Posted on 02/11/2011 11:31:02 PM PST by Slings and Arrows
Too little powder in a case causes aberrant ignition based on what I would call “flash-over-top,” (for lack of a better word).
Imagine a good, 90 to 100% load density powder stack in a case. The primer ignites, sending forward a flash into the stack. The stack burns in a rear-to-front manner, we presume, because the flash can’t project from the rear to the front with all the powder in the way. With me so far?
OK, now take the load density down to 40% (eg). And when we put the round into the chamber, let’s agitate e the cartridge longitudinally a bit to slosh the power from bunch near the rear with a sloping face at the front of the powder column (if we had positioned the case vertically, then brought the weapon down to horizontal to shoot), to a less-than-half cylinder laying across the bottom length of the case. You’d expect the long, half-cylinder configuration of the powder under most normal circumstances, esp. in the situation where a semi-auto is self-loading.
Now light off the primer. The brissiance can likely ignite powder across this entire top horizontal surface of the powder, meaning that the powder is burning across a MUCH larger surface area downward (rather than in a small circular area back-to-front), resulting in a significantly faster burn rate, yes?
In nitrocellulose powders, faster burns means higher pressures, and higher pressures means faster burns. Wheee.... away we go, up to the moon.
I think the problem is less evidenced in handgun rounds than in big rifle bottles. Think about the relative difference in the burning surface area between lighting off a case diameter in pistol rounds vs. the length of the case, and do the same exercise in rifle cases, and you can see a huge disparity there. In rifle rounds, it has been NB’d that the problem is the worst in magnum (ie, big honkin’ bottle) cases and in those older long cases like .45-90 or .50-110 when guys are shooting smokeless in such a huge case relative to the powder charge.
I think this is what causes the too-little powder configuration to develop excess pressures. This is only my theory, I have no data to back this up that I can cite, because no one has yet done a study (that I know of) on this with a pressure measurement and a way to reliably replicate the positioning of the 40% load across the bottom of the case.
BTW - I didn’t say 40k CUP, I said 40K PSI. Big difference. Small nit, but I think an important one.
FWIW, I don’t like using CUP when I’m talking about chamber pressures and will convert them to PSI for readers. Lots of us gunnies who are well read know what a “CUP” is, and have a notion of what “40k CUP” is in pressure terms (viz your “Yeegads!”), so if we gunnies want to use CUP to talk about pressures, I’m absolutely cool with that, but people who don’t know what “CUP” is should NEVER, EVER assume a 1:1 correspondence to PSI. For people who don’t know what CUP is, please, ask how to convert it to PSI to get a “gut check” on what “40K CUP” means.
I used PSI (and will convert CUP to PSI when talking with anyone who isn’t an experienced reloader/gunnie) because it is a unit of pressure that a lot more people understand intuitively in the US, with many people having a good idea of a tire blowout at 80PSI, or an air compressor letting go at 150PSI, etc. Most people would be just as lost if I went all engineer-like on them and started using Bar, or Pascal, etc.
Calling out the numbers as “56K PSI” when they’ve seen a tire blowout at 80PSI gives them religion faster than a discussion with a unit of pressure that they have no basis in their experience for comparison... you see what I’m getting at, right?
Not voodoo if it serves to keep the powder charge in a stack forced rearward in the case against the primer, IMO. But that’s worth only what you paid for it. I’ve used the technique too, but more often I will search for a powder that gives me a 80%+ load density first.
I’ve also read of guys using Cream of Wheat on top of powder in these situations. I’ve never done that myself.
CUP and PSI have a very strong correlation, but the copper crush method measures maximum peak pressure.
Durn... I’ll pass it around. We don’t have any, but it will be good to make sure everybody knows. I wonder if the stuff was hot or cold?
Yes, 1911’s can be blown up and have been. It takes a major issue or someone has to try really hard, but they do blow on occasion.
The first shot of a 20 round box of 7mm Remington Mag. seemed normal until I tried to open the bolt. It was locked tight. I had to take a mallet and tap the handle to open it.
The case showed obvious signs of high pressure such as the primer falling out of the pocket.
I phoned Remington. (this was back in 1979 when they were still part of Dupont) The person I talked to was politely bored and didn’t ask for any information such as lot number etc. so I just hung up and never used any more from that box.
The gun, a 700BDL was undamaged.
Around the same time, I had a cast bullet stay in the barrel of a mint Colt Series 70 M1911. I still am not sure what happened. I was shooting rapidly and noticed a squib load but had fired again before stopping. The second shot kicked about like a .44 mag.
I took the gun home and checked it out. The barrel had a bulge about midway. Besides that, no damage. I kept using it for a while until I got a new barrel.
The thing I could never understand is the squib was so weak as to leave the bullet in the barrel yet had enough power to cycle the slide.
Never seen it happen.
Thoughts on kabooms?
Interesting. All I know is that when I place a 308 to my face I want things right
I have never seen nuclear fusion, but I believe those who have.
That's a very good policy to have.
If you are going to blow a gun up do it with factory fresh ammo so you don't look too stupid.
fair enough, I just ended a legal battle
I didn’t know. Concerning what exactly?
Durus: struwwelpeter nailed the answer to your question above, in spades. Even in states with an amended Castle Doctrine -- so called "stand your ground" or "shoot first" (as the antis love to call them) states, a zealous prosecutor could turn your justifiable shooting into something going before a Grand Jury based on specific intent to kill rather than provide for lawful self defense!
BTW: For serious SHTF I prefer a revolver. 30 years of range time on active duty and reserves with 1911's and M9's leaves me a bit wary of automatics.
Ever experienced an extruded primer that hangs up on the revolver backplate? I have. A gunsmith had to detail strip it to clear the hung round. Had that happened in a confrontation my revolver would have become a paperweight. I've only experienced that particular issue once in my life -- that's about 40 years of shooting experience. That's once too many for me. I use revolvers for teaching only. I've been a certified instructor for 22 + years. Any weapon I choose to carry for the purposes of self defense has to be 100% reliable in it's essence. That of course doesn't include a faulty magazine or even operator error. But in any case, I have to be able to clear a jam in an emergency and put the piece back into service to defend my life or the lives of others. I can always do that with a semi auto. I have not yet run into a jam that I couldn't rapidly clear, again in that 40 year span, if I do my part.
Ummm, I have seen what happens when folks who roll their own or try to shoot a Glock on the cheap use exposed lead loads instead of FMJ ammo. The octagonal barrels tend to build lead residue extremely fast and that will raise chamber pressures at an almost geometric progression. I have seen such a buildup in a Glock lead to a catastrophic KA-BOOM resulting the trigger group blowing straight down and nearly removing the trigger finger of the shooter. I still enjoy shooting the Glock (but wouldn't own one again for other reasons), I just understand they have to be properly fed to offer consistent and safe results for the operator.
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