Well, I broke the AR in today and the problem is no longer there at all!
I happen to have, on hand, about 2k rounds of 5.56 tracers - and I happen to live on a rather wet farm in the middle of nowhere. So, with my suppressor attached, I decided to do a bit of shooting.
All-in-all, I think it was about 400 rounds I ended up firing, with 95% being tracers. The can sure makes things much more dirty - but the NiB-x bolt just wiped off. That thing is amazing.
Anyway - fully aware that shooting tracers through a suppressor completely negates your "reduced signature" it was fun none-the-less. And I'm now much more confident in the weapon.
No FTF; no FTE - just smooth and fun.
Edit - and before someone says that I am ruining my barrel or my can:
First off, want to establish credibility:
I am a Federal Firearms Licenced Manufacturer with an Special Occupational Tax Stamp (basically I can make surpressors and machine guns) and have been around firearms 30 years now. I went to Naval Gunnery School and have worked for a major firearms manufacturer (KAC). I have also fired tracing rounds (in the hundreds of thousands) of many different types (i.e. infrared and foreign military).
Why did I bring up this statement of credibility? Because I'm about to completely dispell some "misunderstandings" posted here about your tracer rounds.
I like the first post: It is entirely true that tracer rounds do not begin the burn until after they leave the barrel, typically between 25 and 75 yards. Because of this fact the only residue you receive in you barrel is gunpower and primer. With that said you should pay more attention to the type of primer you load into the weapon than the type of ammunition. Also, alot of countries put sealants on the projectiles such as tar or petroliuem which is more damaging than anything that tracer round is going to emit inside that barrel.
You will not hurt your barrel by sending a tracer round though it at all. Well, anymore than a regular round. The projectile does "grind in" carbon from preceding rounds, but you should know that the carbon that is present is much less dense than the steel the barrel is made of and therefore the reprocussions of such actions are minimal. This goes for ANY round. A tracer round is no more damaging than any other normal round.
Tracer rounds are illegal in some states. Which states I will not begin to list for fear of omitting one or two and therefore misleading you. I will say, that in Florida the tracer ammunition is illegal to fire, not own. So, yes and no to the legislative standing of ownership of tracer ammunition.
Just as a Post Script, I want to add that the year/type of the tracer ammunition does have bearing on the preceding statements. Older tracer ammunition (before the Korean war, and throughout) does have a tendency to ignite inside the barrel, but still typically has little impact.
To sum it up: Fire away.
Found that online today - found several other sources that back that up.