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Old November 20, 2012, 10:08 AM   #14
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 10,247
Yes. I think the engine cylinder is apropos. Knocking is detonation of the gasoline and air mix. It used to be hypothesized that it was initiated by echoing waves in the cylinder forming additive nodes, but the advent of transparent cylinders and ultra high frame rate cameras showed that, more mundanely, it simply initiates at the hottest point in the cylinder. Well, we've known since the Civil War that nitroglycerin would detonate spontaneously when it got warm enough, so perhaps that shouldn't have been such a total surprise.

It seems likely to me that detonation in firearms is likewise initiated by heat once some portion of the powder gets hot enough to fuse but is delayed in receiving an ignition spark. Filling the case better helps gets it to light up before fusing. It also increases the capacity of the charge to absorb heat by phase change as fusing tries to start (enthalpy of fusion). You see that in squibbed out loads, where what looks like sintered chunks of powder of powder have formed, but are still porous as fusion is incomplete. More complete fusion would provide the contiguous medium a detonating compression wave likes to travel through, and may be what can happen to smaller charges.

I'll add a warning that, despite conventional wisdom to the contrary, SEE's can happen with fast powder, too. The Finnish Gunwriter's site describes a .308 disassembled by a load of 3 grains of N320 (pretty fast pistol powder; about 10% case fill in that example). Fast powder detonations are not as commonly reported as for small loads of slow powder, but when all the stars line up wrong. . . KB. Since reading about that particular event, I've made it a personal rule of thumb that even my loads of fast powders have to fill a case at least 20%.

I don't actually know if the reason for fast powder SEE's being less common is actually because of the powder properties or if it's just a self-selecting observation. The latter could happen due to these events requiring a lot of empty space in the case that you only get in rifle cartridge cases and because slow powders are more likely to be what a rifle handloader has on hand when temptation leads him to run experiments with greatly reduced loads.

Chamber ringing happens. It's was described one time in, I think, The Fouling Shot by a fellow who didn't believe in it until he got a bore scope. It can happen very slowly, and I think his was in a chamber with 10,000 rounds of filled case ammo through it that he found the forming. He just hadn't been aware it was there before looking through the scope. That said, I've never heard of the light plastic fillers causing a problem. Polyester (e.g., Dacron) pillow stuffing, for example, or the commercial Puff-lon filler. These seem to melt down to very little volume, so its as if, once ignition gets under way, they get out of the way.

Rapid chamber ringing seems to me more likely to happen to fellows who develop a load without filler, then add filler without reducing the charge weight, not realizing that robbing the chamber of empty air space will greatly increase his peak pressure for that same charge. The air-filled pillow stuffing doesn't take away nearly so much total air space, and that may be another reason you don't hear about ringing with it. It allows for larger charges.
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Last edited by Unclenick; November 20, 2012 at 10:17 AM.
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