I have an 11.5" barrel on an SP-1 lower that is at least 20 years old that has not been shot much over the years. I recently got interested in it again when I started rifle shooting at work. I went through an Armalite armorer seminar and probably learned enough to be dangerous but I've got a leg up on most of the people I work with. I don't know the barrel/gun's history before I bought it except that I had some serious maintenance issues to take care of before the gun became reliable.

Since I have keep an eye on fourteen rifles at work, I thought I'd get a few tools and gauges to track wear and tear on the work guns. One thing I got was a throat erosion gauge from Brownell's. There are a bunch of rings around a rod that is gently inserted into the chamber area and you count/mark where the rings are in comparison to the forward edge of the ejection port. Well the work guns only go into about the first ring. On my old barrel, the gauge goes five (5) rings deep.

Does this mean I should start thinking seriously about a new barrel?

First of all, don't get hung up on a throat erosion gauge. Don't get me wrong, they are wonderful tools but it is a relative measurement. It is only valid for the most part if you took a throat measurement when the barrel was new and unfired. Who knows, the thing may have throated a 4 when new. Due to the vast number of throats and chambers with the AR it is very hard to take a reading on a used barrel and have it mean a whole lot. I would be more worried about the muzzle, that is where accuracy is more affected. The best way to tell if your barrel is shot out is to shoot it after a thorough cleaning and see how it does on paper with some decently accurate loads. Test drive it first and see how it does.

HAPPY NEW YEAR TO ALL, MAY IT BE SAFE AND SECURE!
JACK

I haven't had the opportunity to shoot at anything over 25 yards in a while so I'll have to wait for the chance to test it out and see from there. If I'm all over the place, and I know I'm better than that, I know it's time to replace.

Thanks for the assist.

I shot one of these 11.5" uppers back in the early 80's, this gun would not group at all. The bullet would tumble in flight and on a paper target some times you could see the bullet profile. I did not look into the reasons why it did this just wrote it off as a bad barrel and replaced it with a 16".