substance on the barrel under the FSB

[under9]

I went to the range this weekend with my new m4 type ar15 to sight in my Aimpoint. I shot a 120 rounds of different ammo(30 green tip M855 and the rest winchester xm193). The gun function flawless. The bolt locked back after every mag(pmags and cproducts with magpul followers. When I went home to clean my rifle I noticed a yellowish dust substance on the barrel under the FSB. I was wondering if this is normal or a potential problem? Like I said the gun function without any flaws just wondering what the substances was.
thaks josh

[Blankwaffe]

Ive seen some rust develop around and under the FSB on a couple uppers.Its not an issue...just make a habit to wet the area with CLP from time to time and allow it to penetrate under the FSB.That will keep the brown stuff away.

[under9]

this is a brand new upper made in May of 08. This is the first time I shot the rifle. I bought it a couple weeks ago. Could this be a gas leak?

[spamsammich]

All of the uppers I have (4), have a small amount of gunk coming out from under the FSB. None have cycling issues in the small amount of test firing done on them. Just keep it lubed up, displace any water under there, and monitor for excessive gas leaking. You're going to give yourself an ulcer fretting over your gun. Just shoot the shit out of it and keep it clean and lubed.

[under9]

thanks for the replies I didnt think there was a problem because the gun shot great. The gun is a LMT that I bought the upper and lower seperate. I put a LMT FA BCG and a LMT H buffer. I recently put on KAC RAS that looks awesome. I am very happy with the overall performance of the gun I was sighting the Aimpoint in. I zeroed in somewhere between 25 and 30 yards. what will the impact be if I shot 50 yards and 100 yards with this zero? All I need now are few more pmags, Surefire Scout 600c, Troy BUIS, lots of ammo, and training.
thanks again for your knowledge

[Blankwaffe]

The LMT uppers are not parkerized under the FSB.Ive even see some that are parked under the FSB grow rust in there.My new LMT did the same thing that your seeing.Like I said just put a few drops of CLP around the FSB everytime you clean.It will penetrate under the FSB and keep it from happening again.
Otherwise dont worry about it as its fairly normal to see.
I would probably start my zero at 50 meters and fine tune from there.