciprocating mass fueled by a likewise honking amount of propellant gases.

Not everyone likes them. I'm in that group. Compared to AR-15s, which are known far and wide for their owner-side friendliness, big-chassis rifles are hard-hitting firearms, both downrange and internally.

Most factory-built big-chassis rifles use an AR-15 gas system configuration, and that's one root of its problems. Gas port pressure, not chamber pressure, is the concern. The ultimate fix is relocating the gas port ahead toward the muzzle. This reduces port pressure and likewise reduces the amount of pressure entering the system. Without this modification, a big-chassis rifle can succumb to the same function problems befalling carbines. It's frequently referred to as "over-function" and, while maybe not perfectly accurate, is closely descriptive of the symptom. I prefer to call it what it really is and that is too daggone much gas. The bolt unlocks too quickly and rearward carrier velocity after opening is too high. The case may still be swelled up against the chamber wall and resist extraction.

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Too Much Pressure

Smaller calibers compound port pressure problems due both to diminished bore volume and also to the generally slower-burning propellants behind most smaller-caliber .308 derivatives. Slower-burning propellants produce their peak pressures farther down the barrel, nearer the gas port. On top of it, most big-chassis rifles are going to have 24" barrels which, by itself, increases gas pressure inside the bore.

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To give an idea of just how much extra pressure there can be, it's routine-standard when building a custom AR-15 Match Rifle to move the gas port ahead 1". That's for a .223 Remington and a 24" to 26" barrel. Now, the routine change for a custom big chassis gun chambered in, say, .260 Remington, is 4" farther ahead. There's that much difference. There's that much pressure.

The most easily or readily effective way to tame its function is through propellant selection. The handloader should choose a propellant on the faster-burning end of suitability. Know your factory ammo! Mil-spec rounds most often have a more suitable port pressure peak than many hunting-use factory loads. This is even more an issue with the .260 Remington and .243 Winchester since both these rounds respond so well to relatively slow-burning propellants.

Delay

Delaying bolt unlocking, again, is the goal, and other means to attain it include increased carrier weight via aftermarket inserts, a stouter buffer spring, and incorporating some means for bleed-off m the gas system. It's not that difficult to install an adjustable gas tube or gas manifold and these are effective. Each provides a means to limit the volume of gas going into the gas tube.

Increasing carrier weight, which is a trick commonly done for competition-use AR-15s, has two edges in a big gun. It will delay unlocking but then increases the influence of the forward hit. The stout buffer spring, I think, is about mandatory.

The bolt carrier assembly on most big-chassis rifles weighs around 20 ounces, or nearly double that of an AR-15. It's noticeable. The bolt carrier coming back home is quite a shock. Polishing the extractor so it slides more easily over the cartridge rim helps reduce the frequent experience of the case shoulder taking a hit upon chambering. Compounding this problem is the tendency for many to install the D-ring inserts to increase spring pressure in the extractor. A good chamber polish also works wonders for extraction ease.

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Bolt stop breakage has plagued this rifle configuration. The stops break because they get hit so hard by the extra-heavy bolt carrier assembly. Reducing bolt stop overrun, which is the amount the bolt face is able to clear beyond the stop, is the only solution I know.

Get it to about a nickel's width and that should be adequate. It's most easily reduced using adhesive-backed phenolic pads intended to cushion buffer impact.

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