25 or so yrs ago my Uncle short started his brand new rifle.....

[Ironoxide]

W

Would someone please explain the physics behind the theory that an air gap creates a “pipe bomb” To my knowledge pipe bombs are usually stuffed full (no aireap) and in the bp rifle the potassium nitrate is producing all the oxygen the sulphur and charcoal need to burn. So pleas explain how a little extra air causes such a violent rise in pressure so as to damage a barrel. As far as I’m concerned the bore obstruction theory is pure BS unless there is another projectile loaded behind it. No anecdotes here please, just pure physics.

A pressure rise does happen with an air gap. I mentioned the reason in my other post, but I'll try to do it more clearly.

The reason is inertia of the stationary bore obstruction, the kinetic energy of a powder column rapidly moving up the bore, and possibly a bit of potential energy of the yet unburned powder.

First a disclaimer. I'm not a structural engineer and it's been over 20 years since I had physics at school, but let's do few "back of an envelope" kind of calculations I marked the tedious bit as quote so you can skip it easily for your convenience.

Few assumptions, 70 gr of powder, 50 cal lead ball. 1471 fps normal muzzle velocity(851ft/lbs muzzle energy) . Ball located 3 inches before muzzle.

We assume the kinetic energy that usually is contained in the projectile is now contained in a powder column rapidly moving up the bore to meet a stationary projectile. Let's calculate the speed of powder column before it hits.
70 grains of powder is 0.00454kg in SI units. Sorry I learned physics in SI units. 851ft/lbs is 1153 Joules.

Let's assume that when it slams into that ball it will transfer 80% of its energy to the projectile. The other 20% will get lost as heat, residual pressure discharge etc.

So the ball will end up with a muzzle energy of 922.4J and velocity of 400m/s (50 cal ball, 178 grains or 0.0115kg mass). The ball has to accelerate to that speed in 3 in or 0.076m so if it starts at 0 speed ends at 400m/s after 0.076m it's average speed is half the maximum therefore 200m/s. We need this to calculate time for the ball to exit the muzzle from the moment of impact, it is t=S/V so 0.00038s. For the ball to reach that speed in that time it has to experience acceleration of a=v/t = 1052631 m/s2. From this we can calculate the force acting on the ball a=f/m or f=am 12105N.

Now the final part, to experience this force a 50 cal projectile with a cross section area of 0.000126m.sq. has to have pressure acting on it to be P=F/area P~96Mpa or almost 14000 psi.

So our no doubt flawed back of the envelope calculation results is 14k psi near the muzzle at point of impact where barrels are usually weaker. Mind that we also assumed constant acceleration at impact over whole 3 inch. Almost certainly it is not constant but significantly higher at start and lower towards the end so I wouldn't be surprised if the real pressure was at least few times what was calculated (50~60k psi) . Normal pressure at the breech for this load given in Lyman's black powder handbook it 5400psi. So we're talking 10 times normal pressure. Definitely et least bulge-worthy.

Also I as well as others have experienced bulges by short starting.

Good steel will just deform. Bad steel with flaws (for example original wrought iron) might crack.

My questioning is not that it couldn't happen, but that a good Italian barrel shouldn't experience such a huge failure with bp pressures. However, crappy products slip through best quality check systems from time to time, corrosion happens etc so anything can happen. All I'm saying is that the extent of that damage is a freak accident rather than norm.

Op, I'm sorry if you felt my questioning was accusatory in tone. That wasn't my intention.

 

[Springerpanhead]

Not a scientist or engineer so I really can't do that . Pipe bomb was a slight exaggeration on my part , but maybe not by much . Black powder needs to be compressed to be a propellant so that it burns progressively going down the barrel . Left loose in the barrel due to a ball not being seated it detonates rather than burning . The ball will come out of the barrel ahead of the gas but the huge pressure spike created by the gap Can burst or bulge the barrel . Not guaranteed to do either but it can and does do it . My point is , don't take the chance and end up being the one that it does burst on .

So why doesn’t it explode when a pile is loosely set on a rock and ignited. Having done that quite a few times it seems to burn more slowly when NOT compressed.

 

[Ironoxide]

So why doesn’t it explode when a pile is loosely set on a rock and ignited. Having done that quite a few times it seems to burn more slowly when NOT compressed.

Detailed explanation one post above yours. It is not the air gap itself that matters. Its the extra space that allows the powder column to become a projectile on its own within the barrel. Then once it is going really fast it slams into the short started bullet.

 

[Flintlock Whiskey]

A pressure rise does happen with an air gap. I mentioned the reason in my other post, but I'll try to do it more clearly.

The reason is inertia of the stationary bore obstruction, the kinetic energy of a powder column rapidly moving up the bore, and possibly a bit of potential energy of the yet unburned powder.

First a disclaimer. I'm not a structural engineer and it's been over 20 years since I had physics at school, but let's do few "back of an envelope" kind of calculations I marked the tedious bit as quote so you can skip it easily for your convenience.



So our no doubt flawed back of the envelope calculation results is 14k psi near the muzzle at point of impact where barrels are usually weaker. Mind that we also assumed constant acceleration at impact over whole 3 inch. Almost certainly it is not constant but significantly higher at start and lower towards the end so I wouldn't be surprised if the real pressure was at least few times what was calculated (50~60k psi) . Normal pressure at the breech for this load given in Lyman's black powder handbook it 5400psi. So we're talking 10 times normal pressure. Definitely et least bulge-worthy.

Also I as well as others have experienced bulges by short starting.

Good steel will just deform. Bad steel with flaws (for example original wrought iron) might crack.

My questioning is not that it couldn't happen, but that a good Italian barrel shouldn't experience such a huge failure with bp pressures. However, crappy products slip through best quality check systems from time to time, corrosion happens etc so anything can happen. All I'm saying is that the extent of that damage is a freak accident rather than norm.

Op, I'm sorry if you felt my questioning was accusatory in tone. That wasn't my intention.

If you like I can post a close up picture of the breaks. One thing that is noticeable is the crystalline structure of the metal. It very well may not have been tempered correctly.

 

[Eddie Southgate]

So why doesn’t it explode when a pile is loosely set on a rock and ignited. Having done that quite a few times it seems to burn more slowly when NOT compressed.

It does explode. Goes up in a puff , even makes a little noise and you can feel the pressure coming off of it as it goes. Ignited on a flat rock in an open pile it is not contained so does not build near as much pressure and builds what pressure it does spread out over a much larger area.

 

[Ironoxide]

If you like I can post a close up picture of the breaks. One thing that is noticeable is the crystalline structure of the metal. It very well may not have been tempered correctly.

Yes, please if you can take a closeup that would be great. Perhaps using a flash will show it better. It is very interesting to see.

 

[rhett45acp]

Detailed explanation one post above yours. It is not the air gap itself that matters. Its the extra space that allows the powder column to become a projectile on its own within the barrel. Then once it is going really fast it slams into the short started bullet.

Its is very interesting the photos look very close to a "squib" event. Which, doing what I do, have seen a few.

Your explanation of the column of air is something to ponder. Acting like the 1st projectile slamming into a 2nd projectile (squib).

I am one that my eyebrow raises when I hear about light loads blowing up guns or short started balls. Its that I do not believe, but just the curiosity and the science behind the events. So far this thread is a good read. FYI I have seen the OPs post on the sister site. I do believe what he witnessed that day from his perspective.

I wish I had the $$ to do some actual test on issues like the above. Its fascinating.

 

[Okie Hog]

Explained here:

Short-Started Ball (celnav.de)

 

[Britsmoothy]

Explained here:

Short-Started Ball (celnav.de)

I have also pondered this many times and Fadala makes valid theoretical possibilities. However, shotguns have been known to burst via a blockage impeding a shot and wad columns journey thus no spread out powder!

One thing underestimated is the compression of air alone, or in this case gasses, rapidly expanding gasses. Airguns only compress a small volume of air but give a good return from a rapid moving piston. In crease the speed of that piston on the same volume and a greater return is yielded!
Time also is a huge factor. The obstruction although not totally stuck does not have enough reaction time to respond and may as well be a welded obstruction! In fact, it is possibly the differing amounts of sticktion between cases that determine zero damage, a bulge or total failure! All because of the more the resistance, the longer the time the pressure can rise before the obstruction moves.

In the past I have distinctly noticed how much hotter a barrel feels shooting blank! Leading me to believe a greater release of energy over a shorter time period.

My advice, chuck the short starter and develope a thumb started load

 

[Okie Hog]

Years ago, something similar sometimes happened with the US Army 155mm howitzer when firing reduced charges. The propellant was detonating, destroying the barrel.

The old iron muzzleloader barrels were less likely to rupture. i've seen iron barrels with huge bulges that never burst.