The bore-scope view pictured below shows the gas port of a 20" AR-15 barrel with a rifle length gas system that has not yet been fired.
The next bore-scope view shows a 20" barrel with a rifle length gas system after only 1000 rounds have been fired through it. This view illustrates one of the reasons that certain high-end barrel manufacturers make the extra effort to ensure that the gas port is drilled in the center of a groove.
The shorter the gas system, the more rapid the erosion. The bore-scope view below shows the gas port of a 14.5" barrel with a carbine length gas system after only 500 rounds have been fired from it.
Wow… I never would’ve thought that there would be that much erosion that fast. The erosion pattern makes sense. I wonder what the gas block side of the port looks like…
Yes. mis-matched bullet weights and powders can make a dramatic difference in carbine port erosion. Some industry guy (a few years back) posted info on the firing line. He was testing finishes or treatments for a manufacturer, and found that, for example,… 55 gr bullets, when paired with Varget (a relatively slow powder) yielded dramatically increased port erosion.
Port erosion, from what I’ve read over the years, is most dramatic in the early life of the barrel. It tends to settle in and slow down as the tear drop shape of the erosion sets in.
I’ve pulled the FSB off of my highest count barrel… 30k plus rounds (14.5" carbine barrel)… and there’s no erosion on the top side of the port. I don’t worry about port erosion at all.
The port erosion posted is normal. What’s interesting is how clean the port is in the unfired pic was, ideally most would be this way, many times it’s not in regular production barrels. Erosion in this area is normal, many factors contribute to the rate that it happens. It becomes an issue when the cyclic rate comes out of a desired specification, and/or required precision isn’t within tolerance after proper(?) cleaning of the bore, The rate of wear does reduce with round count, but continues to do so.
Many barrels are improperly cleaned for their own desired uses. Sometimes things like carbon and copper are dealt with in different ways for different barrels and different loads
To add more to this post from a personal email that I received today from this posting. I should have been clear that barrel bore cleaning should only be done enough to restore the precision requirement and corrosion resistance, not totally stripping the barrel of fouling. Not leaving some fouling to fill voids and used as a sacrificial barrier of additional erosion is counterproductive inside the barrel. Certain areas of the barrel may tend to collect more of on type of fouling than another than others depending on that particular situation and the the trouble spots need to be worked with accordingly to get the required results.
Chrome-lined GI barrels will also show chrome peeling off the highest parts of the lands at the leads and surprisingly at the shoulder at the front of the chamber.
Markm, you said:
Port erosion, from what I’ve read over the years, is most dramatic in the early life of the barrel. It tends to settle in and slow down as the tear drop shape of the erosion sets in.
That is very true. It’s like if you took two machetes and crossed them, edge to edge, and started whacking them together. The first whack, big notches. Second, gets deeper, but with each whack the damage is less and less.
Exactly, it is seeking equilibrium with the path of least resistance. Gas is a fluid and acts somewhat like water. It is making what we failed to give it in the first place. Nature generally abhors right angles.
The ultimate solution would be to do what LMT did, drill the damn thing at 45 degrees.
It’s like water, you can build with it or fight against it but ultimately, it will wear you down if you chose the latter option.
