As an avid AFV modeler and a combat vehicle crewman (Artillery) in the US Army I have learned some very interesting things when it comes to "blackening". The reality is I have not seen soot build up on tanks and artillery tubes.
Anything firing a rocket usually does leave black marks on the vehicle. However this is only in areas exposed to the exhaust of the rocket or missile. Thus looking at the modern M270 MLRS you would see some black soot around the front of the LLM (the box containing the rocket pods) but not in the back.
This is probably due to the fact that soot is deposited where combustion happens - at the tail end of the rocket at the moment it departs the Rocket Pod/LLM. This is part a probable explanation as to why the ends of gun tubes do not blacken - because the projectile exits the tube but combustion takes place inside the chamber.
This idea is not only backed up but given exception by the experience of a friend of mine, a Bradley crewman, who said he does get soot buildup on the suppressor of his 25mm cannon. For those who are not aware every 25mm round from the Bradley IFV has a tracer element on the back of it. This re-enforces the idea that where combustion takes place soot appears. A side note for the M2 AFV builders out there - don't go overboard with the soot on the end of the 25mm as its minor in appearance.
However from experience with small arms I have learned part two of gun tube blackening. If you don't believe me buy your self a Mini-14 and a 10/22 and try this. My Mini-14 does not gather soot on the end of the barrel after an afternoon of shooting (and I mean a LOT of shooting). My 10/22 however does gather a ton of soot on the end of the barrel. I came to realize the reason is probably ammunition quality. .223 Rounds are more expensive and generally of better quality (at least the brands I use, about $10.00 for 15 rounds) but I purchase the 500 rounds for $8.00 of .22. These .22 rounds are not even jacketed, just lead on the end of .22LR. I'll admit that maybe 50 of those rounds are duds, but that is beside the point.
Most militaries won't depend on shoddy tank ammunition so it's a good bet that this is the reason we don't generally see the build up. After all, soot is partially burned material; the higher quality the ammo the more efficient the burn and therefore the less unburned powder (thus less soot). Smokeless powder probably affects this as well.
However when it comes to the proper weathering on a model one should consider that the removal of this soot (when present) requires a stiff brush, water, simple green (or other house hold cleaner) and sometimes a paint scraper. Water will remove it but the scraper is faster. I have also come to notice that CARC Paint (used on NATO vehicles) dries out over time and the heat and blast from the rockets may assist in the disappearance of paint in the area.
This fact accounts for the occasional appearance of bare metal in certain areas - the paint has been scraped off. Likewise, on the first photograph on the right, this actually appeared to me as paint removed by the bore brush during clumsy insertion during cleaning - and not blackening. Perhaps something else that should be weathered in?!
Taking into account the bare metal idea and the lack of soot we still see in some pictures black barrels. I have a last theory into the appearance of this and it relates to military experience. The military loves everything to look new and generally subdued. I cannot count the number of times that paint got scraped off a rucksack frame or a part of a vehicle - and the solution was clean it and spray paint it. Odds are that in some units damage to the barrel has prompted the crew to paint the barrel end. Black paint is probably just the most versatile color available to a military unit in the field.
So, while some of you out there might really love the idea of weathering a sooty barrel. My advice is to weather bare metal from the bore brush on the barrel end. In my modeling experience when I paint a camouflage patterns I try my hardest to leave the end of the barrel black in the scheme; as this avoids the whole argument all together. Additionally most modern armies clean their weapons daily and any soot that might be left behind would be cleaned up.
All contents written by Christian Ankerstjerne unless stated otherwise