Revolver pressure question

[tundrawolf]

Hi,


So I was thinking about the design of the revolver. Of course, there is a gap between the cylinder and the breech end of the barrel. This is with every revolver.

So, when a round is fired, as soon as the bullet leaves the case, there is a place for the gasses to go. From between the cylinder and the barrel.

Now, with other firearms, the cartridge is inside of the barrel when it is fired, there is nowhere for the gasses to go but to propel the bullet out of the barrel.

My question is, how much velocity is lost because the propelling gasses are vented in part from between the cylinder and the barrel?

 

[MrBorland]

how much velocity is lost because the propelling gasses are vented in part from between the cylinder and the barrel?

Very little. By all accounts, it's a few fps and not enough to matter.

 

[Sam1911]

We've had a few threads that discussed this. Jim Watson once reported an actual test:

Theory aside and sorry it is not with a more usual powder but it is what I can readily find in my notes as a direct comparison...
.45 ACP 230 gr LRN + 5.0 gr Green Dot
1911 w 5" Briley barrel -- 792 fps
S&W M25-2 sawn to 5" -- 752 fps

So there's 40 fps difference. Basically no difference at all. Variations between one gun and another of the same type will often create more difference than that.

 

[Nom de Forum]

Everything I have seen on the subject confirms that any velocity increases are so trivial from sealing the cylinder gap of a revolver that it is not worth the the effort to do it. Anyone have any data on the approximate extra barrel length need by the revolver to make up the velocity loss between the examples given by Sam1911.

Does anyone have any data on how cylinder gap affects the pressure curve of a cartridge fired in a revolver compared to a pistol? Anyone have any data on gas escaping through the cylinder gap before the bullet has entered the forcing cone?

 

[rcmodel]

http://www.ballisticsbytheinch.com/gaptests.html

rc

 

[Sam1911]

Anyone have any data on gas escaping through the cylinder gap before the bullet has entered the forcing cone?

Just for clarity, there isn't any gas escaping before the bullet has entered the forcing cone, and the bullet enters the forcing cone well before its base has cleared the throat of the chamber.

 

[Jaymo]

Comparison between a 1911 and a.45 revolver is not valid.
The grooves, lands, number/depth/width/twist are going to impart some differences that testing with an identical barrel does not.

The best test would be with a Dan Wesson revolver, since it would allow for testing with zero gap and with different gaps.
It would need to be loaded/cocked and hammer blocked before screwing the barrel tight against the front of the cylinder.
All other cylinder gap tests would be performed as per normal shooting.

 

[Nom de Forum]

Just for clarity, there isn't any gas escaping before the bullet has entered the forcing cone, and the bullet enters the forcing cone well before its base has cleared the throat of the chamber.

Ideally. Is the seal always prefect between the cylinder wall and throat in all situations? I can't agree that the bullet always seals the chamber before the bullet clears the throat. I suspect short bullets, especially in short cartridges in some calibers having very long chambers in comparison can experience gas leakage. Examples would be revolver competitors shooting shorter cartridges in .45 Colt and .357 chambered cylinders. Of course this is very trivial and I am only wondering about it out of intellectual curiosity not practical concern.

 

[Nom de Forum]

Comparison between a 1911 and a.45 revolver is not valid.
The grooves, lands, number/depth/width/twist are going to impart some differences that testing with an identical barrel does not.

The best test would be with a Dan Wesson revolver, since it would allow for testing with zero gap and with different gaps.
It would need to be loaded/cocked and hammer blocked before screwing the barrel tight against the front of the cylinder.
All other cylinder gap tests would be performed as per normal shooting.

I agree, but still think the results would be very similar.

 

[Sam1911]

Well, "ideally," the bullet is within 0.001" of filling the chamber throat, and I think we can assume there aren't any bullets at all that have a driving band shorter than the 0.01" (or less) of a cylinder-barrel gap.

 

[Nom de Forum]

Well, "ideally," the bullet is within 0.001" of filling the chamber throat, and I think we can assume there aren't any bullets at all that have a driving band shorter than the 0.01" (or less) of a cylinder-barrel gap.

I follow what you are saying about throat diameter. Here is better example of what I am questioning. People in competitions are using .38 Short Colt in revolvers with .357 Mag cylinders. The case length difference is more than half an inch. Could some gas leak around the bullet base before the bullet even enters the throat? Granted it would be a trivial amount, but again this is a question derived from intellectual curiosity not practical concern.

 

[Sam1911]

I see what you're saying, and sure! Every time you see that black smudge on the OUTSIDE of one side of your brass, that's gas that snuck around the case mouth, so no question that has to happen a bit more when the case is a lot shorter than the chamber.

 

[Nom de Forum]

^^^^^ Exactly, just going the other direction in front of the bullet before a seal is formed between the bullet and throat.

 

[Slamfire]

This is one of these mechanisms, that philosophically, should not work. I would have been one of the nay sayers: it is intuitively obvious this will fail unless you adopt a Nagant style cartridge. I am glad Samuel Colt did not listen because I love my revolvers.

 

[Nom de Forum]

Does anyone know how much the average cylinder gap changes the peak pressure and pressure curve in comparison to an integral chamber and barrel firing the same cartridge? Intuitively I am thinking it lowers the pressure and flattens the pressure curve somewhat.