Keeping Hot Barrels Accurate
BY Herschel Smith
6 years, 4 months ago
Anyone who has spent time at the range shooting high-powered rifles knows that sustained fire or continuously shooting without letting the barrel cool down between strings of shots will get the barrel so hot that accuracy will suffer and shots can be thrown off a couple of inches. It isn’t hard to fire a military-style rifle in semi-auto mode at 100 rounds a minute, and many owners of these rifles do just that at times when shooting at the range.
Even when shooting a semi-auto rifle in rapid fire mode, an enormous amount of heat is generated, which can quickly ruin a rifle barrel .
The leade, which is the unrifled portion of the barrel just forward of the chamber as well as the first few inches of rifling, are subjected to enormous temperatures approaching those on the surface of the sun as well as pressures exceeding 50,000 PSI during rapid-fire exercises.
During slow-fire conditions, this area is allowed to cool sufficiently between strings of fire.
Under sustained rapid fire, however, there is no time for the heat to dissipate and temperatures soar into the thousands of degrees Fahrenheit.
Currently there are four very different methods used by shooters to protect and extend the service life of their barrels.
Those who participate in bench-rest and long-range completion use very heavy and long barrels which can last much longer than the barrels found on most sporting and hunting rifles. Still, those big heavy barrels have to be replaced when accuracy begins to deteriorate.
Currently, the military hard chrome lines the barrels of their rifles to protect them from the excess erosion that occurs during sustained fire. This greatly extends the barrel life of rifles that are fired for prolonged periods in full auto or semi-auto mode. It takes a very knowledgeable professional person to evenly apply the hard chrome lining to the inside of a barrel, but the barrel will have approximately twice the service life of an unprotected barrel. Both the military and civilians who shoot semi-auto versions of military style rifles swear by the hard chrome lining of the barrels.
Some claim that hard chrome lined barrels aren’t as accurate as unprotected barrels because the rifling of hard chrome lined barrels is not as sharp as in unprotected barrels. This is true, but the difference in accuracy will never be notice by the majority of shooters. One MOA is pretty common in most of the military rifles and their semi-auto counterparts being built with hard chrome lined barrels today.
Another method of dealing with the heat and pressure that rifle barrels can be subjected to is a process where the un-blued barrel is immersed in a very hot liquid nitride salt bath for a period of time. The process is known as “ferritic nitrocarburizing.” This is not a new technology but has recently been applied to rifle barrels to protect them from the heat and pressure from sustained fire.
Most people will recognize terms like Melonite, Tennifer, Ni-Corr, Blacknitride or Salt Bath Nitride. They are all variations of the same process …
Read the rest. I always like to catch everything that Smokey writes. I’ve exchanged email with him and find him to be a very nice guy, and always very knowledgeable.
From what I understand it’s a good idea to keep muzzle velocity under the 3000 FPS – 3200 FPS threshold, and that guys who shoot the .243 in competition approach 4000 FPS with the lighter loads and have to change out barrels every several hundred rounds.
Those folks would have to be sponsored.
On August 12, 2018 at 11:14 pm, Georgiaboy61 said:
Carbon fiber barrel sheaths deserve a mention… apparently, graphite or similar carbon-based polymers dissipate heat well, which is why they are used as barrel coatings. I haven’t dug deep into the chemistry, but it appears these compounds are excellent heat sinks, and when applied to a standard rifle barrel, they transmit heat away from the barrel better than steel alloys alone. Thus coated, standard bolt-action rifles deliver sub-moa accuracy, even in strings of heavy fire. Make of that what you will…
On August 13, 2018 at 5:16 am, Ratus said:
I’d recommend watching the series of videos by InRangTV did discussing basically everything you’d want to know about barrels with someone from Faxon.
It’s four videos running around three hours total of Ian and Karl asking interesting and probing questions and the Faxon guy doing a good job answering.
On August 13, 2018 at 8:07 pm, =TW= said:
@Georgiaboy61
The carbon hybrid barrels are gaining a wider following.
Pencil barrels heat up real quick and POI shifts as a consequence. (You will notice this on your Mini after a mag dump.)
Bull barrels are generally more stable, at the price of added weight.
Fluted barrels shave some weight and expose more surface area to dissipate heat. But I reckon if the flutes are not machined with precision, or if concentricity is not maintained, groups will suffer.
Proper heat treat schedules and stress relief during manufacture, and subzero quench afterwards may stabilize barrels.
Barrel resonance (whip) is another variable to consider.
Various bedding techniques and devices have been employed to increase accuracy with some success, and attention to detail is a key ingredient.
But the process of improvement affords a certain satisfaction, incremental gains along the way enhance satisfaction as well as accuracy.
On August 17, 2018 at 10:46 pm, Blake said:
Rule of thumb for match barrels: 800 rounds and it’s time for a new barrel.