3 Feb 16
Barrels and Chambers on Serious Rifles:
A friend in our State Prison System passed-on information to me about Central Armory’s experience with department AR barrels.
He has been tracking service life and found that original Colt chrome-lined barrels show less throat erosion and are staying in the “good range” far longer than are untreated barrels. New barrels that were installed, which were not chrome-lined, were discovered to be “shot-out,” (ie: “insufficiently accurate”) and thus in need of replacement, at as few as 4k rounds. By contrast, chrome-lined barrels are nearly all still serviceable after 10k rounds. Of course, the foregoing is greatly influenced by kinds of ammunition used and how hot rifles get during certain exercises.
Here are sage comments from several noted colleagues and gunsmiths:
“When we ream a .223 chamber to correct 5.56 dimensions, the question is often asked, and rightfully so:
‘Will that process remove chrome plating in the reamed area?’ The answer is yes. I leave it to the individual to choose between keeping the chrome and suffering the tight-chamber issue, or having a barrel that is not restricted to .223 ammo and may be fired safely with 5.56, although it might not last as long.
‘It is safe and reliable for the life of the barrel,’ as opposed to ‘neither safe, nor reliable, but it will be unsafe and unreliable for a longer time.’
Colt 5.56 barrels are my choice for chromed barrels, as they always get chamber-dimensions right, so there is seldom any need for subsequent reaming described above.”
Freddie Blish from Robar:
“Good information that confirms what most gunsmiths know about hard-chrome-lined military rifle barrels, and thus often specified in the Technical Data Package for the M4/M16 family of firearms.
However, ferritic nitrocarburizing, often abbreviated “QPQ,” (Glock calls it ‘Tennifer,’ S&W and others it ‘Melonite’) treatment represents the future for military rifle chambers and barrels.
Here is why:
Hard-chrome plating requires barrel manufacturers to bore barrel and chamber to an oversized dimension. Thickness of the subsequent plating then restores correct dimensions. The problem is that during the electrical plating process the amount of chrome deposited onto the metallic substrate is seldom even. Electroplating, no matter how well done, is subject to build-up, thin spots, and pin-holes. Build-up on corners, like lands, negatively impacts accuracy. Some barrel manufacturers are better at this than others, as Ned noted!
By contrast, when you nitrocarburize a rifle barrel and chamber, you can cut chamber and barrel to the exact dimension you want, then nitrocarburize it. The treatment causes no change in dimensions.
We have found the useful life of nitrocarburized barrels to be at least twice that of hard-chrome-lined barrels.
I believe corrosion resistance in serious rifle chambers and barrels is at least as important as is erosion resistance, described above. Surgical accuracy enjoys some importance, but not nearly as much as does rugged reliability.
I like serious rifles that are impervious to chemical corrosion, inside and out! “Normal” barrel and chamber erosion I can live with, so long as the rifle runs reliably, despite heavy use, relentless exposure to the elements, and more or less continuous neglect. I have absolutely no use for moody, temperamental prima donnas!
That is why I’m a fan of Robar’s polymer treatment of externals, combined with NP3 treatment of internal parts. I’m also a fan of Anderson Arms “RF85″ surface treatment, and STS Arms ceramic treatment. All are recommended!
However, NP3 is not suitable for barrel and chamber, nor is polymer, nor is ceramic. RF85 and nitrocarburizing are.
Tight-chambered rifles, that Ned mentioned above, are both dangerous and unreliable. Unfortunate owners of these guns are continuously harassed with feeding issues, blown primers, and broken extractors. With those issued plaguing you, “accuracy” becomes almost a moot point!
Not recommended for any serious purpose.
“Many are the beautiful theories that have been demolished by ugly facts.”