Has anyone had rust form under their FSB? I removed the visible rust that was in the edges where the FSB and the barrel meet, and have applied CLP. I am hoping this will prevent any more rust and keep this from being a problem. I don’t want the rust to eat into my barrel :eek:
Almost all barrels have a little bit of rust under the front sight base.
This is not a problem though because it does not “grow” in the presence of oil.
Keep it oiled down.
If you want, you can knock out the pins and slide it forward off the barrel.
Wipe off the rusty oil that’s there now from both the FSB and the barrel and re-oil with clean oil.
I do this on almost every upper I tear down and rebuild.
The reason it rusts is that most barrels are parkerized as an assembly.
The parkerizing does not get under the FSB, but the remaining parkerizing solution DOES.
It’s this remaining solution that causes the problem.
The post-treatment oil is much thicker viscosity than the parkerizing solution.
The port-treatment oil preserves all the exposed freshly parkerized surfaces, but it does not leech into the nooks and crannies of the FSB like the parkerizing solution did.
Here is a brand new LMT barrel, only weeks old from the factory:
If you look closely, you can see how the barrel is discolored in the un-finished area.
Thank you very much for the informative reply. I don’t really want to remove the FSB from the barrel, as I bet I would just mess something up, but it’s good knowing that the rust is pretty common and that oil of course helps.
I do happen to have an LMT, so I guess if the barrel decides to fall off I can take advantage of their warranty 
Thanks again
Great reply. Thanks for doing the work.
I personally don’t lose any sleep over it. Anyone with experience on seeing any rust under theirs or someone else’s?
BTW, AR15 barrels has seen the bare steel under my old FSB when it was replaced with a LaRue gas block. I doubt he remembers it though, just yet another satisfied customer.
I remembered to take a picture of a barrel BEFORE I wiped out the rusty oil today.
While this particular barrel is a Bushmaster, almost any barrel will be just like this.
They also will mostly clean-up to look just like the LMT barrel pictured a few posts up before I re-oil them during re-assembly.
Quick question, won’t a Colt barrel be parkerized in those spots that are not on the LMT and BM barrels?
A colt barrel is parkerized on the gas block seat, but not inside the front sight base or on the handguard cap as those parts are parkerized as an assembly and will show similar rusting.