A worn out AR and Throat Erosion

We have a rental AR that was rebuilt in 2014.  After that rebuild it was practically a new gun.  The only parts it retained were the bolt carrier, the lower receiver, and the small parts typically included with a lower parts kit (trigger, hammer, detents, springs, etc).  Since this gun had been at the store long before I got here I don’t know exactly what brand those parts were but the lower receiver was an Essential Arms and it was a 20” A2 fixed carry handle rifle.  It seemed nobody wanted to shoot a fixed carry handle, 20″, fixed stock rifle so we converted it to a 16″ with an adjustable stock.

We replaced the upper with an Anderson Manufacturing Sport upper.  The barrel used was a 16” used DPMS chrome lined 1:9 twist M4 profile barrel I took off one of my personal rifles several years before.  We swapped the fixed stock for an Anderson M4 adjustable stock and the handguard was a custom one made for us with our logo by Unique ARs.  We topped it off with a set of Magpul MBUS sights and we put it back into service.

For the last three years this rifle has been our primary rental rifle chambered in .223/5.56 and it is rented and shot every single week multiple times.  Based on the number of times it is rented, and the average number of rounds fired through it, the round count on this rifle is estimated to be between 100,000 and 120,000 rounds.  That is a substantial amount of ammunition through a rifle by anyone’s standards.

At around 80,000 to 90,000 rounds we had a bolt break.  It broke at the cam pin, a common point of failure for these guns.  We decided to swap the entire bolt carrier group with one from Ballistics Advantage (made by Aero Precision) and it has been running with that bolt carrier group since.

Aside from needing to be cleaned regularly, and needing new gas rings periodically, the gun requires very little maintenance.  Over the last few weeks, however, we have noticed that the rifle regularly gets taken down for service.  It was experiencing failures to extract, to eject, and to feed.  We would clean it thoroughly and find that it would run again, only to go down after a few rental sessions.

I took the gun out and shot it and it ran fine for me, but I noticed that it was cycling hard.  The recoil impulse was off and it was ejecting brass at a short forward angle hard enough that it was actually chewing into the upper receiver a bit at the forward edge of the ejection port.

Once apart we found that the gas block, and gas tube, were both in very good condition.  This was surprising given that the gas block was an aluminum block from Anderson Manufacturing.  We have seen aluminum gas blocks erode terribly and with the round count on this particular gun we expected it to be in rough condition.  It turns out, that concern was unfounded.  We did find that the gas port itself was eroding.  Not horribly, but it was over spec for a carbine length gas system.  This was likely a contributing factor for many of our cycling issues and made sense given what we noticed while shooting it.

The most interesting thing we found was the throat erosion on the barrel itself.  If you aren’t familiar with the term, throat erosion is just the throat of the chamber eroding down the barrel.  This increases the distance the bullet must travel after leaving the brass case before it contacts the lands of the rifling.  When measured next to a new barrel, the throat on this barrel had eroded nearly .500 inches (see pics).  Since this jump to the lands of the rifling should start at around 0.100” (it can vary somewhat due to chamber spec) the throat was approximately 6 times longer than it should have been.

Unfortunately we didn’t test the accuracy of this barrel before taking the gun apart.  It would have been interesting to see how poor it had become.  None of our customers complained about accuracy but since our indoor range is only 25 yards and this particular rifle only had iron sights it isn’t surprising that it went unnoticed.

Needless to say we are replacing the barrel and will be replacing the gas block and gas tube as well (they are cheap and owe us nothing).  We’ll be stripping and re-coating the upper, lower, and handguard as they are getting well worn.  We plan to coat it in the new Cerakote Elite series coating to see how it holds up to hard use.  Once this is done the gun should look and shoot like new and we expect many more years of service.  You gotta love how versatile and robust the AR platform is, and how easy it is to breathe new life into a worn out old rifle.

– Aaron