4seventy & tinker,

thanks for the advice on the 'brimstone' appearance'

. Maybe I used a misleading term here. What I wanted to describe is the look of 'burnt' metal. When you overheat metal long enough, it 1) becomes first softer then brittle in the end 2) the surface assumes a typical look because iron oxide builds up - the surface is no longer shiny, but rather dull, 'dry' - maybe 'brimstone' was too strong an image here, initially I wanted to call it a 'shale-like' appearance

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Daryl,

thanks. You made one of the most important points in this tread regarding obturation, which I'd like to take the chance to expand on a little:

Obturation:

A bullet, by being pushed fast enough, get slightly compressed (inertia effect) while it travels through the barrel, which temporarily expands its diameter. This is a however a known and beneficial process that aids the sealing of the bullet in the barrel. Without that, we would have gas leakage as the barrel, being elastic, expands temporarily as well while under pressure (this is why strain gauges can measure pressure). THIS is what obturation means, it is a word with a latin root that means to seal. Obviously, a SOFTER bullet obturates (seals) better then a hard bullet, as softer material is more easily compressed (again: LENGTHWISE compression, because the inertia of the bullet works against its acceleration, which means the diameter of the bullet increases). So harder bullets are by definition LESS prone to that effect. Herein lies the real problem of these bullets: As manufacturers undersize 'hard' bullets to minimize pressure spikes at ignition, they open the window for potentially damaging gas leakage. These leakages, which amplify the gas flow in areas where they occur (in the sense of a jet stream), could over time be damaging to the inside of a barrel. Think of miniature blow-torches passing through those leaks - I think you get the idea.