The larger the bore, the faster the bullet?

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Let’s change it from hydraulics to pneumatics. Think of it this way. Say we have pneumatic air cylinders, one that is 2” diameter and one that is 3” in diameter. Both are attached to a scale that can let the piston move in and out and measure the force on it. Let’s also say both pistons have the same volume under them with the piston retracted, 10 cu inches.

Now we pressurize both to 1000 psi. While holding the piston in place. The force on the 2” piston will be 3140 lbs. (PxR2xpressure)

Because of its larger surface are the pressure on the 3” cylinder is 7065 lbs.

Now we shut the air valve so no pressure can get in our out and we allow the piston to stroke out 20”. As the piston moves down the bore the space behind the piston increases and so the gas expands to fill the larger space and pressure drops. Acounting for the extra 20” of stroke the smaller piston will now have 72.8 cu in of space behind it (62.8+10), and the larger piston will now have 151.3 cu in (141.3+10). If we ignore temperature changes the pressure inside the smaller cylinder will now be 137.4 psi, and the larger cylinder will have just 66.09 psi.

This is why large bore small case cartridges do so well in short barrels, they make the most of the pressure inside the chamber early in there travel down the bore, but they will loose steam quickly and will stop showing velocity gains earlier than smaller bores with the same case capacity because of expansion ratio.

Now as you think about the air space expanding as a bullet moves, add powder burn speed into that thinking as well.
 
Newtosavage said:
So I'm sure some bright person out there has kinda sorta figured out what the most efficient bullet diameter to case capacity ratio is by now...?

Cuz what good is it to push a fatter bullet of the same weight faster, only to generate more recoil and have the lower BC slow it down prematurely anyway?
Click to expand...

Depends what you want from your cartridge. I have rifles in 243win, 7-08, 308, and 338 federal all of these are very different rounds. I have a 6.5 creed and a few 6 Creeds - really can’t say I have great use for the 6.5, but love the 6. I have 7mm, 300wm, 338wm, and 416rm, 458wm, 375 H&H, and 458Lott rifles - all of these very different rifles. A 6.5-06 is a different animal than a 30-06, and different than a 338-06 or 35 whelen... what a guy can do with a 7.62x39 is very different than they can do with a 243 LBC.

So what is most “efficient” in one application isn’t in another. Change powder burn rate slightly and bullet weight, and your case efficiency - powder burn efficiency - changes dramatically.

“What good is it to push a larger bullet with greater recoil?” Well, a 7mm Rem mag isn’t quite as efficient for killing Cape buffalo as its brother, the 458 win mag. The 338 Federal might not be as good for 1,000yrd shooting at a 7-08, but it’s fantastic medicine for 0-400yrd brown bear and moose.
 
Efficiency in terms of energy and velocity retention from MV to whatever distance. Will that always be the smaller diameter bullet from a given case, or is there some happy medium?
 
Newtosavage said:
Efficiency in terms of energy and velocity retention from MV to whatever distance. Will that always be the smaller diameter bullet from a given case, or is there some happy medium?
Click to expand...

If velocity retention is the only measure, then usually a smaller bore with a faster twist and greater BC would be the better option. But long range velocity retention doesn’t mean much when I’m running a bullet into a bison at 120 yards.
 
Hydraulic issue aside, consider friction. A 100 grain .243 bullet compared to a 100 gr 7-08 bullet has a vastly different shape. The .243 will have a much longer cylindrical portion of the bullet which means more area scraping the bore as the bullet travels down the barrel. The 7mm bullet being shorter and fatter will have a smaller friction bearing surface area meaning less drag.

So, hydraulic pressure over larger area means more thrust for acceleration, and a reduced bearing surface means that a bigger bullet is reaping benefits through multiple realms of physics. Too bad that the bigger bullet has to travel through air which imparts drag based on leading surface shape and size... but if we shoot in the alps where the air is thin....


And now I am suddenly back the thinking about sabots and sub caliber bullets. So, if a .224 bullet with sabot weighing a combined 90 gr were fired from a 300RUM...
 
WestKentucky said:
And now I am suddenly back the thinking about sabots and sub caliber bullets. So, if a .224 bullet with sabot weighing a combined 90 gr were fired from a 300RUM...
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I shot a bunch of Remington Accelerators in the late 1990’s in 30-06 with the same attraction to the same thought, but they didn’t group for crap. Sure were fast, but apparently the sabots don’t release evenly - if they even hold the bullet evenly. Hard pass.
 
Varminterror said:
I shot a bunch of Remington Accelerators in the late 1990’s in 30-06 with the same attraction to the same thought, but they didn’t group for crap. Sure were fast, but apparently the sabots don’t release evenly - if they even hold the bullet evenly. Hard pass.
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Maybe it was just a coincidence but around the time those "Accelerators" were on the market, compound bows were all coming equipped with overdraws. That generation of consumers was obsessed with speed for a while. Glad those days are behind us.
 
It's actually fairly even when high energy density slow burning powders are used for the smaller bore diameters. Energy is energy, and E=MV^2. Bore diameter doesn't really come into it. Historically though there weren't many slow powders, and the ones that did exist had poor energy so smaller bores appeared to perform worse.

As a practical observation, a .264WM shooting 150ish grain bullets can match factory velocity for 150gr .300WM (listed 3150 ft/s) while not exceeding SAAMI pressure. So it's close enough to be down to the vagaries of individual powders and load formulas. Chose your bore based on desired sectional density and BC, not a double digit velocity difference.

Having this discussion without considering burn rate misses the point.
 
Newtosavage said:
Thanks for all the thoughtful replies. So what I thought I was seeing, is actually the case. That's very interesting. Makes sense now about the .338 Fed. I was like, how the heck are they getting those speeds out of a .308 case with that heavy of a bullet??? And now I know.
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That's why I have a 338-06. I learned that it was easier to push a 180 grain bullet faster out of a 338-06 than it was with a 30-06. I am an avid 30-06 fan but I'm not too proud to look for the advantage of the bigger bore. I can push a 180 grain bullet up toward 2,800 fps with the increased powder capacity of the 30-06 case. I feel really comfortable shooting the 338 bullet out to 250 yards and I am consistently hitting 10 inch steel targets at that range with only a shooting stick. If I thought I was going to be shooting at 300 yards I would take my 30-06 with 165 grain bullets.
 
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sage5907 said:
That's why I have a 338-06. I learned that it was easier to push a 180 grain bullet faster out of a 338-06 than it was with a 30-06. I am an avid 30-06 fan but I'm not too proud to look for the advantage of the bigger bore. I can push a 180 grain bullet up toward 2,800 fps with the increased powder capacity of the 30-06 case. I feel really comfortable shooting the 338 bullet out to 250 yards and I am consistently hitting 10 inch steel targets at that range with only a shooting stick. If I thought I was going to be shooting at 300 yards I would take my 30-06 with 165 grain bullets.
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When I had my Tikka 30-06, I developed some very accurate 180 grain loads over Superformance powder, and was getting 2800+ fps. with them. Let me tell you, that 6 lb. rifle was not much fun to shoot with those loads. LOL
 
someguy2800 said:
Let’s change it from hydraulics to pneumatics. Think of it this way. Say we have pneumatic air cylinders, one that is 2” diameter and one that is 3” in diameter. Both are attached to a scale that can let the piston move in and out and measure the force on it. Let’s also say both pistons have the same volume under them with the piston retracted, 10 cu inches.

Now we pressurize both to 1000 psi. While holding the piston in place. The force on the 2” piston will be 3140 lbs. (PxR2xpressure)

Because of its larger surface are the pressure on the 3” cylinder is 7065 lbs.

Now we shut the air valve so no pressure can get in our out and we allow the piston to stroke out 20”. As the piston moves down the bore the space behind the piston increases and so the gas expands to fill the larger space and pressure drops. Acounting for the extra 20” of stroke the smaller piston will now have 72.8 cu in of space behind it (62.8+10), and the larger piston will now have 151.3 cu in (141.3+10). If we ignore temperature changes the pressure inside the smaller cylinder will now be 137.4 psi, and the larger cylinder will have just 66.09 psi.

This is why large bore small case cartridges do so well in short barrels, they make the most of the pressure inside the chamber early in there travel down the bore, but they will loose steam quickly and will stop showing velocity gains earlier than smaller bores with the same case capacity because of expansion ratio.

Now as you think about the air space expanding as a bullet moves, add powder burn speed into that thinking as well.
Click to expand...

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.
 
So the difference in bore diameter changes the pressure curves too? Such as in the larger bore, pressure drops more rapidly (because of a larger column of air to fill in the larger bore) therefore the larger bore can have less of a recoil impulse?

Ive never thought about that before.
 
Newtosavage said:
When I had my Tikka 30-06, I developed some very accurate 180 grain loads over Superformance powder, and was getting 2800+ fps. with them. Let me tell you, that 6 lb. rifle was not much fun to shoot with those loads. LOL
Click to expand...
This is in my future.... Need to get that limbsaver for my tikka 30-06...
 
TN Outlaw said:
So the difference in bore diameter changes the pressure curves too? Such as in the larger bore, pressure drops more rapidly (because of a larger column of air to fill in the larger bore) therefore the larger bore can have less of a recoil impulse?

Ive never thought about that before.
Click to expand...
I'm sure the scientists will figure that out, and what the reason is - but that's something for them to look at. The recoil is less on the .338 Fed, over the .308 Win, both guns set up the same. Shocked me, when I took it out for the first time. When I take them out, with new shooters now, I'll have the new shooters fire the .338 Fed first, when it's time for them to shoot the large-frames.

Let the theories flow on that one...
 
98Z said:
I'm sure the scientists will figure that out, and what the reason is - but that's something for them to look at. The recoil is less on the .338 Fed, over the .308 Win, both guns set up the same. Shocked me, when I took it out for the first time. When I take them out, with new shooters now, I'll have the new shooters fire the .338 Fed first, when it's time for them to shoot the large-frames.

Let the theories flow on that one...
Click to expand...

Now that IS bizarre. But also worth looking in to! The idea of a .338 Fed is really growing on me. And since I'm a savage shade-tree mechanic, it's as simple as a barrel swap.
 
A 338 Fed running the same bullet weight at the same speed should have a lesser charge, so the net momentum is less. Simple physics there.

Equally, the duration of the impulse is significant when it comes to FELT recoil. We experience it all of the time with these cartridges - how many times have you read online a 480 Ruger recoils more like a 44mag, despite having energy more similar to 454 Casull? It’s because the impulse of a slower bullet is spread out over a significantly longer duration - literally, the same Work (same bullet weight brought to the same speed) done over a longer duration is less Power. So even when the same Energy is on the ejecta and bullet upon leaving the muzzle, two rifles can have experienced very different Power rates.
 
Varminterror said:
A 338 Fed running the same bullet weight at the same speed...
Click to expand...

Not with mine. I don't shoot factory ammo, and only handload. My range-trash .308 Win is a 150gr Hornady FMJ-BT, and good loads are 178gr Hornady HPBT and 195gr Hornady HPBT. My .338 Fed Loads are 200gr Hornady SSTs and 225gr Hornady SSTs.

The projectile weight difference in those loads is not the difference in the recoil - and it's not "felt recoil." The .338 Fed does not even push as hard as the .308 Win loads, or my 195gr .308 Wins would be near identical to my 200gr .338 Feds. No where near the same. The .338 Fed is the softer rifle, by far.
 
Varminterror said:
I shot a bunch of Remington Accelerators in the late 1990’s in 30-06 with the same attraction to the same thought, but they didn’t group for crap. Sure were fast, but apparently the sabots don’t release evenly - if they even hold the bullet evenly. Hard pass.
Click to expand...

I think I am the only person on earth that liked accelerators :) .

I used them quite a bit for ground hogs and they seemed as acurate as anything. I wasn't into paper punching and group shooting at the time, just hunting and many hogs fell to the accelerators
 
98Z said:
Not with mine. I don't shoot factory ammo, and only handload. My range-trash .308 Win is a 150gr Hornady FMJ-BT, and good loads are 178gr Hornady HPBT and 195gr Hornady HPBT. My .338 Fed Loads are 200gr Hornady SSTs and 225gr Hornady SSTs.

The projectile weight difference in those loads is not the difference in the recoil - and it's not "felt recoil." The .338 Fed does not even push as hard as the .308 Win loads, or my 195gr .308 Wins would be near identical to my 200gr .338 Feds. No where near the same. The .338 Fed is the softer rifle, by far.
Click to expand...

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
 
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.
 
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