No matter how well-lubed, ect. I have noticed this on one of my Dad’s bushmasters. It isnt bad, just a bit of seepage. I seep some CLP under the FSB each time I clean it. Is this how I should deal with it? Will this cause a problem at any point with gas-seal or whatnot? I have heard it is normal, but just wanted a second opinion. My new Sabre does it as well FWIW.
BM does not parkerize under the front sight base. So it is not going to stop.
1- Keep it, keep it lubed and shoot it till it wears out and buy a new one.
2- Get an AR/Gun smith to remove the FSB and clean/remove rust and park.
3- Sell/trade it for a new upper.
?
It won’t really hurt anything but if you’re really concerned with it you could remove the barrel from the upper, remove the FSB. Remove all the rust and refinish the FSB and barrel with GunKote or Norrels moly and reassemble. I’ve sprayed all my barrels except for one (which is on my PWS piston converted Colt).
I would just place 2-3 drops of whatever protectant you use on the front of each fsb ring before putting it into the safe. When vertical, it will weep down and under the fsb to keep it in check.
Shoot the gun, enjoy it and when you have to replace it, get one with the park done before installing the fsb.
hummmmm ! are you certain it is “RUST” ? i have seen this brown stuff myself on two of my ARs, and i have come to the conclusion that the “brown stuff” is actually burned powder residue, what made me come to this conclusion is changing the ammo in one of the carbines, for one shooting session, which was 250+ rounds fired, when i got home i took my magnifying glass inspected the area, no “BROWN STUFF”…, but “GREY STUFF”.
kinda like checking the tail pipe of your vehicle, if “black” you are running “rich” if “whiteish gray”, you are running “lean”, if “gray”, the fuel/air mixture is just right for that engine.
I get some grayish residue around the FSB on my 10.5" LMT uppers.
It’s minimal, and seems to be self-sealing, in that it once a little bit is established, it doesn’t get any worse.
However, if I disassemble the upper and remove the FSB (and clean that prior to reassembly), the gray residue returns after the first range session.
Not a big deal to me.
from this thread:
https://www.m4carbine.net/showthread.php?t=138&highlight=short+stroking
Oddly my Sabre defence does it too, and I thought they parked under the FSB. It is doing it from both bases, but appears to be doing it moreso from under the base closest the barrel.
Calling Sabre, they have always been honest and very informative with me. I think they will have some ideas about whether it is powder, rust, normal, abnormal. Once I called them and the tech simply admitted to not knowing the answer to a question I asked and had to go look at the part to verify. They are the only company to have a tech say to me “I don’t know, let me find out”. I really admire that.
Randall from AR15barrels.com posted that Sabre does a “light” park under the FSB.
So is this brown stuff oozing out rust…or…something else?
so this is leakage and i should be concerned?..
cant see it well(on the rear),but its coming from both of the FSB rings/mounts.
Mine is very similar to the above ^, except I am a CLP freak so it kinda runs instead of forming up around the crevices. It looks a lot like rust, is it, or is it not?
PS. what maker AR is that?
Mine is very similar to the above ^, except I am a CLP freak so it kinda runs instead of forming up around the crevices. It looks a lot like rust, is it, or is it not?
PS. what maker AR is that?
i just recently noticed it(from those pics actually),so its been getting the heavy CLP lately as well.
i thought it was just rust,but from the description in the other thread(that was quote)it sounds like a leak.
LMT
How can the front "leg’’ leak?
I don’t know. If Randall was still around he might be able to shed some light. 
How can the front "leg’’ leak?
good question(maybe?).
i wouldnt know,never seen under a gas block or FSB.i dont even know where the ports are located exactly.
i just clean/lube and shoot…some basic trouble shoooting.
Called Sabre they and:
The brown stuff I am seeing is the blasting media they use to clean things up right before attatching the FSB. They flash-parkerize under the FSB and they have assued me there is no bare metal. They say when things heat up and there is CLP under there, the media weeps out. They clean it out with an air-hose before it leaves the factory, but some still stays under there and it will just weep out with the CLP when things heat up.
That looks like the brown stuff that LMT uses in the finishing process. My everyday AR has it all over the barrel and inside the chamber. Some get it confused with rust.
Is this the same stuff Sabre uses as blasting media or what?
That looks like the brown stuff that LMT uses in the finishing process. My everyday AR has it all over the barrel and inside the chamber. Some get it confused with rust.
yeah,LMT says nothing to worry about…no explanation though.
unless it starts causing issue’s i think thats good enough for me at this point.
there appears to be some of it in spots that wouldnt be explained by a gas leak.
which was my main concern.