Swampmouse,
You’re right, I think someone has gotten their terms mixed up in an attempt to answer. A "flash-over" as the poster was trying use it is actually a sympathetic detonation which has nothing to do with a problem in a single chamber. It is basically a secondary explosion of a separate explosive charge caused by debris, shock wave concussion or heat from a primary explosion. In most cases it is a concussive "flash-over." A lot of modern explosives have been developed specifically to have resistance to sympathetic detonations.
Detonation as it pertains to small arms ammunition was used back in the '60s and '70s to describe a phenomenon with some fast burning smokeless powders that had very low volume ratios to their energy densities. People were getting unexplained ruptured cylinders while firing very light target loads. The primary offending powder was one that originated in 1897 and is still in use today. It has gone through two major evolutions and remains one of the primary pistol target powders today. The other was a "Winchester" powder which has since been reformulated (and renamed) into a powder with more bulk to help prevent the problem. Some writers erroneously reported it was to help with double charging which doesn't make sense because the volume is still very small and not enough to prevent double charging.
The problem existed with relatively long cartridges like the .38 spl shooting light PPC and other very light target loads. There was so little volume with the powder it would lay on the bottom of the case as the revolver was aimed and the primer "flash" (brisance) would literally shoot over the top of the charge and ignite it from the front. A “flame front” creating a pressure wave would actually travel to the rear and if it met a wave coming from the rear there was a huge spike in pressure because the normal path to relieving the building pressure would be to the front as the bullet was pushed forward. This usually resulted in a ruptured cylinder. Some have argued that even the wave moving to the rear and reflecting off of the base wall of the case was enough to create the pressure spike. I am in the camp for opposing shock waves.
It has been reported that in some cases the bullet would move forward slightly and the barrel would rupture at the forcing cone area and threaded extension in the frame, which is usually the thinnest and weakest portion of a barrel. This was reported to cause a failure cascade and the frame would break at this point as well. I will add that most failures of this type are caused by a first bullet (usually from a squib) already being in the bore. I have never read of a controlled test where the frame failed in this manner, but they were definitely able to reproduce the "detonation" and split cylinders apart.
The layman's term ended up being “detonation” because they attributed it to simultaneous ignition of all or most of the propellant, it should more properly be called "simultaneous contra, or opposed ignition". I guess you can see why it has taken on the name “detonation.” This also sometimes happens in solid fuel rocket motors which often have long cores through the axial length of the propellant to give a greater burning surface. The motors normally burn from the inside out (down the entire length), or from the inside of each core in the case of the multiple core bodies away from the burning surface. The problem arises when an area towards the rear burns faster and usually burns through to one of the other cores (this is also caused by cracks in the propellant) this may "block" the pressure from the areas burning at the front of the motor. You once again have opposing flame fronts. Motor bodies are lightweight and fragile compared to a firearm and the increased pressure will often rupture the body.
I doubt a low charge in a cylinder would cause a crack in a barrel, if anything it would cause a cylinder rupture long before a barrel was affected. Based on the research of w44wcf and others concerning BP cartridges being less than full in some 19th century reduced power loads I'm starting to even doubt some of the commonly held beliefs on air gaps with BP. But I'll reserve my comments until I have more data. If you put so little powder in a '58 chamber that you would get a cap's brisance going over the main charge and igniting it from the front then there would be so little powder it would be an anemic squib load, not something dangerous. BP is already very low pressure.
You are right about the term flash-over as it applies to cap and ball revolvers. Chain fires are from flash-overs where burning gasses from one chambers firing will “flash-over” to another chamber and set it off as well. “Flash-overs” on cap guns don’t cause ruptured cylinders they just send multiple projectiles out of multiple chambers upon one pull of the trigger.
I also wholeheartedly agree with you voids in the bullet will not be a problem. Bullets are almost always deformed in some fashion and any void to the outside or even an inclusion would be so insignificant that it pales in comparison to the pressures of the burning gases or pressures that would be created in a bore as in the case of a “pneumatic ram.” Even with an entirely hollow ball the volume is very small.
There are two failure mechanisms from blockage in a barrel:
A ball, bullet or even an obstruction like water, ice or mud at the muzzle end of the bore creates blockage at one end of what becomes a pneumatic cylinder. The oncoming ball acts as a piston compressing the gas (some escapes around the sides of a bullet and precedes it) and air building pressure. The pressure created by a bullet acting as a piston accelerating through the bore is tremendous. This phenomenon was well understood even in the 19th century.
The second mechanism pertains to barrels that are sufficiently thick to withstand the initial pneumatic pressure from an obstruction. In this case the “stuck” bullet in the bore has enough inertia not to move forward or only begins to move before the second bullet strikes it. The second bullet is decelerated deforming both bullets. This causes a spike in the pressure behind the fired bullet and is usually enough to add to the strain the barrel has already been exposed to from the previous pneumatic pressure to cause a rupture.
The crack may not be from a defect, in addition, lengthwise fracture is actually very common. The rifling creates stress raisers. I wish the pictures were in better focus.
Tom, can you give us one that is sharply focused even if it isn’t as close up as the ones you have provided? Just move back a little (keep it as large as possible) until you get one I can blow up on my screen. I really appreciate the time you have taken to give is these photos already. Please do one more thing for us, when you look in the bore, how does the crack correspond to the rifling?
If the barrel had been subjected to an earlier over pressure situation the barrel may not have failed catastrophically that time, but a crack(s) could have been initiated. Continued firing continues to stress the barrel until the cracks finally propagate through the barrel thickness. Steel is a wonderful material, even when damaged it often slowly fails unless the ultimate tensile strength has been severely exceeded. When the tensile strength has been exceeded it just means the material begins to neck or plastically deform, not rupture. It won’t spring back, the elastic limits have been exceeded.
If I had to venture a guess I would say either an inclusion in the steel or a previous overpressure. I tend to lean towards a previous overpressure because a fracture, inclusion or something else would have to go almost entirely through a piece of steel to be a problem with the low pressures we normally get with a 30 grain load of BP and a .45 (.44) caliber bullet or ball. The inclusion or manufacturing defect differs from a crack created by an overpressure situation because the surrounding material has not been stressed from a previous event experienced by the entire barrel. In short the surrounding unaffected steel would act as a reinforcement for the weaker section. We see this in composite materials every day.
Regards,
Mako
News & Blogs
Home
Posts
Recent Posts
Help
Business Directory
Calendar
Advertising Rates
Contact Us
Login
Register
Topic: 1858 remington barrel split (Read 21250 times)