Good barrels aren’t cheap. Here are a few ideas on getting the most accurate life from your investment. READ MORE
Rifle barrel chamber throat erosion was the topic last time, and mostly its causes and the effects. Short retake: The barrel “throat” is the area directly ahead of the case neck area cut into the chamber. This is the area that receives the majority of the “flame cutting” created by burning propellant gases. When a barrel “quits” it’s from deterioration in the throat. The greatest enemy to sustained accuracy is the steel surface roughness.
The throat is also advancing, getting longer, as the steel deteriorates; it’s wearing in little bit of a cone shape. The gap, or “jump,” the bullet has to cross before engaging the lands or rifling therefore is increasing, and also plays its part in poorer on-target performance. Last time I talked about using a gage to measure and record the actual amount of this increased gap. One way to preserve more consistent accuracy, which means not only group size on target but also shot impact locations (zero) is to adjust seating depth for the lengthening throat.
A chronograph also comes into this picture.
Routinely chronographing your load will show that velocity drops as the round count increases. Since the throat is getting longer (and slightly larger) there is more and more room for expanding gases. Pressure will, therefore, be lower and, along with that, so will bullet velocity.
Increasing the propellant charge to maintain original velocity is a tactic used by a good many good NRA High Power Rifle shooters. Bumping the charge in this way to maintain velocity is a safe and sound practice, by the way. I mention that because, over enough rounds, you might be surprised just how much change is needed. Middleton Tompkins, one of the true Jedi Masters of competitive rifle shooting, used this — propellant charge level increase — above all else to determine when a barrel was “done.” On a .308 Win., for example, when Mid was +2.0 grains to keep the same speed, that barrel became a tomato stake.
Moving the bullet forward to maintain the same amount of bullet jump, or distance to the lands somewhat offsets the result of reduced pressure and velocity as the throat lengthens, but, overall, and if it’s done in conjunction with bumping up the charge, both these tactics are a safe and sage help to preserve on-target performance for a few more rounds, maybe even a few hundred more rounds.
Either of these tactics, and certainly both together, requires a level of attention that many (like me) might not be willing to give. To actually see some reliably positive effect from maintaining velocity and jump consistency, you’ll need to make checks at least every 300 rounds. That’s a fair amount.
Another point I need to clarify is that moving the bullet out to maintain jump only matters to rounds that don’t have some magazine box overall length restriction. Otherwise, propellant charge for loads for rounds constructed with box restrictions can be wisely increased to maintain velocity, but the increased jump will take its toll on accuracy sooner than it would if jump could also be adjusted for.
A few more ideas on keeping a barrel shooting better longer: Bullet choice can matter, if there’s a choice that can be made. Flat-base bullets will shoot better, longer in a wearing barrel. Trick is that when we need a boat-tail we usually need a boat-tail! Flat-base bullets “obturate” more quickly. Obturate means to “block,” and here it means to close a hole, which is a barrel bore, which means to seal it. The angled tail on a conventional boat-tail creates a “nozzle” effect intensifying the cutting effect. Flat-base will result in a longer barrel life, and, in the way I’m approaching it here, is that they also will extend the life of a barrel after erosion might otherwise have taken its toll. Erosion tends to, at least effectively, become exponential: the more it wears the faster it wears more. An obscure but well-proven boat-tail design does increase barrel life, and also usually shoots better though a worn throat, and that is a “rebated” boat-tail. This design has a 90-degree step down from the bullet body (shank) to the tail. It steps down before the boat-tail taper is formed. These obturate quickly. It is common for competitive shooters to switch from a routine boat-tail to a rebated design when accuracy starts to fall of. Sure enough, the rebated design brings it back for a couple hundred more rounds.
A Welcome Set Back
Another common way to (really) extend barrel life for a bolt gun is to “set-back” the barrel. Pull the barrel, cut some off its back end, and then re-chamber and re-thread, and re-install. New barrel! Well, sort of. Given that there’s no significant wear on the barrel interior elsewhere, overwriting throat erosion does put that barrel almost back to where it started, except being overall shorter. That tactic works very well for chromemoly barrels but not so well for stainless steel. The difference is in the “machine-ability” of each steel. It is possible to set back a stainless barrel, but it’s difficult to then get a “chatterless” cut when the reamer engages. A little more usually needs to be removed to get good results with stainless, and this, of course, is making the barrel overall that much shorter. You have to plan ahead for a set-back, and that means including enough extra length to compromise. Usually it takes a minimum of 1 inch to get a worthwhile result with chromemoly.
In case you’re wondering, coated bullets don’t have any influence on throat erosion, but they do seem to shoot better through a roughening throat. Boron-nitride is the only bullet coating I will recommend.
One last for the semi-auto shooters. Throat erosion is also creating more volume to dissipate more pressure, which reduces the pressure that gets into the gas system. If you’re running an adjustable gas block, it’s liable to need readjustment, or, as also suggested, altering the propellant charge should likewise overcome any issues. This is one reason that savvy builders tended to increase gas port diameter on an NRA Service Rifle, for instance, to ensure good function after a fairly high number of rounds had done downrange.
The preceding is a specially-adapted excerpt from Glen’s book Top-Grade Ammo. Available HERE at Midsouth Shooters Supply. Visit ZedikerPublishing.com for more information on the book itself, and also free article downloads.