Tag Archives: throat erosion

RELOADERS CORNER: Extending Barrel Life

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Good barrels aren’t cheap. Here are a few ideas on getting the most accurate life from your investment. READ MORE

barrel life
Flat-base bullets obturate more quickly than boat-tails, and that reduces some of the flame-cutting effect from propellant gases.

Glen Zediker

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.

barrel life
Use this gage, along with a chronograph, to adjust the load to maintain the “same” as the barrel throat erodes. More propellant, longer cartridge length to maintain jump. GET ONE HERE

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.

Other Ideas
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.

barrel life
Uncommon design, but very effective, all around: DTAC 6mm 115gr RBT (rebated boat-tail). The step-down to the tail mimics a flat-base in its capacity to seal the bore. It’s a sort of “best of both worlds” design.

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.

barrel life
And make sure you’re not eroding your own barrel! Get a rod guide and a good rod and keep the rod clean! A log of throat damage can result otherwise.

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.

RELOADERS CORNER: Barrel Throat Erosion

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How long does a barrel last? About 5 seconds. KEEP READING

throat erosion
Well, it’s hotter than this, but it’s flame cutting over time and distance, and hotter for longer is the issue.

Glen Zediker

As is by now common enough in this column I write, ideas for topics very often come from questions that are emailed to me. As always, I figure that if someone has a question they want answered, then others might also like to know the answer. This question was about barrel life and, specifically, this fellow had been reading some materials on the interweb posted by some misinformed folks on the topic of bullet bearing area and its influence on barrel life: “Is it true that using 110 gr. vs. a 150 gr. .308 bullet will extend barrel life because of its reduced bore contact?”

NO. Not because of that.

However! The answer is also YES, but here’s why…

Wear in a barrel is virtually all due to throat erosion. The throat is the area in a barrel that extends from the case neck area in the chamber to maybe 4 inches farther forward. Erosion is the result of flame-cutting, which is hot gas from propellant consumption eating into the surface of the barrel steel. Same as a torch. There is very little wear caused from passage of the bullet through the bore, from the “sides” of the bullet, from friction or abrasion. The eroding flame cutting is at or near the base of the bullet.

When the propellant is consumed and creates the flame, the burn is most intense closer to the cartridge case neck. There are a few influences respecting more or less effect from this flame cutting. Primarily, it’s bullet weight. Time is now the main factor in the effect of the flame cutting. Slower acceleration means a longer time for the more intense flame to do its damage.

The slower the bullet starts, and the slower it moves, the more flame cuts in a smaller area for a longer time.

Bullet bearing area, therefore, has an influence on erosion, but that’s because it relates to acceleration — greater area, more drag, slower to move.

The amount of propellant, and the propellant nature, do also influence rate of erosion. Some assume that since there’s more propellant behind a lighter bullet that would create more erosion, and that’s true, but that is also not as great a factor as bullet weight. Other things equal, clearly, more propellant is going to cut steel more than less propellant. A “lighter” load will have a decidedly good effect on barrel life.

throat erosion
It’s heavier bullets that have the most influence on shortening barrel life.

Heavier bullets, without a doubt, are a greater influence than any other single factor. “We” (NRA High Power Rifle shooters) always supposed that it was the number of rapid-fire strings we ran that ate up barrels the most, but that was until we started using heavier bullets and found out in short order that our barrels weren’t lasting as long. That was moving from a 70gr. to an 80gr. bullet.

The “nature” of propellant is a loose reference to the individual flame temperatures associated with different ones. There have been some claims of greater barrel life from various propellants, but, generally, a double-base will produce higher flame temperature.

Even barrel twist rate plays a role, and, again, it’s related to resistance to movement — slower start in acceleration. Same goes for coated bullets: they have less resistance and move farther sooner, reducing the flame effect just a little. And, folks, it’s always “just a little.” It adds up though.

There are bullet design factors that influence erosion. A steady diet of flat-base bullets will extend barrel life. There’s been a belief for years and years that boat-tail bullets increase the rate of erosion because of the way the angled area deflects-directs the flame. And that is true! However, it’s not a reason not to use boat-tails, just a statement. We use boat-tails because they fly better on down the pike, and, ultimately that’s a welcome trade for a few less rounds. An odd and uncommon, but available, design, the “rebated boat-tail” sort of splits the difference and will, indeed, shoot better longer (they also tend to shoot better after a barrel throat is near the end of its life).

The effects or influences of barrel throat erosion are numerous, but the one that hurts accuracy the most is the steel surface damage. It gets rough, and that abrades the bullet jacket. The throat area also gets longer, and that’s why it’s referred to as “pushing” the throat.

The roughness can’t much be done about. There are abrasive treatments out there and I’ve had good luck with them. Abrasive coated bullets run through after each few hundred rounds can help to smooth the roughness, but then these also contribute their share to accelerated wear. I guess then it’s not so much a long life issue, but a quality of life issue. I do use these on my competition rifles.

lnl gage
Use the Hornady LNL O.A.L. gage to record and then track barrel throat wear. This isn’t technically a “throat erosion gage,” which do exist, but I’ve found it an easy and reliable way to keep up with an advancing throat. As the seating depth gets longer, it’s indicating how far the throat is advancing. Get one HERE 

Keeping in mind that the throat lengthens as erosion continues, using something like the Hornady LNL tool shown often in these pages can let bullet seating depth that touches the lands serve as a pretty good gage to determine the progress of erosion. On my race guns, I’ll pull the barrel when it’s +0.150 greater than it was new. Some say that’s excessively soon, and a commonly given figure from others in my circle is +0.250. One reason I pull sooner is that I notice a fall-off in accuracy sooner than that since I’m bound by a box magazine length for my overall cartridge length for magazine-fed rounds with shorter bullets, and I’m already starting with a fairly long throat (“Wylde” chamber cut). And another is because gas port erosion is having some effect on the bullet also by that number of rounds. Which now leads into the “big” question.

So, then, how long does a barrel last? Get out a calculator and multiply how many rounds you get before pulling a barrel by how long each bullet is in the barrel and barrels don’t really last very long at all! At full burn, maybe 4-6 seconds, some less, or a little more.

Another misgiven “fact” I see running rampant is associated with comparing stainless steel to chromemoly steel barrels for longevity. Stainless steel barrels will, yes, shoot their best for more rounds, but, chromemoly will shoot better for an overall longer time. Lemmeesplain: the difference is in the nature of the flame cutting effect on these two steels. Stainless tends to form cracks, looking like a dried up lakebed, while chromemoly tends to just get rough, like sandpaper. The cracks provide a little smoother surface for the bullet to run on (until they turn into something tantamount to a cheese grater). The thing is that when stainless stops shooting well it stops just like that. So, stainless will go another 10 to 15 percent more x-ring rounds, but chromemoly is liable to stay in the 10-ring at least that much longer than stainless steel.

throat erosion
Stainless steel barrels keep their “gilt-edge” accuracy for about 15% more rounds, but hit the wall head-on and in a big way when they reach their limit. Chromemoly steel tends to open up groups sooner, but also maintains “decent” accuracy for a longer time, by my experience — the groups open more slowly.

Do barrel coatings have an effect? Some. A little. I’ve yet to see one that made a significant difference, or at least commensurate with its extra expense. Chrome-lined barrels do, yes, tend to last longer (harder surface), but they also tend not to shoot as well, ever. Steel hardness factors, but most match barrels are made from pretty much the same stuff.

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.