Sinclair Wrote:Gunpowder does not explode, it burns at a controlled rate just under detonation (unless you are loading squib loads of Bullseye).
Sinclair,
I believe the term is deflagration. Not 'detonation' and like you say, not burning like uncontained gunpowder.
My garage went up in flames a few years ago. When the Fire Brigade arrived they wanted to know if there was anything flammable in the garage before they went in. I said ammunition stored in a safe. The guy dismissed it immediately and repeated the question, No, anything seriously flammable like gas bottles? After the fire all the ammunition had cooked off, but the door to the safe was still closed. Not like the movies where you throw ammo in the fire and everyone runs for cover.
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As for the ops question...Any length less than 30" is a compromise in velocity. As the U-shaped velocity/length curve implies, the shorter the barrel the more severe the compromise. It's a qualitative decision weighing up how much compactness and weight is vs. velocity. You could answer it by considering what velocity you want to hit the target with (e.g. below 2K fps,or 1.8Kfps and ballistic tips reportedly act like monoliths when hunting. Or, will it make the range I want it to go before going subsonic?). Then there are issues like unburnt powder (flame and less consistent velocities) and muzzle pressure (noise, and if you are using a suppressor, over-pressure). Not all powder is burnt in 3" but as the Quickload graph in my previous post shows, most of it will be. Plus it's complicated by burning gasses expanding due to firstly a chemical reaction, then expanding further by heat.