I have a Sig P556 that I turned into an SBR about 5 or 6 years ago.  I shot it a bunch at first, but it hasn't come out of the safe very often the past few years. I noticed what looked like rust around some of the edges of the gas block about two years ago.  I scrubbed around the sides and sprayed some Rustoleum epoxy paint on the gas block as good measure.  I pulled the Sig out of the safe tonight and it looks like there is a tiny bit of rust around the screw holding the Troy rail on.








Picture might help.




http://imgur.com/OrD9i7a



 

The only  way I know to stop that rust problem is to send it to me here in the cool, dry high country of the Colorado Rockies. I promise to never take it below 7,000 feet.??

Seriously, I would get it hot and try some Cano Kroil or  PB Blaster and let it penetrate. That looks like it could be a problem long term if not stopped.

That's a good idea.  I'm waiting on another AAC 51t mount.  Once that comes in, I'll do this.

 

I did take the rail off and I couldn't see any rust except for what's in that picture above.  I sprayed it down with Hoppes 9 and scrubbed as much as I could. It hasn't returned yet, but it hasn't been long.

I can't tell if that is rust, but the gas block is very easy to remove.  Much, much easier than on an AR15.  After removing the gas valve and gas tube (And bolt/piston assembly of course), simply tap out the two roll pins and the gas block slides right off the barrel.  The pins do not need to be pounded with force like an AR.  Trust me, it is very easy.   In fact, you don't even have to completely remove the pins- I tapped these out just enough to slide the gas block off the barrel.  That way, reassembly was easier.  (I sometimes have problems getting a roll pin started back into the hole after it was completely taken out).  

US and Swiss gas blocks:




Removing the block will help you inspect and clean it up if its rust.  
Good luck!

Thank you!  I've put together quite a few uppers for ARs, but the Sig just always looked intimidating to me so I didn't want to mess with it.

 

Pull gasblock, soak in kroil for 2-3 days, use a wire brush to remove rust, refinish the gasblock and reinstall. The gasblock is very easy to remove and reinstall. Make sure you buy dedicated roll pin punches to avoid marring the spring pins. I lube the pins overnight before removing and put some moly grease and oil on them before reinstalling.

You weren't kidding!  The roll pins came out very easily.  There was zero muscle involved.  It was light taps and they came right out.



Once everything was taken apart, I didn't see anything resembling rust or any other abnormalities.  I really have no idea what that rust-colored residue was.  I'll get some pictures up shortly.






I had been contemplating taking off the Troy rail that I have on it and putting the original handguards.  Lining the gas block back up, however, was made extremely easy due to the Troy handguard since it attaches to the upper receiver and the gas block.  I was able to put the rail back on and simply line up the gas block to the hole in the receiver to reattach it.






Now, to get my mount on there so I can put my can back on it.  I moved the 51t mount to an AR and have just been shooting using the A2 FH.  Even though the Sig doesn't suppress as well as my AR, it feels much handier on a 10" barrel as opposed to a longer one.






Pictures of Barrel and Gas Block:


http://imgur.com/a/umd1W





 

It's very easy to line up the gasblock. Find a roll pin punch that is almost the same, but just slightly smaller diameter than the I.D. Of the pin holes. Place the punch in through the gasblock to index the gasblock to the barrel. Get the other pin started and once about half way in, you can remove the punch.