A question about Pressure

JWFilips

Well-Known Member
If one were to compare a S&W 2" barrel revolver to a S&W 4" barrel revolver ( both of the same caliber) for a "given load" Is the pressure any different because of barrel length or is it all within the cylinder chambers?
 

John G

Active Member
I would think the pressure difference would be very small as once in the barrel the relief valve of the cylinder gap would balance out the possible increase. Cylinder gap and the bullet in motion would negate any notice pressure increase. IMO.
 

Missionary

Active Member
I do know the slower the powder the higher the barrel pressure especially with an adjustable barrel gap... like the Dan Wesson
But with those short of barrels a case full of H110will light up the neighborhood.
 

JWFilips

Well-Known Member
Mitty,
Just interested in the wheel gun>
I know many pressure readings are from universal receivers. Just wanted to know how much of the pressure of the load in a revolver is based on the cylinder chamber vs the length of the barrel & does the length of the barrel play any part in the Pressure the cartridge generates
 

Ian

Notorious member
Peak pressure happens within an inch of bullet travel even with the slowest powders.
 

Ian

Notorious member
No, it doesn't all happen in the cylinder, the peak can be anywhere from before the bullet base leaves the case (Bullseye and similar) to just as the base is leaving the cylinder (Unique) to about the time the base band is getting fully engraved (H110). After the peak there's a decay which can be gradual or abrupt.
 

Petrol & Powder

Active Member
I believe it would be affected by where the bullet is when the peak pressure is reached with that given load. If a particular combination results in the peak pressure occurring while the bullet is still in the cylinder (and I'm not claiming that is reality) then barrel length would not be a factor. If however, the peak occurs when the bullet is 3" down the barrel (again just tossing out hypotheticals here) then barrel length would play a role. In that situation when the bullet exited a 2" barrel, the pressure would cease to increase because the pressure would be vented out the end of the barrel.
NOW, in reality, the peak occurs before the bullet mores very far as Ian stated. And as the bullet moves farther away from the case the volume that the expanding gas must fill becomes greater. In fact, the bullet isn't just moving away from the casing, it is accelerating as it moves forward. This means the rate at which the burning powder creates expanding gas would have to be greater that the constantly increasing volume in order for the pressure to continue to rise as that volume gets larger.
 
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fiver

Well-Known Member
Boyles law says pressure drops double for every single that the volume is increased.
but, if you look at pressure and velocity with fast and slow powders the greater gas volume will in every instance create a higher velocity.
it simply pushes longer at the same peak pressure.
it will generally start pushing at peak pressure later in the sequence too, otherwise you'd have to have a perfect pairing of powder and pressure in every instance of top end performance to not exceed the MAP of the rounds designation.
in other words you couldn't use powders outside of the 2-3 that fit that perfect window.

we know in say the 38 special we can use everything from bulls-eye through H-110 and stay under the MAP
so the only variable left to change velocity is gas volume.[time to accelerate the bullet under pressure]

how much extra gas volume/pressure does the slowest powder create?
enough to cut/etch steel top straps.
if there is that much gas volume there, there is certainly enough to continue pushing on the bullet just past that point in the barrel.
if there wasn't,,, barrel porting would be pointless on a revolver.
 

JWFilips

Well-Known Member
OK I'm am learning here! I like that!
So let me give you guys the reason I was asking:
As we all know older loading manuals usually show higher charges for a given load combo than the more modern manuals of the same Brand.
I have a bunch of Old and New Lymans at my disposal.
OK above scenario was about the 38 Special however the cartridge I'm working with is the Old 38 S&W For which I have very good light target loads and Heavy CC loads for my wife to use in her S&W Mod 32 -1 Terrier 2" barrel!
Of course she seldom shoots large numbers the CC loads because they are for protection and they are a bit heavy for everyday practice Granted these are safe loads well within specs using 158 grain SWCs and Unique.
Her normal practice load In a 125 gr TC cast with 2.5 gr BE .....this are light and easy to shoot all day. We both decided to come up with some new medium loads to more closely match the CC loads POI and add a bit more recoil over the light load.
So The problem with loading for the venerable old .38 S&W is there is a lot of bullet choices in the old manuals but far less if any loads in the more modern manuals!
Working from the 43 edition Lymans loading manual in the 148 grain class I have 2 choice for which I have proper sized Cast bullets
the 358495 Wad cutter ( @ the proper .361" size) as well as the 358246 RN....load data is simple 2.0 gr BE = 700 fps to Max 3.0 gr BE = 810
for both bullets Seated to crimp groove ....but this sent up a read flag because the wad cutter would be set farther into the case to use it's crimp groove! So common sense tells you that isn't correct so in seating that bullet I measured the length that the 358246 RN bullet extended in to the case
and used this data to set the wad cutter depth to the same! This did extend the bullet out longer into the cylinder but there is plenty of room and they drop right in! So I felt good about loading them both with 2.7 gr BE OAL of .1.010": Lyman's data for the 38 S&W has always been a 4" barrel revolver.
The S&W mod 32 is a 2" barrel so I was thinking that the pressure would even be less...so I guess not.

The later lyman manuals dropped the wad cutter loadings completely...so that was a bit unnerving to me!
We have shot a lot of the swaged HB wad cutters with 2.5 grain BE in the past so I know long loaded wad cutters will chamber.

Then if I go to the Lyman Cast bullet Handbook 3rd ed They again have the 358246 wad cutter listed but they have it set deep into the case at .990" This is crimped in the real crimp groove and they have the max load for it at BE at 2.4 gr so the bullet is set very deep in the case.
Anyway sorry to ramble on......
Will be testing them soon
 

fiver

Well-Known Member
run the long and short over a chrono.
you have the same gas volume to work with so the only thing left to explain any changes is more [sooner] pressure.
 

Ian

Notorious member
The energy released from a given amount of nitrocellulose/nitroglycerin is the same. What is controlled when we choose BE or 2400 is the rate of that release via deterrent coatings and kernel shape/size (surface area exposed).
 

Ian

Notorious member
It all boils down to (pun intended) thermodynamics. Boyle's law applies, but our systems are dynamic in several ways such as cartridge/bore cross-sectional ratios, bullet moving, bullet accelerating, rate of deterrent being burned off the surface of the powder kernels, shot initiation pressure (system begins as a closed one and then energy is dumped as the bullet's static inertia and engraving resistance is overcome), and orobably some I missed. It's complicated but not beyond understanding. Quickload operates on basic thermodynamic formulas, specific heat, and physics 101.
 

Mitty38

Seeker of knowlege
Thanks for asking this question JW. Did not know what you were getting at.
I too load for a revolver and have picked up a bit of knowledge from this conversation.
Never considered the gap having an effect on cylinder pressure or how fast a powder, when the projectile passes the gap along with barrel length being a contributor in a revolvers cylinder pressure. Hope this conversation continues a bit so some more info can sink into my hard noggin.
 

Petrol & Powder

Active Member
I do think it's incredibly complex because as the bullet accelerates down the bore, the volume of the "cylinder" behind that "piston" gets larger and that increase is an acceleration not just a linear movement. So while the increase in gas being produced may initially be great enough that the pressure continues to rise even as the volume becomes greater (the bullet moves farther down the bore) you will very quickly reach a point where the increase in volume outstrips the burning powder's ability to make more pressure in that larger space. When that happens, you will have reached the peak pressure.
In a small bore rifle with a bottle necked cartridge and a slow burning powder, the bullet may travel farther from the casing before that peak pressure is reached than if compared to a short, straight walled, large caliber handgun cartridge with a smaller amount of a fast burning powder.

There are a lot of variables in play. I agree with Ian and I also think that with handgun cartridges the peak pressure is reached before the bullet moves any considerable distance. However, even if peak pressure is reached early in the bullet's travel from the casing, the amount of time the pressure acts on the bullet is still a significant factor. Even if peak pressure is achieved early in the bullet's travel down the bore, a longer barrel can increase the bullet's muzzle velocity simply because the pressure could act upon that bullet over a longer period of time.
 

Rally

NC Minnesota
There wouldn't be much actual difference in expansion chamber with a hollow base wad cutter seated at crimping groove and a semi wadcutter seated out farther. There was quite a bit of discussion of this when folks started reversing hollow base wadcutters for self defense use in the gun rags.
 

Ian

Notorious member
I posted a bunch of Quickload screen shots here one time comparing different powders and seating depths of a couple of handgun calibers. I'll see if I can dig it up.