The Shoot2hunt Podcast

[wind gypsy]

At an equal pressure, volume does not matter when comparing velocities. A larger volume will only mean that gasses will exit the barrel for a longer duration.

Granted, as pressure decreases, velocity will also decrease.

Why wouldn't volume matter? More volume at same pressure = more energy doesn't it? At least from a recoil standpoint.

I can relate to this kid.

 

[huntnful]

Kinetic molecular theory and Graham's Law of Effusion were definitely not on my Rokslide bingo card for today.

Bro is just making up big words to make us feel inferior right now. Not gunna work on me pal, because I can’t even read.

 

[alexnelon]

Why wouldn't volume matter? More volume at same pressure = more energy doesn't it? At least from a recoil standpoint.

I can relate to Bobby.
View attachment 1056741

Volume doesn't affect velocity of the gas at the same pressure.

I think the big point is that Ryan and Jake don't understand physics well enough to explain why different guns have different levels of recoil, and everyone is more confused because of it.

Pressure and volume of gas affect recoil, gas velocity during effusion doesn't (much).

Comparing 6UM and 6.5PRC is a false equivalence. You can't use external ballistics as a baseline to explain differences in internal ballistics.

 

[Ryan Avery]

Volume doesn't affect velocity of the gas at the same pressure.

I think the big point is that Ryan and Jake don't understand physics well enough to explain why different guns have different levels of recoil, and everyone is more confused because of it.

Pressure and volume of gas affect recoil, gas velocity during effusion doesn't (much).

Comparing 6UM and 6.5PRC is a false equivalence. You can't use external ballistics as a baseline to explain differences in internal ballistics.

So what's the answer? I've made it clear I don't know why, but none of you nerds have explained it to my simple brain either, so that tells me no one knows....

 

[Firth]

I can see why they would think a smaller hole would produce less velocity. If you think of a tank of water with a hole in it under pressure, it will spray water if the hole is big enough, but as the hole gets smaller it will drop to a drip. Basically, the energy loss gets high enough that there isn't much potential energy from the pressure left to be velocity after the exit. I don't think that applies here though due to the relative sizes in holes.

My answer for gun barrels: I'd have guessed the same pressure would produce about the same velocity. Pressure is potential energy and it's getting converted to kinetic energy (velocity).

But I'm a civil engineer where everything is steady state, incompressible, and the temperature is constant, and I have no idea on the relative magnitude of those effects. Pretty big would be my guess though.

@Jordan Smith

He's a physicist, let's see if he can resist.

 

[alexnelon]

So what's the answer? I've made it clear I don't know why, but none of you nerds have explained it to my simple brain either, so that tells me no one knows....

It technically is rocket science, and I'm not a rocket scientist, I'm just a nerd, but basically;

Mass flow rate X Gas velocity + (area X pressure) = thrust (or close to that)

In rifle terms;
mass flow rate = Gas volume
area = barrel diameter
gas velocity and pressure mean the same thing in both aspects

a 20 inch 6.5mm barrel holds ~15% more volume than a 6mm. Which (loosely) equates to 15% more gas leaving the barrel with the same powder at the same pressure. But you have to burn more powder to get that pressure, which is how you get more gas.

The problem with comparing recoil between different cartridge diameters is you can't really have equal gas pressure AND equal gas volume unless you compare different powders and charge weights.

A 6UM at 62k PSI with N570 is actually closer in gas volume to a 6.5 creed at 62k with 42 grains of H4350, which I bet your shoulder has told you already.

 

[nm.otter]

I'll wait for DAS captures of the recoil impulses. If the other variables are controlled and they feel different, we should learn something useful.

Based on what I heard, my working hypothesis is that smaller hole = longer time to vent gases = spread out impulse.

 

[mt terry d]

Can someone give me a website for Foster Huntington?
Great guest.

 

[Wprinkle]

When you dorks are done dorkin, send me the cliff notes.. i engaged both brain cells reading that, imma need you to explain to me like you would a 5 year old that English is a second language. Pictures help lmao

Joking aside, physics and “feel” both point to the 6.5 saum improved necked down to 6 (6um) is the best return on investment cartridge speaking. Wonder how the 6.5 saum compares to other 6.5’s.. is it case/ chamber design?

 

[wyosam]

One thing is clear in this conversation. It must be about time for spring bear, because cabin fever is getting pretty intense.


Sent from my iPhone using Tapatalk

 

[wy_will]

So what's the answer? I've made it clear I don't know why, but none of you nerds have explained it to my simple brain either, so that tells me no one knows....

You are comparing a 115 grain to a 147 grain. We all know heavier bullet is more recoil.

Of course that is actually not always true, but I’m just giving you a simple answer for your brain to understand.

 

[WDO]

Can someone give me a website for Foster Huntington?
Great guest.

For the range cards and stuff

Sobchak Security LLC

Sobchak Security LLC

waltersobchaksecurity.com

I’ve read his articles for a few years on other topics. Interesting reads in my opinion.
Here’s his other. I get the newsletter/email that goes once a week or so.

https://arestlesstransplant.substack.com/

 

[mt terry d]

Thank you

 

[4th_point]

Sorry if I missed it.

What are the rifle parameters?
-Barrel lengths
-Chamberings - 6UM and 6.5 PRC?
-Overall weights
-Same stocks? Same scopes, mounts, butt pad?
-Same muzzle treatment - same can design?

What are the load details?
-Bullet type and weights
-Case capacity
-Powder charges, and what powder
-Muzzle velocities

 

[Bittereinder]

Dude talking about the way the tacticool guys speak had me cracking up. “You’re utilizing?”

Also, "knowledge transfer".

 

[Jordan Smith]

There's a lot of conflicting opinions on this topic, primarily because the physics strongly depends on the set of assumptions a person chooses. Are you interested in comparing calibers that have identical cartridge cases? Identical external ballistics? The list goes on. The variables that you choose to keep constant, and those that you choose to vary completely change the outcome. As an example, assume we are comparing identical case designs but different calibers. The gas velocity is a function of muzzle pressure, so we'll assume the same type and quantity of powder. We're also assuming the same peak pressure and pressure curve (not a realistic assumption, given the difference in the volume of the gas container). A larger caliber at the same muzzle pressure (and therefore, gas velocity) and amount of gas has a larger impulse imparted to the rifle, increasing recoil velocity.

For conversation, here are a couple of links:

- SAAMI discussion of recoil: https://saami.org/wp-content/uploads/2025/03/Gun-Recoil-Formulae-2018-07-9.pdf
- Another thorough (although with some minor errors) discussion of recoil: https://robrobinette.com/Gun_Recoil_Calculator.html

Note that caliber is not mentioned in the SAAMI discussion. This is because it uses phenomenological gas velocity, which depends on muzzle pressure, which depends on caliber, powder type, and powder quantity.

 

[MEdude]

Yup, spot on. Couldn’t have splained it better myself.

 

[Jordan Smith]

To elaborate a bit more on why this can be confusing, there are several confounding and competing variables in determining relative recoil between chamberings of different calibers. In the SAAMI document mentioned above, we are primarily concerned with how this expression compares between 6mm and a 6.5mm cartridges:

(W_E*V_E + W_PC*V_PG)

where,

W_E is the bullet weight
V_E is the bullet muzzle velocity
W_PC is the powder charge weight
V_PG is the muzzle velocity of the powder gases

Now, V_PG is not only a function of muzzle pressure, but also the bullet's muzzle velocity, a dependency derived phenomenologically for rifles to be V_PG = 1.75*V_E, and the above expression becomes

V_E * (W_E + 1.75*W_PC)

Let's assume, as an example, that we are comparing two cartridges of identical case capacity (e.g., 6 CM and 6.5 CM). Practically speaking, bullet weight is greater in the larger caliber (e.g., 120 gr ELD-M versus 105 gr Horn BTHP, again assuming comparable SD and similar bullet design); muzzle pressure is similar but bullet muzzle velocity is less for the larger caliber; an identical case, heavier bullet, and greater chamber volume in a larger bore, means that to achieve identical muzzle pressures a comparable charge weight is used. So, approximating the above expression with relative relations, for the larger caliber:

less * (more + 1.75*same)

So, we can see that the relative recoil depends on exactly how much lower is the muzzle velocity, how does the powder charge compare, and how much greater is the bullet weight. In this practical example comparing 6 CM and 6.5 CM, we can use the following values (per Hodgdon data maximizing muzzle velocity for each while keeping peak pressure comparable),

6CM (105 BTHP, 43.7 gr SB6.5, 3156 fps at 60.2k psi peak): 3156 fps * (105 gr + 1.75*43.7 gr) = 549,461 gr*ft/s
6.5CM (120 ELD-M, 45.3 gr SB6.5, 3004 fps at 60.5k psi peak): 3004 fps * (120 gr + 1.75*45.3 gr) = 598,622 gr*ft/s

So, in this case the 6.5CM produces ~4.5% more recoil than does the 6CM. A different set of assumptions (not comparing similar bullet SD, identical case capacity, heavier bullet in the larger caliber, etc.) may produce a different outcome. If we assume maximal external ballistic performance of each caliber, and allow the other parameters that we previously held constant to vary, then

6CM (110 AT, 42.7 SB6.5, 3046 fps at 60k psi): 3046 fps * (110 gr + 1.75*42.7) = 562,672 gr*ft/s
6.5CM (150 MK, 42.6 gr H100V, 2713 fps at 60.3k psi): 2713 fps * (150 gr + 1.75*42.6) = 609,204 gr*ft/s

Now the 6.5CM has 8.3% more recoil, or about double the relative increase from before.

 

[WyldGoose]

 

[Jordan Smith]

I can see why they would think a smaller hole would produce less velocity. If you think of a tank of water with a hole in it under pressure, it will spray water if the hole is big enough, but as the hole gets smaller it will drop to a drip. Basically, the energy loss gets high enough that there isn't much potential energy from the pressure left to be velocity after the exit. I don't think that applies here though due to the relative sizes in holes.

My answer for gun barrels: I'd have guessed the same pressure would produce about the same velocity. Pressure is potential energy and it's getting converted to kinetic energy (velocity).

But I'm a civil engineer where everything is steady state, incompressible, and the temperature is constant, and I have no idea on the relative magnitude of those effects. Pretty big would be my guess though.

@Jordan Smith

He's a physicist, let's see if he can resist.

@Firth - I couldn't resist.