NRA American Rifleman by Justin Dyal - Friday, September 7, 2018
Conventional wisdom holds that rifle bores must be relatively clean to shoot well. But how clean? Or better yet, how dirty is unacceptable? That is one of the questions that is virtually guaranteed to produce as many answers as shooters you ask. And when you ask for hard data to support the number asserted, there is usually not even so much as a first-hand anecdote grounding the answer.
As a young Marine captain, I was the new officer on a rifle team and remember asking the grizzled old salts who had shot in many an inter-service championship or Camp Perry what the proper cleaning interval was for those incredible Quantico-built National Match M16s. The answers varied from daily on one end to at the end of the season on the other. There didn’t seem to be any real testing to support any given answer, and I accepted that you punched the bore whenever it seemed right.
I recently thought back on that experience as I finished up an endurance test on a Bravo Company Manufacturing (BCM) upper receiver. Over the course of a little more than three years, I had logged the lubrication intervals using FireClean to see how far the AR would run as it got dirtier and dirtier. At the end of the test, I was in possession of a barrel through which I had logged 10,000 rounds and had never cleaned in any manner. In shooting the last thousand rounds or so, I had noticed that the rifle seemed to be still shooting quite well and thought it would be interesting to do a formal accuracy workup. I borrowed a Bushnell Elite 4.5-18x44 LRTS riflescope to give the barrel every chance to succeed, and grabbed some quality ammo.
All firing was done from the prone position with the rifle supported by a Harris bipod in the front and some bags under the toe of the stock. The rifle had a Geissele Super V trigger, which is an excellent duty and snap-shooting unit, but not normally associated with group shooting. The BCM wore a free-floated KeyMod aluminum KMR-A rail and the barrel was a basic government profile 16†with a mid-length gas system and a 1:7†twist.
I fired a couple of sighters to get the LRTS on paper and then the very first five-round group of Hornady Steel Match clustered five .224-cal. holes into a tight .84†group that could be covered by a nickel. That was pretty close to prophetic, as the average of all five groups with the Steel Match ran .89 from the filthy barrel, with the series tallying .84, .85, .86, .72, and one lonely group over one minute of angle at 1.17.
More:
https://www.americanrifleman.org/articles/2018/9/7/filthy-firearm-ar-accuracy-testing-at-10-000-rounds/
