Rejuvenating A Worn Throat
Rejuvenating A Worn ThroatBy John Barsness
©Copyright The Varmint Hunters Association, Inc.
One of the saddest aspects of shooting varmint rifles is that eventually the chamber throat becomes worn and ragged. Prairie dog rifles suffer the most, because we shoot them a lot in warm weather.
A bore scope is handy during the process, but not essential.
Heat is the villain that burns barrels. The heat inside our barrels is created primarily by smokeless powders burning at temperatures up to around 6,000 degrees Fahrenheit, more than twice the 2,795 F melting point of iron. Cartridges with a relatively high powder-to-bore ratio do the quickest damage. The hot gas-flame created by 40 grains of powder in the throat of a 220 Swift burns longer than 13 grains of powder in a 22 Hornet.
The 223 testing was done with some Black Hills ammo. Three five-shot groups averaged under half an inch, about like the rifle shot when new with good factory loads.
Burn-out accelerates when the barrel heats up during repeated shooting, because really hot steel is far more vulnerable to gas erosion. At its most basic, steel is iron alloyed with up to 2.1% carbon. (This slight amount of carbon hardens the iron, while more carbon weakens its structure, turning it into “cast iron.”) Tiny amounts of other elements are added to the basic iron-carbon formula, resulting in various characteristics, including rust-resistance and machinability.
Barrel steel often contains many elements with a slightly lower melting temperature than iron, including manganese, nickel and silicon. Some even melt at very low temperatures, including phosphorus (111 F) and sulphur (235 F). Consequently, when powder gas heats the throat, not all the elements melt at the same rate. The surface of the steel actually contracts and cracks, like the surface of damp earth as it loses water on a warm day.
The fliers disappeared from the 221's targets after the throat was smoothed with 15 shots of 220-grit fire-lapping compound.
Through a bore scope light cracking can be seen in as few as 100 rounds in a barrel chambered for the 220 Swift, and cracks can appear even in the throat of a 223 Remington in a few hundred rounds. Cracking takes a lot longer to appear in milder cartridges: I have a Ruger No. 1B 22 Hornet that’s been fired over 2,500 times, often with the barrel really hot, and there’s no visible trace of throat erosion through my Hawkeye bore scope.
Visible cracking is only the first stage in throat erosion. Eventually the cracks grow so deep and wide that chunks of steel break off. Whether cracking or chunking, erosion is always worst right in front of the chamber, where gas temperature is highest. A really worn throat can be chunked for half an inch in front of the chamber, deeply cracked for an inch or so beyond the chunking, and then show only very fine cracking for another couple of inches.
This article originally appeared in Varmint Hunter Magazine, and appears courtesy the Varmint Hunters Association, Inc. The VARMINT HUNTER Magazine is a popular publication of the VHA. Each year, the VHA hosts several 600-yard IBS matches, a coyote calling contest, and an annual Jamboree in Fort Pierre, SD. The Jamboree is a week-long shooting event known as "a summer camp for shooters".
Many methods have been tried to reduce throat erosion since smokeless powder became the primary rifle propellant, including platings and coatings ranging from chromium to ceramics. These can slow down erosion, but the steel underneath still gets hot. Coating bullets with molybdenum disulfide doesn’t slow down erosion much either, since it only reduces bullet friction, a relatively minor source of the heat generated when a rifle goes bang.
One of the problems with an eroded throat, of course, is that accuracy starts eroding too. Exactly how long this takes depends on the cartridge, the steel, and how long and often the barrel is shot hot. Much also depends on the shooter’s definition of accuracy. A benchrest shooter often considers a barrel shot out in fewer than 1,000 rounds, but I’ve known varmint hunters who’ve gone far longer before replacing the barrel.
My old friend Chub Eastman, who worked for Leupold and then Nosler for decades and turned gun writer in his semiretirement, once had a prairie dog rifle chambered for the 223 Remington. Chub kept pretty close track of the number of rounds shot through the rifle, and the barrel finally quit shooting minute-of-prairie-dog at around 20,000. Chub then had the rifle rebarreled – and the original barrel cut in half lengthwise – to see what remained inside. The answer was “not much”: The rifling was completely gone for six inches in front of the chamber. Now that’s a long throat!
For decades the common belief has been that throat erosion can be compensated for by seating bullets out farther (though it would have been hard to seat a 50-grain Ballistic Tip far enough out in Chub’s barrel). This often helps accuracy, but several of the lab ballisticians I’ve ever talked to have noted that peak pressure rises and becomes erratic as a throat first starts to erode.
At first this increased pressure is because of increased bullet friction in the cracked throat, but can become much worse when steel starts chunking, probably because the diameter of the throat grows enough that thin-jacketed, lead-cored bullets actually “bump up” in diameter as they enter the worn throat. The bullet then must be squeezed down again as it enters the rifling, increasing the peak pressure.
One solution to throat erosion is to cut off a couple of inches of the rear of the barrel, then refit and rechamber the shortened barrel, eliminating the eroded section. Depending on the contour and length of the barrel, this sometimes can be done two or three times.
Lately I’ve been experimenting with another delaying tactic that’s less expensive. For a number of years the well-known centerfire target shooter David Tubb has been selling his FinalFinish kit with abrasive bullets to help break-in new barrels and to smooth out the throat area in eroded barrels. I’ve used both the FinalFinish system and a NECO fire-lapping kit to smooth out the tool marks from the chamber reamer in several rifles, and even had used the FinalFinish system to remove some light pitting in the throat of my New Ultra Light Arms 30-06.
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