Hot Barrel
I was out plinking with some friends and one of them had an AR-15 clone. He ripped 3 30-round magazines through it in rapid succession. The rifle is an Oly Arms PCR01 with a 16 inch HBAR, A2 upper. After the quick 90 rounds, the barrel was very hot---I would guess that if someone touched it, they would have left charred skin on it.
My question is can this type of rapid fire damage a rifle such as this? What are the visible signs that a barrel is overheating? Is it even possible to overheat a modern barrel in semi-auto shooting? What is a safe number of rapid-fire shots before a rifle needs to cool down. We were shooting NATO surplus XM193 ammo.
My question is can this type of rapid fire damage a rifle such as this? What are the visible signs that a barrel is overheating? Is it even possible to overheat a modern barrel in semi-auto shooting? What is a safe number of rapid-fire shots before a rifle needs to cool down. We were shooting NATO surplus XM193 ammo.
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When you heat up a barrel, it expands which can cause the bolt assy to start binding and the bullet to not fit tightly in the bore. This is what eats bores when doing this, the high pressure gas will seep aroung the bullet and erode the barrel. Smoke, melted handgaurds, and guns getting hastily dropped are good signs of overheating. You also increase your risk of cooking off a round in the chamber. Even LMG's have max firing cycle specifications as guidelines to how many rounds to fire before letting it cool. Obviously in a combat situation you probably won't find anyone giving a damn about "duty cycle" on a rifle. I try to limit firing cycles on the AR because the barrel profile is usually pretty narrow to cut weight so the barrels heat up quikly. I have two SKS-M rifles that have bipods and 75rd drums and I ran two drum through one just to see if it would work OK (in the event I ever needed to do so) and it functioned fine, but was too hot to touch for about 15 minutes. I have seen handgaurds or stocks start to melt/smolder/ignite from folks spraying away so take it easy on your rifle. Three 30rd mags would be about the extent of what I would run through an gov't profile AR or an M4
Love them Beavers
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As a general rule of thumb, when the barrel begins to glow a dull red, you're shooting too rapidly. Ripping through several mags in a short time won't hurt anything...the first time. If you make a habit of it, barrel life WILL be drastically shortened. 0 -
Heat in ANY rifle barrel is the cause of EROSION. Steel melts at a given temperature, nuff said. Now, when you continually fire rounds in a bore, you are increasing it's residual heat each time. That being said, the supercompressed gasses (actually a plasma) exiting the case mouth are now flowing over the already superheated steel. THUS, LESS plasma is required for the same amount of erosion, therfore, the hotter you get it and continue to fire, the greater the erosion rate.
simple as this:
500 rnds fired allowing adequate cooling vs 500rnds fired rapidly, will yield (If I were to guess) a 300%+ increase in the throat erosion measurement.
why chase the game when the bullet can get em from here?....
Got Balistics?0 -
The Tinman,
It's difficult to form an opinion regarding so many of the physical things that take place in a barrel when you can't actually see what goes on inside. Part of the information has been covered here but I'd like to add some comments from Boots Obermeyer, who is one of our premier barrel makers. Boots has made more barrels than most of us combined, have seen or handled. These comments are part of a series of articles that Boots has written and I'll post the link at the end.
"A mistaken view is that loss of barrel life is due solely to the abrasive action of the projectile with the barrel's rifling. This is partially true; however, the main culprit is bore erosion followed by bore wear. Bore wear and bore erosion are not synonymous. Bore wear results from the removal of metal from the bore's surface by mechanical abrasion. Bore erosion results from the removal of metal from the bore's surface by hot propellant gases. Abrasive wear is gradual and usually even to the surface of the bore. On the other hand, erosion can be rapid, particularly when the bore is exposed to continuous high temperatures. Erosion is the major cause of barrel wear with the greatest focus directed toward the area where the rifling begins (Figure 1). The hot metal from the bore's surface reacts with the propellant gases to produce a brittle state that results in metal pieces being removed during subsequent firings. In addition to bore wear and erosion, corrosion should be considered as a contributing factor even though it plays a minor role with non-corrosive propellants and primers."
"What is thermal stress and how does it affect accuracy? Thermal stress is due to the production of very hot propellant gases as well as to the friction created when the projectile first engraves into the lands of the rifling. Approximately 30% of the energy available in the propellant is transferred as heat by both convection and radiation to the barrel within milliseconds after powder ignition. With ignition of the first cartridge, the barrel chamber is shielded from the newly created heat by the cartridge case. So the duration of the heat build up is very short with one firing and is restricted to the bore surface. With continuous ignitions, heat builds up as it is transferred from the chamber to the rest of the barrel. Rifles chambered for a cartridge with muzzle velocities that excel 3000 fps can burn out after a few hundred rounds if shot consecutively without cooling. Heat build up can be contained, with time interruptions between ignitions -- allowing the barrel to cool down between firings.
Thermal stress of barrels can be divided into two categories: 1) uninterrupted multiple firings over a short period of time without cooling the barrel, and 2) few firings over a long period of time with cooling the barrel between each firing. The first category is characteristic of machine guns (as well as over enthusiastic prairie dog shooting), while the second category is characteristic of rifles used for sniper, competition and less enthusiastic varmint shooting.
Either multiple or few firings result in a transfer of heat into the interior of the barrel wall. To appreciate the dynamics of the events, let's focus on multiple firings without cooling. This scenario reveals an increased barrel wall temperature, which is transversed three-dimensionally within the steel. The maximum temperature of the barrel's internal surface depends upon the type propellant being used, barrel composition and duration of firings. With long durations, the elevation of barrel temperature results in an increase of bore diameters. Simultaneously, a change in the depth of the land engraving to the projectile's surface occurs, which results in the projectile leaving the muzzle with an undesirable yaw. Is there any way to overcome or to reduce these changes? Current methods used to dissipate heat from barrels include graphite coating the barrel, fluting the barrel, liquid holding jackets (e.g., water liquid nitrogen etc.), increased barrel diameters etc."
John: Boots, over the years as you have made thousands of rifle barrels, the topic of rifle barrel life must have arisen a number of times in a number of different ways. Can you provide the reader with an overview of your experiences with rifle barrel life?
Boots: My experience with barrel life comes from two areas. The first is with pressure barrels, while the second is with competition rifle barrels. When the Amron operated in Waukesha, Wisconsin, I made their test barrels, which allowed me to observe problems as they developed. A major problem was erosion loss of the piston holes located in the chamber. There would be some leakage to the rear of the gas port because the thickness of the cartridge case wall at that point didn't allow it to seal perfectly. Often the test barrel could be set back an inch and be used again simply by adding a correction for the velocity loss due to shortening of the barrel. I noted that as the bore became eroded the velocity spread became a problem. In the .243 Win. caliber at approximately 2000 rounds, the velocity spread widened greatly even though the pressure remained constant. Most of the ammo that was being tested was made for a large company that sold under their own brand name. They demanded tests for each lot of ammo to insure that it had reasonably low velocity spread. Simply applying a correction factor wasn't good enough for them.
The other area is competitive high power rifle shooting. John, as you know, not only do I make barrels for sport shooting; I'm a competitive shooter as well. I believe that this gives me special insight to a number of problems that might be missed by others. I've concluded that barrel life has a time volume relationship.
There is a lot more to this article and it is definitely worth reading. It is not hard to understand but it will take a few minutes to read and more time to sift it down.
Best.
http://www.snipersparadise.com/tsmag/june2001.htm0 -
Great article---should be required reading for any serious shooter. Thank you.
Tinny0
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