http://www.mssblog.com/2016/08/12/factors-in-barrel-life/

FACTORS IN BARREL LIFE: THROAT EROSION PART I
AUGUST 12, 2016 MIDSOUTH SHOOTERS 18 COMMENTS
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The preceding was a specially adapted excerpt from the book The Competitive AR15: the ultimate technical guide, by Glen Zediker and Zediker Publishing. Available at Midsouth Shooters or BuyZedikerBooks.com

By Glen Zediker

Last time the topic was finding the bullet seating depth that touches the lands or rifling. In discussing the tool featured, the Hornady LNL O.A.L. gage, I also mentioned how this appliance can also be used to document barrel throat erosion. A little more about this…

As rounds go through the barrel, one after another, the chamber throat is advancing, moving toward the muzzle. The “wear” in a barrel is all right in the throat. The influence is “flame cutting” by the high-temperature gases that result from burning propellant. The steel is burning up. At some point, it quits shooting well. The reason for a fall-off of accuracy is a combination of excessive free-bore (space between end of the chamber neck area and the first point of contact a bullet will have in the barrel) and also roughness, plain and simple. Bullets won’t enter the rifling as smoothly and the rough surface rips at the bullet jacket. Any wear along the remaining length of the barrel is insignificant, and not influential; it’s mostly from simple friction.

So how long does a barrel last? About 5 seconds. Let me explain.

Of course, that’s spread over a scant few milliseconds at a time over a number of rounds. There are two main influences in the progress of erosion: bullet weight and amount of propellant. The more of each, the faster the deterioration, but bullet weight factors mostly. As was introduced in the material way on back about gas port pressure, if we plot out pressure levels against bullet movement through the bore, we get a “pressure-time curve.” Pressure levels are associated with respective levels of flame cutting. A steep P-T curve (slow bullet acceleration) means more concentrated cutting over a shorter distance. It’s clear, then, that lighter bullets will do less damage than heavier bullets, even though the lighter bullets mean burning more propellant. In a .223 Rem., for example, a steady diet of 77-grain bullets will shorten barrel life compared to using mostly 55-grain bullets. Fortunately, .223 is one of the kindest to barrel steel of popular rounds. I expect about 5000 good rounds from a quality barrel (about the same as .308 Winchester). Some folks offer a much higher figure than that, but, again, “shoots well” is subjective. Shoot it until it doesn’t shoot well. The cartridge factors mightily: .243 Win.? Maybe 1200 rounds with 107-grain bullets.

Barrel steel material most definitely has an influence on life. Short answer: get stainless steel. Comparing true “match-grade” barrels, stainless will not shoot one bit better than chromemoly, but will shoot its best for longer, about 10-15 percent more accurate rounds. The reason is in how the steel “wears” as throat erosion progresses. Chromemoly tends to get rough (like sandpaper) whereas stainless steel tends to form cracks with still-smooth areas between them (like a dry lake bed). The stainless erosion is less disruptive to the bullet jacket.

However, that’s not a blanket recommendation of stainless. When a stainless barrel quits shooting, it quits right then and there. Accuracy fall-off is abrupt. Like in the middle of a string… Really. Chromemoly group sizes cone outward more slowly. Chromemoly tends to continue to shoot “better” after it’s lost its gilt-edge. Some will shoot a very long time at only a minimum fall-off from its best performance. I have to recommend chromemoly for a semi-auto, unless, that is, the semi-auto is strictly a competition rifle. Then, just take your medicine.

A bolt-gun can have its chromemoly barrel pulled and “set back” to prolong its life. Simple: cut a half-inch or so off the chamber-end of the barrel, rechamber it, back in business. Can’t do that with stainless. Of course, semi-automatics can’t get this treatment because of gas system orientation.

Chrome-lined barrels do, yes, tend to last longer (slower erosion), but they also tend not to shoot as well, ever. Steel hardness also factors, but most custom match barrels are made from pretty much the same stuff.

Back, finally, to using the gage: Take a measurement every now and again. Experience will tell you when. My standard is +0.150 inches, compared to the reading I got on the barrel when new. That’s a max. Sometimes they quit before then, sometimes not, but no matter how it’s shooting, I rebarrel at +0.150. I use stainless and I don’t want the next target to be when it quits. I’m a little conservative, by the way… Some very good shooters comfortably extend that to 0.250.

Keep a worn barrel clean! Scrub the throat area (carefully of course). The cracks and roughness attract and retain fouling. There are other “tricks” to help preserve accuracy for a longer time, and I’ll talk about those next time.