Alright, Ladies & Gents...grab a cup of you favorite beverage, be seated and get comfortable. (There may be a quiz at the end of the lecture period).
First, I will agree that without seeing the rifle in question, analysis of the problem is difficult. An OOB firing sound possible if not probable. The original Winchester '73's design was far superior to the current batch of reproductions. Slam fire due to inadvertent release of the follower on M1860-design Henry Repeating Rifles
has definitely happened! My solution is to load with the rifle almost horizontal, and to firmly wrap the fingers of the other hand around the barrel/magazine prior to releasing the follower.
Now, as to "detonations": By ordnance definitions, a "detonation" is an explosion that produces a shock front in excess of 25,000 ft/sec. There is
insufficient chemical energy in small arms smokeless powder to cause a "detonation"! HOWEVER...in smokeless powder bottleneck cartridges, especially ones that have sharp shoulders and are overbore capacity, reduced loads of slowburning smokeless powder
can cause overpressure conditions strong enough to exceed the ultimate tensile strength of the brass cartridge, thus releasing high pressure gas into the action, with bad results! What causes the high pressure with reduced loads of slowburning powder is creation of a shock wave that, if the powder is in a certain configuration in the case, can reflect off the front of the shoulder in the case. When such a shock wave reaches the smoldering or partially burned powder, the pressure increases, which, in turn, increases the burning rate of the power. This increases the pressure, which increases the burning rate exponentially until the cartridge ruptures. This phenomenon has been known by Naval Ordnance engineers since at least the 1960's and probably before.
Moral: DO NOT ATTEMPT TO REDUCE LOADS OF SLOW-BURNING POWDERS BELOW THE RECOMMENDED CHARGES! If you want reduced loads, go to a faster-burning powder!
Revolver blowups: There are three potentials for overpressures in revolvers: 1) Double-bulleting due to failure to notice what's happening either by the handloader not paying attention to what he/she is doing; or malfunctioning of a progressive loader. (I have never had the latter happen...because I don't use a progressive press!)
2) Double-charging of a smokeless powder, either through carelessness or malfunctions. However, experiments have shown that some double charges may not result in destruction of the gun! Just be careful!
3) A phenomenon of premature shotstart in revolvers, where the bullet moves out of the case, and into the forcing cone where the bullet stops, allowing the pressure to build up until the case ruptures, allowing high pressure gas to do a number on the gun!
Now smokeless powder requires between 5,000-7,000 psi (NOT CUP!) to commence stable burning. If the bullet starts to move before this pressure level has been reached, the pressure will drop until the remaining powder is smoldering, but no extinguished. If the bullet lodges in the forcing cone and stops, so that the barrel-cylinder gap is blocked, the pressures will rise. This will cause the burning rate of the powder to increase, increasing the pressure, etc., exponentially. Depending on how sloppy the chamber is compared to the cartridge case prior to firing, the increasing pressure will expand the case until it ruptures, releasing hot gases that act like a cutting torch on the steel of the chamber wall. With many .45 LC guns, the oversized chambers allow the cartridge to initially lie against the bottom of the chamber. If the bullet stops in the forcing cone, as outlined above, the case will usually stretch upward until it ruptures at the 3 o'clock and 9 o'clock positions, releasing the flame and cutting the chamber walls. The pressure also causes the top of the cylinder to come off, usually taking the topstrap with it. Experiments I ran some years ago, measuring pressure-time curves in .45 LC ammo, purposely loaded to allow premature shotstart, without causing destruction, as well as experiments by others, confirm the presents of the pressure spikes and drop off until the bullet hits the forcing cone.
There is a simple way to prevent this phenomenon: Insure that "bullet pull", the force needed to move the bullet out of the case is high enough so the powder gets burning fully enough to move the bullet: Reduce the diameter of the expander plug sufficiently so there is good friction between the case wall and the bullet; AND CRIMP, CRIMP, CRIMP! Roll crimp the case mouth into a crimp groove enough so you can see and feel it, but not so much as to collapse the case or bulge the case wall away from the side of the bullet!
Now, I will entertain questions until the end of the class period. Go ahead! Flame away!