The larger the bore, the faster the bullet?

[Llama Bob]

Saying “energy is energy” without considering expansion ratio and pressure dependent progressive burn rates of modern smokeless powders kinda misses the point too.

If you pick a burn rate suited to your application, energy is energy. In fact, "energy is energy" is a reasonable description of all non-nuclear physics

 

[Vern Humphrey]

Necking up a cartridge -- for example, going from .308 Win to .358 Win, results in faster expansion of the powder gasses -- there is more volume in the .35 caliber barrel than in the .30 caliber barrel. Now when a gas expands, the pressure drops. That's not what we want -- we want to keep the pressure as high as we can (within safety limits) for as long as we can.

To do this, we add more powder. The necked-up case can safely burn more powder than the original case. And when you add more powder, you add more energy.

Normally, the result of this is not more velocity, but the necked up case can push a heavier, larger diameter bullet to nearly the same velocity as the original case.

 

[someguy2800]

Necking up a cartridge -- for example, going from .308 Win to .358 Win, results in faster expansion of the powder gasses -- there is more volume in the .35 caliber barrel than in the .30 caliber barrel. Now when a gas expands, the pressure drops. That's not what we want -- we want to keep the pressure as high as we can (within safety limits) for as long as we can.

To do this, we add more powder. The necked-up case can safely burn more powder than the original case. And when you add more powder, you add more energy.

Normally, the result of this is not more velocity, but the necked up case can push a heavier, larger diameter bullet to nearly the same velocity as the original case.

And if you can't add more powder such as going from 308 to 358, you go to a faster burning power.

 

[Newtosavage]

One powder vs another can make a huge difference in felt recoil as well. I have a load in 270 win that I load for my father in law and brother in law that for whatever explainable reason recoils about 3/4 of their usual loads. They both swore I gave them puffballs but the chrono and the POI shows them to be the same.

Very true. I've experienced this many times myself. The difference in LVR powder vs. 8208 in my Grendel is very noticable. If not for the chrono, I'd never believe it.

 

[98Z]

Are your .308win and 338fed in similar size/wt rifles? Or is your 338fed in a heavier gun?

If the 338Fed is in a heavier configuration or with a better designed stock, that would largly affect FELT recoil.

This is interesting

Almost identical. 16" barrels, midlength gas, 1~4 scope on one, 1~6 on the other. Both running AR-10 Carbine recoil systems, both rails are similar, same brake on both. Damn near twins.

 

[Jessesky]

If I’m not mistaken, it is relative to the barrel length of the firearm. The larger the bore diameter, the better the expansion ratio which allows an efficient powder burn.

 

[98Z]

Are your .308win and 338fed in similar size/wt rifles? Or is your 338fed in a heavier gun?

Figured I'd toss these two in here, since these are the ones I'm talking about. .308 Win on top, .338 Fed on bottom. Almost identical.

P1060969

• 98Z
• Feb 28, 2019

 

[WestKentucky]

#overdrawanonymous LOL

I still shoot an overdraw but only because I learned to shoot on a bow that is ungodly big for me. 33” when I measure at about a 30-1/2. Shooting even a 31 feels really awkward, but the 28”s are easy to find and a lot easier to pack in a quiver, so overdraw it is.

 

[TN Outlaw]

Figured I'd toss these two in here, since these are the ones I'm talking about. .308 Win on top, .338 Fed on bottom. Almost identical.

P1060969

• 98Z
• Feb 28, 2019

Sure are. Twins!

Other than shooting hot loads for one and mild for the other, those should tell the tale.

Id be curious to shoot and chrony say a 180gr bullet from each at basically the same speed and know what the difference in recoil is then?

Pretty cool stuff, and nice looking rifles there 98Z.

 

[Newtosavage]

As an aside, I've never understood the cantilever scope mount solution for AR's (the non-AR owner says...). Always looks like a weak point in the system to me.

 

[Matt.Cross]

As an aside, I've never understood the cantilever scope mount solution for AR's (the non-AR owner says...). Always looks like a weak point in the system to me.

You gotta try it out brother, if you have one really nice optic and a cantilever mount that returns to zero, you can move your optics from gun to gun with no more effort expended than simply recording the zero shift for each gun.

 

[Newtosavage]

You gotta try it out brother, if you have one really nice optic and a cantilever mount that returns to zero, you can move your optics from gun to gun with no more effort expended than simply recording the zero shift for each gun.

I suppose I would if I was an AR fan, but I'm not. They just don't do anything for me at all.

 

[MistWolf]

To expand a bit on this - a hydraulic piston is supplied with (relatively) constant pressure. A rifle bore is not. So the case capacity has to be sufficiently large to suit the bore diameter, otherwise the above generalization I described won’t be true. Rifle cartridges all reach their peak pressure within a couple inches of bullet travel down the bore. Once that peak is reached, if a bore is too large for the case, it won’t have sufficient expansion ratio to sustain the pressure curve as high in the larger bore.

In the case of a large bore, small case, it’s like a hydraulic piston with a low oil condition - the smaller cylinder would stroke fully and at full speed, whereas a larger cylinder wouldn’t have sufficient volume to fully stroke, and power would be reduced during travel.

The gasses are a highly compressed gas and as the bore volume expands, the gasses expand with it thus accelerating the "piston".

Hydraulic fluid is non-compressable. It has to be pressurized by a pump or a charge of compressed gas. If the system does not contain enough fluid to fill the volume of the cylinder as the piston moves, the piston will slow and just come to a stop. For example, if the hydraulic system of an A-10 were pressurized to 2500 psi and the pump turned off, the system will show 2500 psi. But, the pressure can do no work because without the pneumatic pre-charge, there would be nothing to expand and push the fluid. If a piston were activated (such as stepping on the brakes during an aircraft tow) the piston would move only enough so that the minor expansion of the hydraulic lines & accumulator tank was relieved.

However, the accumulator has a pneumatic pre-charge that will expand and push the hydraulic fluid enough that the piston will move and activate the brakes. Basically, the principle of using pneumatic pre-charge to activate a hydraulic system is the same as using gas pressure to push a bullet down a bore. The difference is how the pressure is generated.

 

[MistWolf]

Great point, right here, and it explains the recoil differences between .308 Win and .338 Fed, if you think about it - .338 Fed recoils less than .308 Win. I have two 16" guns set up almost identically, even down to the same brake on both. The .338 Fed recoils less than the .308 Win. I know that's contrary to some of the comments previously, but that's reality.

If both calibers were to launch the same weight bullet at the same muzzle velocity using the same weight powder charge out of rifles of the same weight, free recoil numbers for both will be the same.

The 338 may feel softer. But the free recoil would be the same.

Now, if the 338 needed less powder to achieve the same velocity, it's free recoil would be less.

 

[98Z]

If both calibers were to launch the same weight bullet at the same muzzle velocity using the same weight powder charge out of rifles of the same weight, free recoil numbers for both will be the same.

If I can find .308 and .338 projectiles that are the same weight, I'll make up loads for them and try it out. Getting them to the same muzzle velocity will require different powder charges - that's a given. The same powder charge is not going to launch both projectiles at the same velocity. I'll need to come up with a way to specifically measure the recoil pressure at the buttstock.

The closest I have right now are the 195gr .308 projos and the 200gr .338 projos. Both these rifles are pretty close in weight, but not identical. I won't be changing the rifles to do this, but I might try to find a way to add weight to the one that might be lighter. Even static weight, attached to the rifle anywhere will do the trick.

 

[Newtosavage]

If I can find .308 and .338 projectiles that are the same weight, I'll make up loads for them and try it out. Getting them to the same muzzle velocity will require different powder charges - that's a given. The same powder charge is not going to launch both projectiles at the same velocity. I'll need to come up with a way to specifically measure the recoil pressure at the buttstock.

The closest I have right now are the 195gr .308 projos and the 200gr .338 projos. Both these rifles are pretty close in weight, but not identical. I won't be changing the rifles to do this, but I might try to find a way to add weight to the one that might be lighter. Even static weight, attached to the rifle anywhere will do the trick.

I'd be interested in that test. The .338 interests me as a short to medium-range elk and bear round. Why, I'm not sure, but it does.

 

[Varminterror]

If both calibers were to launch the same weight bullet at the same muzzle velocity using the same weight powder charge

With the same powder, suitable for the cartridge cases, all 3 of these don’t happen at the same time. Same bullet weight and velocity will mean different powder charge, same velocity and charge would mean different bullet weight, or same bullet and powder charge would mean different velocity.

 

[MistWolf]

With the same powder, suitable for the cartridge cases, all 3 of these don’t happen at the same time. Same bullet weight and velocity will mean different powder charge, same velocity and charge would mean different bullet weight, or same bullet and powder charge would mean different velocity.

That may be true, but I'm talking about the math. The point is, bore diameter does not directly affect level of recoil, although it may affect factors that do directly affect recoil energy, such as the powder charge needed for the velocity generated. The math proves this.

 

[Varminterror]

That may be true, but I'm talking about the math. The point is, bore diameter does not directly affect level of recoil, although it may affect factors that do directly affect recoil energy, such as the powder charge needed for the velocity generated. The math proves this.

Great addition - Which I stated 2 pages ago...

Recoil doesn’t care about bore or pressure in a literal sense, but secondarily so. Same charge, same bullet weight, then the faster of the two will have the greater net momentum, and such impart greater recoil momentum to the rifle. Equal but opposite, and all that... If the case has sufficient capacity to capitalize on the larger bore and produce greater velocity, then the larger bore will have greater recoil.

 

[MistWolf]

My response to 98Z was for clarification and to nudge his critical thinking process in the right direction.

 

[d2wing]

It was commonly thought and an idea pushed by old timers that if you neck down a cartridge you get higher velocity, We know that is not always true. There are several factors, but in a necked down cartridge it internal shape of the case and opening size determine how fast the exploding charge can escape the case out the neck and push the bullet. A narrow opening can restrict that flow. Bullet weight makes a difference, type of powder. In fluid dynamics the smaller the opening, the greater the pressure, but in the case of an explosion the rate of expansion can over ride pressure and the narrower neck and chamber shape as in sharper shoulders can restrict expansion rate. This is why some of the short magnums failed. In some cases they are close to offsetting so a bullet of the same weight goes close to the same speed regardless of caliber.

 

[Newtosavage]

So I still want to know where peak efficiency is attained. What case to bore ratio?

 

[.308 Norma]

So I still want to know where peak efficiency is attained. What case to bore ratio?

I'd kind of like to know that too - if such a ratio even exists. I've heard that the .22LR is about as efficient as cartridges get...and it's a straight walled cartridge.

 

[someguy2800]

So I still want to know where peak efficiency is attained. What case to bore ratio?

I don't know a scientific answer to that, but I think the most efficient cartridges in terms of muzzle energy per grain of powder are short high pressure pistol cartridges like a 9mm luger. For a rifle cartridge with a desired velocity over 2000 fps though I think the most efficient case size/length/bore ratio is about the size of a 22 K hornet or a 300 blackout. Basically a short fat powder column with a small bottleneck. If you compare what those two cartridges get for velocity with a certain bullet weight they are just astoundingly efficient. My 300 blackout for instance does 2400 FPS with 19 grains of powder and a 125 grain bullet from a 16" barrel. My 7.62x39 takes 25 grains of powder to match it, and 30 grains of powder to do just 200 FPS better, also from a 16" barrel. Even a 357 magnum according to hodgdon's rifle load data takes 22 grains to hit 2275 fps. Now that's efficient!

Of course if you add down range energy that's a whole different story.

 

[Varminterror]

So I still want to know where peak efficiency is attained. What case to bore ratio?

Define “peak efficiency” and your question would be much more easily answered.