Finding peak pressure points

[CANNONMAN]

I used to work for a guy who would determine where peak pressure would be in a rifle. He would then terminate the barrel and replace it with smooth bore. The thought being that the bullet would not be retarded post peak pressure and would actually accelerate past this point. Accuracy should also increase. Now for BP. I understand how to calculate peak pressure with nitro cellulose and the rest of the knowns. However, BP differs considerably. Darn stuffs basically explodes. I want to build my next cannon with consideration of where , if it exists, the peak pressure point will be. Any help or ideas, as always, would be great.

 

[BCRider]

When correctly packed it doesn't just about explode. Look for a pressure curve for black powder vs smokeless and you'll find that BP has a rather long build and decay compared to smokeless when used in firearms.

I'm no internal ballistics engineer but I'm guessing that when we pack the stuff like we do that it burns from the point of ignition forward into the pack of powder for a controlled burn. Sort of like a solid fuel rocket motor. Which is also why we don't want to see much or any air gap. With a significant air gap the flame front can travel easily through the air gap and the whole upper face of the powder laying in the casing or chamber can light off at the same time, or near enough. And THAT is going to give an explosion, burst barrels and generally produce pipe bomb like results.

A controlled burn from end to end in a correctly packed charge might just be why so many burning particles are ejected at a pretty high velocity along with the ball or bullet. The last portion that is pushed up to the base of the projectile is moving as fast as the projectile as it is ignited near the end of the travel so these burning particles get ejected at a good rate of speed.

When I look around at all the options for bench rest rifles and other high accuracy firearms such I don't see a lot of rifled to smooth bore transition solutions. So I'm not sure if this theory of his holds much water.

Besides, to avoid the propelling gases blowing past the projectile and causing it to tumble in the barrel the fit would need to be snug. And a snug smoothbore portion is going to drag down the spin rate so the ball or bullet won't have as much spin for stabilizing as it leaves the crown. So there's another reason I don't think that this is a viable way of doing much of anything for good shooting.

All in all it sounds like an idea worth of Rube Goldberg. I'll be interested in seeing if anyone knows of this idea and if there's any existing present or historical examples.

 

[Jim Watson]

The peak pressure occurs very early in any firearm, black or nitro powder.
That doesn't mean there is no longer enough pressure to keep accelerating the projectile, there is plenty.

 

[J-Bar]

I would like to see an objective experiment that tests the hypothesis that a section of smoothbore barrel following a rifled barrel section improves accuracy and increases velocity.

Until this hypothesis is proven experimentally (and I doubt that it can be) any discussion is purely speculation.

 

[ofitg]

You can make measurements, but it'll set you back a few hundred dollars -

http://www.shootingsoftware.com/pressure.htm

 

[CANNONMAN]

Thanks guys! One of my interest in finding/guessing at a cannons peak pressure point is trunnion placement and the overall balance of the cannon. Or, I can balance the cannon and add the trunnions where I want. This , depending on design, may add additional structural strength where I want. The smooth bore conversion was not BP. It did work... until the "other" variables showed up. Most notably, after the barrel heated up, the diameter of the bore increased and accuracy suffered. Fist five shots, pretty impressive. The military turned down an offer to buy.

 

[Jim Watson]

You could rifle the other end, like a Paradox.

 

[Slamfire]

I would like to see an objective experiment that tests the hypothesis that a section of smoothbore barrel following a rifled barrel section improves accuracy and increases velocity.

Until this hypothesis is proven experimentally (and I doubt that it can be) any discussion is purely speculation.

I agree.

 

[CANNONMAN]

Sorry guys but it has been done. The problem here is that it was not done within a BP format. If the variables remain static, the improvement is impressive. They don't. Hence the improvements quickly degrade. This was some 30 years ago. I was at a 1,000m NRA shoot hired as a tech photographer. A modified rifle was entered and the shooter, an Air Force Academy marksman from Colorado Springs was called off. I was asked if I would like to shoot in the competition. Never shot anything but a .22 until then. I took second. When my turn came up to work the pits, the gun was shot by another guy. The pit crew all noted how you could tell which gun it was by the delay between the bullet and the bang. The other guy took first. There are a whole bunch of data and numerous patents on this concept. It is not BP and this no longer belongs here.

 

[J-Bar]

Sorry guys but it has been done. The problem here is that it was not done within a BP format. If the variables remain static, the improvement is impressive. They don't. Hence the improvements quickly degrade. This was some 30 years ago. I was at a 1,000m NRA shoot hired as a tech photographer. A modified rifle was entered and the shooter, an Air Force Academy marksman from Colorado Springs was called off. I was asked if I would like to shoot in the competition. Never shot anything but a .22 until then. I took second. When my turn came up to work the pits, the gun was shot by another guy. The pit crew all noted how you could tell which gun it was by the delay between the bullet and the bang. The other guy took first. There are a whole bunch of data and numerous patents on this concept. It is not BP and this no longer belongs here.

I wonder why rifle manufacturers, in their never ending quest for more velocity and accuracy, aren't taking advantage of the phenomenon?

Honest, not trying to be snotty, just curious. Seems like everyone would be doing it if it leads to superior performance.

 

[CANNONMAN]

Last time:1. When the - numerous - variables change, it does not improve anything. 2. Numerous patents govern the concept. 3. When conditions are nominal, lots of awesome things happen. Not mentioned, Post peak pressure the bullet accelerates, no rifled barrel to "kick" bullet, bullet actually begins to pull, reducing recoil. I ask the moderator to move this to an appropriate site. Perhaps there you might find an appropriate discussion. I was a photographer on a government test base. I believe no expense was spared and even less was shared. Thanks all for taking an interest.

 

[4v50 Gary]

Wasn't the Rodman Gun or Dahlgren gun designed around the pressure peaks?

 

[Slamfire]

Wasn't the Rodman Gun or Dahlgren gun designed around the pressure peaks?

Probably around the pressure curve. This is a 7.62 pressure curve and you can understand how the pressure relates to distance down the barrel.



Where pressure is highest, at the breech during the ignition, that is where you want the most steel, so if you notice, your chamber is the thickest part of the barrel. As the projectile goes down the barrel, pressure drops. You only need the steel thickness necessary to keep the barrel from rupturing for safety considerations, so barrels taper towards the muzzle. I suspect rifle barrels are actually a bit thicker than needed for weight (rimfires particularly) and to keep the things from getting bent, and so would have been those cannons you mentioned. The Dahlgren cannon has a most pleasing contour with a bit of muzzle flair, probably so a hoisting rope would not come loose.

 

[4v50 Gary]

Thanks Slamfire!

 

[CANNONMAN]

Ditto on the appreciation. I could not have asked for better!

 

[Jim Watson]

There are a whole bunch of data and numerous patents on this concept.

I would sure like to see a cite for the data. It is not described clearly enough here to search for. Patents are not a whole lot of use because they usually claim the whole world.

 

[Officers'Wife]

I used to work for a guy who would determine where peak pressure would be in a rifle. He would then terminate the barrel and replace it with smooth bore. The thought being that the bullet would not be retarded post peak pressure and would actually accelerate past this point. Accuracy should also increase. Now for BP. I understand how to calculate peak pressure with nitro cellulose and the rest of the knowns. However, BP differs considerably. Darn stuffs basically explodes. I want to build my next cannon with consideration of where , if it exists, the peak pressure point will be. Any help or ideas, as always, would be great.

My uncle was working of a theory of pierced BP propellents the year before he died. Using a bore sized "puck" pierced in the center his thinking was as the propellent burned in the center the hole size increased giving more surface area burning. He believed that would produce a steadily increasing pressure curve through the length of the barrel rather than a peak. IIRC he got the idea from WWI era "rosette" cordite powders used in naval vessels. Research on those powders might give you a start on calc of the pressure points since they where designed to eliminate them.

 

[Bezoar]

its not easy to determine. The whole barrel is considered the chamber. and the chamber doesnt end until the muzzle crown.

A gentleman named Dreyse solved this problem out. He discovered that when the bulk of the powder charge was kept at the back of the barrel, the pressure created was far more consistent and higher then what was currently done by standard muzzleloading guns using black powder. Its why his rifle outshot everybody until metal cartridges were introduced.

I believe the muzzleloadingforum website determined, volume with steady pressure beats out high pressure with low volume.