Rejuvenating A Worn Throat O
John Barsness
ne of the saddest aspects of shooting varmint rifl es is that
eventually the chamber throat becomes worn and ragged. Prairie dog rifl es suf- fer the most, because we shoot them a lot in warm weather.
Heat is the villain that burns
barrels. The heat inside our barrels is created primarily by smokeless pow- ders 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-fl ame created by 40 grains of powder in the throat of a 220 Swift burns longer than 13 grains of powder in a 22 Hornet. Burn-out accelerates when the
barrel heats up during repeated shoot- ing, 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
A bore scope is handy during the process, but not essential.
elements with a slightly lower melting temperature than iron, including man- ganese, nickel and silicon. Some even melt at very low temperatures, includ- ing 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. Through a bore scope light crack-
ing 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 fi red 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 fi rst
The 223 testing was done with some Black Hills ammo. Three fi ve-shot groups averaged under half an inch, about like the rifl e shot when new with good factory loads.
Page 114 Spring 2012
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 re- ally 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 fi ne
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