Will Pressures Go Up, Down Or Stay The Same As Projectile Is Loaded Further Down The Barrel?

[arakboss]

I will be using my smokeless powder muzzleloading 50 Beowulf project as an example for this question. In that project, the bullet was loaded just in front of the case mouth by an 1/8" or so. I had no way of measuring pressure of that load but let's pretend that it was 25,000 PSI. If I had only pushed the bullet half way down the barrel so it was 8" from the case mouth would you expect pressure to go up, down or stay the same? What if I only pushed the bullet an inch or two down the muzzle?

Edit: Under the same conditions as above what would you expect velocities to do?

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[arakboss]

My money would bet pressures will go down the further the bullet is loaded towards the muzzle. Velocities would go down too. All that free space in the barrel between the powder charge and the bullet, would be comparable to having a really long case.

 

[DizzyJ]

My money would bet pressures will go down the further the bullet is loaded towards the muzzle. Velocities would go down too. All that free space in the barrel between the powder charge and the bullet, would be comparable to having a really long case.

This would be my guess as well.

 

[Deathgrip]

The powder produces a specific amount of gas so if it has to fill a larger volume the maximum pressure will decrease proportionately. The velocity would be much lower because not only would the pressure start lower but the bullet would have a 'shorter barrel' to travel before it was in free flight.

 

[arakboss]

The powder produces a specific amount of gas so if it has to fill a larger volume the maximum pressure will decrease proportionately. The velocity would be much lower because not only would the pressure start lower but the bullet would have a 'shorter barrel' to travel before it was in free flight.

That is what I was thinking but a lot of others here disagree and feel that something to due with pressure waves will cause catastrophe instead? Someday I may remotely fire some projectiles in this manner to see what the real world results are but it's down at the bottom of my list of activities to do.

Edit: I have heard of cases where ringing or splitting of barrels occurred because of bore obstructions, so there is something going on that I don't understand. The stories I have read about barrel ringing and barrels being split open have all involved somebody firing a projectile behind the obstruction. My question here and in the other thread about clearing a obstruction with a projectileless cartridge (essentially a blank) is different but may result in the same problems? I would think we would have seen a bunch of catastrophes surrounding the can cannons, golf ball launchers, etc if it was a common danger.

 

[dangerranger60]

I would expect that over all pressure will be about the same,BUT, It will also have a pressure spike at the obstruction.
In normal firing the pressure spike occurs near the chamber, firearms designers make the area thicker around the chamber to combat this. the typical barrel tapers to cut weight. This means the farther down barrel you go the thinner and weaker it gets.
in normal firing the pressure spikes as the bullet engages the rifling, and every inch down bore the bullet goes has the pressure diminishing.
This is also why longer barrels are quieter.
An obstruction at half way down barrel, means that the powder ignites and starts expanding, it hits a standing wall [ the base of the bullet] and at the same time the powder is still building pressure. Thats a pressure spike.
In normal firing, the bullet starts to accelerate as the powder starts its burn. but as pressure builds it is already in motion and creating an ever bigger cylinder to contain the pressure.
I have seen a few rifles blown up by having an obstruction, but many more shotgun barrels. [they are much thinner]
At least one rifle had a bore sighting devise left in the muzzle. It pealed back like a cartoon banana.
I would not mess around with an obstructed bore. DR

 

[arakboss]

I would expect that over all pressure will be about the same,BUT, It will also have a pressure spike at the obstruction.
In normal firing the pressure spike occurs near the chamber, firearms designers make the area thicker around the chamber to combat this. the typical barrel tapers to cut weight. This means the farther down barrel you go the thinner and weaker it gets.
in normal firing the pressure spikes as the bullet engages the rifling, and every inch down bore the bullet goes has the pressure diminishing.
This is also why longer barrels are quieter.
An obstruction at half way down barrel, means that the powder ignites and starts expanding, it hits a standing wall [ the base of the bullet] and at the same time the powder is still building pressure. Thats a pressure spike.
In normal firing, the bullet starts to accelerate as the powder starts its burn. but as pressure builds it is already in motion and creating an ever bigger cylinder to contain the pressure.
I have seen a few rifles blown up by having an obstruction, but many more shotgun barrels. [they are much thinner]
At least one rifle had a bore sighting devise left in the muzzle. It pealed back like a cartoon banana.
I would not mess around with an obstructed bore. DR

I would be curious as to what percentage of the barrel splitting, ringing, etc were caused by firing a cartridge with no projectile behind the bore obstruction? It seems to me that would not be a normal circumstance as happened in the case of Brandon Lee getting killed. I don't doubt that a pressure spike would still happen, I just wonder if barrel damage results would be different?

What if a really long case cartridge was created, let's say 12 inches in overall length. Would that create the same conditions as a bore obstruction or?

 

[arakboss]

This is my idea for safely testing the potential for barrel damage. I would perform this with a 223/5.56 barrel as that is what I have an abundance of. The rig would be set up for remote firing.

I would use a rod with drill stop installed to seat projectiles at the various depths starting an inch off the case mouth. Then two inches, three inches, etc. This would continue until barrel damage occurred or the bullet was seated near the muzzle.


Edit: The powder would be secured in the case with a thin piece of cereal box type cardboard. All loads would use the same charge weight. If I had a chronograph I would expect velocities to go down as the bullet was loaded further down the barrel. Given that I don't have a chronograph and trying to remotely fire through one, would be tricky, I will settle for barrel damage assessment only.

 

[dangerranger60]

I would be curious as to what percentage of the barrel splitting, ringing, etc were caused by firing a cartridge with no projectile behind the bore obstruction? It seems to me that would not be a normal circumstance as happened in the case of Brandon Lee getting killed. I don't doubt that a pressure spike would still happen, I just wonder if barrel damage results would be different?

What if a really long case cartridge was created, let's say 12 inches in overall length. Would that create the same conditions as a bore obstruction or?

I'm pretty sure Brandon Lee was shot with a revolver. The cyl gap would vent a lot of pressure. and they suspect there was only a partial blockage. still the piece of debris was enough to kill.
I doubt the case length would change much. its having a bullet already lodged in the rifling and enough space behind it to build pressure before the bullet is moving. DR

 

[dangerranger60]

I would think long and hard before experimenting with barrel blockages.
When pressure vessels burst it is always a spectacular explosion.
I doubt the walls of your shoot house are built to stop shrapnel or the back wall would stop the bolt and receiver as it exits the gun. And you are talking about making it progressively worse with each try until it bursts.
I like fun gun experiments but this is one I would run from. DR

 

[Xaevian]

You should read up on physics. All that empty space is not a vacuum. It is filled with air. Air is compressible and acts as a fluid. The compression is not instantaneous. It is a wave. Suddenly stop or alter the relatively predictable rate of compression, and bad things can happen.
Math > basing so much on Brandon Lee's demise.
But hey, who knows. I'm not a physicist or an engineer.

 

[Mark W.]

everything about this line of experimenting seams VERY dangerous and of no actual value. You are not going to improve on anything and could damage equipment and in the worst case your self or others. Granted this is just my opinion and you can do what you want but I sure won't be encouraging you.

 

[RX-79G]

Barrels burst because one projectile hits another one.

However, minimum cartridge pressures (start loads) are important because you can have a condition where the powder is insufficient to get the projectile moving in a timely manner, but it still prevents venting of chamber pressure and the pressure "slowly" goes above max.

I would doubt this would happen with the bullet lodged 8" from the chamber, since there is a lot of volume available. In that scenario I would likely use a very light load to get it out. If it was stuck in the throat, a full load.

 

[Yarome]

One projectile hitting a squib will certainly increase the likelihood of a barrel bulge caused by bullet expansion on impact, but as others have stated, the lack of a bullet in the case doesn't negate the possiblity of a catastrophic failure. The pressure spike is real and occurs at the static obstruction. Further up the barrel also adds more air volume compression.

I agree with others, also, that the pressure spike is designed to occur within the chamber... which is the stongest part of the barrel and designed specifically to tolerate those pressures. Your rifle barrel is not. It is designed only to contain a consistantly expanding wave in motion as it travels and exits the barrel with resistance within allowed tolerances (a bullet moving down the barrel). Also, strong enough to maintain integrity while under pressure and accounting for accumulated heat buildup and distribution within established perameters. It includes a safety margin.. but it is "finite" and degrades with usage.

That doesn't mean a barrel will experience a catastrophic failure the first time it's fired under full load with an obstruction, but the risk is there. It depends on the barrel but no matter how strong it might be... each subsequent occurance will stress the barrel and increase the risk exponentially.

Personally, I wouldn't, simply because I don't see any value, need or performance reason to do it... unless I just wanted to test how much abuse a specific barrel can take before it explodes on me.

YMMV

 

[red442joe]

Regarding the thread title; yes.
I imagine the plotted pressure curve would look different than a normal cartridge. Also, I highly doubt a pressure difference "at" the obstruction.
"Chamber" volume with a bullet down the bore will obviously be much greater than with a bullet at the case mouth; smokless burns differently under pressure, in fact needing that pressure to develop its designed burn characteristic; what is the pressure curve like for a case full with no bullet?

Joe

 

[arakboss]

Regarding the thread title; yes.
I imagine the plotted pressure curve would look different than a normal cartridge. Also, I highly doubt a pressure difference "at" the obstruction.
"Chamber" volume with a bullet down the bore will obviously be much greater than with a bullet at the case mouth; smokless burns differently under pressure, in fact needing that pressure to develop its designed burn characteristic; what is the pressure curve like for a case full with no bullet?

Joe

Good point, that powder may not fully ignite in a cartridge with no projectile. I can tell you from experience that having the bullet just outside of the case did not noticeably prevent the burning of Unique powder in my 50 Beowulf muzzleloading experiments. Maybe the Unique powder doesn't require the pressure other powders do to burn. Or maybe my bullet was loaded close enough to the case mouth that it was still able to achieve the necessary pressure. Either way the bullets left the barrel and no evident damage occurred to the barrel. It is a heavy profile barrel.

 

[Deathgrip]

OK, I see the reasoning behind the pressure wave or pulse issue. And I understand that the space between the cartridge head and the base of the projectile, wherever it is, isn't a vacuum. But here's why I have my doubts about the destructiveness of a projectile partway down a rifle barrel (and I would truly love to perform experiments under controlled conditions, and being proven wrong is a great learning process in itself - I guess -):
1. The primer pressurizes the cartridge to several thousand psi, but there isn't terribly much space to pressurize. The nearly instantaneous pressure wave from that would be greatly reduced;
2. I wouldn't be all that sure the powder would ignite normally - the primer blast might push a lot of it out into the space between the cartridge and the projectile, where it might not ignite at all.
3. Here's the interesting conjecture... Would the brass case expand enough to seal the gas that IS generated within the available space or would there be a (dangerous!) blowback?
The powder, being contained, may burn, but not like it would at 'normal' pressure...

So conjecture #2: perhaps there's a 'danger zone' of a few inches beyond the chamber where the powder ignites normally, the case seals the gases, but the slug remains static too long and the pressure rises too far and exceeds the strength of the thinner steel beyond the chamber. After all, jamming a bullet into the riflings by seating it out too far is a bad idea... but putting the slug quite a bit farther down the barrel might not be as dangerous due to 1 and 2 above?

Sorry for verbosity... things like this get me wound up.

Deathgrip

 

[Yarome]

3. Here's the interesting conjecture... Would the brass case expand enough to seal the gas that IS generated within the available space or would there be a (dangerous!) blowback?

Obviously it would depend on the wall thickness of the brass, but standard casings of standard composition may expand with as little as 1050psi. That's kind of moot though when talking "excessive blowback" since cartridge to chamber tolerances as tight enough not to allow dangerous blowback even without expansion. Much like shooting sub sonic steel.

 

[RX-79G]

3. Here's the interesting conjecture... Would the brass case expand enough to seal the gas that IS generated within the available space or would there be a (dangerous!) blowback?

Unless it is a blowback operated firearm, there should be no "dangerous blowback" forces. Guns (generally) do not function by using the drag of the expanded case to resist primary extraction. If an action opens under too much pressure the brass case extracts until a weak section is exposed and a blowout happens. If the case doesn't seal well (steel case ammo?) there might be some gas blow-by, but that doesn't cause the action to open. So I would say that all normal cartridges "blow back" when fired. But with a locked breech there is nowhere to blow back to. Less pressure just decreases that force.

 

[arakboss]

Unless it is a blowback operated firearm, there should be no "dangerous blowback" forces. Guns do not function by using the drag of the expanded case to resist primary extraction. If an action opens under too much pressure the brass case extracts until a weak section is exposed and a blowout happens. If the case doesn't seal well (steel case ammo?) there might be some gas blow-by, but that doesn't cause the action to open. So I would say that all normal cartridges "blow back" when fired. But with a locked breech there is nowhere to blow back to. Less pressure just decreases that force.

Edited to clarify my lack of blow back concerns only applies to locked breech firearms:

In a locked breech firearm, I would not expect blowback of gases to ever be very dangerous when it comes to ill fitting cases or cases that don't expand properly. Eye protection should be sufficient to protect against most blowback issues.