that soot inside your cases is being put into your gun everytime you pull the trigger. That means your barrel is getting dirtier faster, and your guns mechanism is being exposed to extra filth which leads to premature wear and shorter life of the firearm.
If you want your guns to stay clean and hold value you cant put dirt in them.
Haha, oy... I guess if soot is so evil, then we shouldn't be using "dirty" powders, or use ammo with dirty burning characteristics. (Winchester, Remington, Aguila, and others come to mind.) But the fact is that all powders produce some solid by-products when they are burned. No matter how immaculate a case may be before firing, when the powder charge gets burned, a whole new batch of soot will be formed along with the hot, expanding gases. Most of the soot (solids) gets blown out with the projectile. If any of you folks are afraid that increasing the quantity of soot involved will cause dire consequences, then by all means, clean away. Heck, swab the gun after every firing, I promise I won't feel a thing!
If you mean real dirt, as in soil (a combination of sand, silt, clay and organic matter), I agree, that is an issue. An issue that a good rinse in warm water (Dawn may help, but even plain water works) or a wipe with a good cloth is very good at rectifying. Real soil particles are small and have abrasive properties. Silicon dioxide in particular. (Sand or silt from weathered quartz is not a desirable lubricant for your firearm.
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Also your dies suffer and how can you expect to load quality ammunition with your essential tools not in top shape.
You dont clean brass just for looks
Same deal here... I could see the dies suffering from soil particles, but not soot. And again, I rinse off brass with dirt, or at the very least wipe each case with a rag.
Just for grins, I cracked open the Sierra 5th Ed. to see what their thoughts are on the subject. The opening line begins... "Although not absolutely essential..." On pg 123 for those that have one or care to look. The Section is headed "Tumblers and Polishers". To summarize, they can be a good thing, but aren't necessary.
They mention surface grit, and that, for example, wiping off cases with a rag can increase the life of the sizing die. (Tumbling is also of course mentioned as a way to remove surface grit.)
They do recommend tumbling briefly after sizing, as it can help remove lubricant applied before sizing. Obviously this isn't much of an issue if no lube is used, as in with carbide dies. Although again, they say that a wipe with a rag will also work.
I'm not saying that you can't or shouldn't tumble or vibe your brass. I'm just saying that I don't, and that I'm not the only one who realizes it isn't a necessity.
All this being said, when I got home with my last batch of range brass, it was dirty. Actually had some soil stuck to the outside. It all got a nice warm bath before it got sorted and stashed.
Happy shooting folks.