Velocity difference 7mm08 vs 338 Fed

vpsalin

vpsalin

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How is it that the 338 Fed can sling 185gr pills at 2750fps when the 7mm08 dishes out 140gr pills at 2860fps and both are using the same 308 case? Where does the 338 get so much velocity from? Even the 308 shoots out 180gr pills at only 2600fps...!?
 
The key is that 338 Federal has a much lower expansion ratio than 7mm-08.
7mm-08 expansion ration: 3.5
308 Win expansion ratio: 3.0
338 Fed expansion ratio: 2.5

This means that for a 24" barrel (22" of barrel and 2" of chamber)
a 7mm-08 propellant will expand to 22" / 3.5 = 6.25 times
a 308 propellant will expand to 22" / 3.0 = 7.33 times
a 338 propellant will expand to 22" / 2.5 = 8.8 times
As in a car engine the expansion ratio (or the inverse compression ratio) determines the efficiency of the transformation compressed hot gas into a bullet's (piston's) kinetic energy.

As simpler way to put this is that all of these barrel length are as efficient:
a 20" 338 Fed (180gr 2770 fps)
a 23" 308 Win (180gr 2660 fps)
a 27" 7mm-08 Rem (175gr 2550 fps)
The 338 Fed get an extra 7" of "equivalent barrel" for free (7 x 25fps = 175fps).

Alex
 
I'm still trying to figure out why the 338 Fed isn't more popular? Seems like a far more practical calibre than say a 270 WSM. I think we have all gotten obsessed with speed and everyone thinks they can shoot out to 400 yards.

Just my opinion. George
 
g-manz35 said:
I'm still trying to figure out why the 338 Fed isn't more popular? Seems like a far more practical calibre than say a 270 WSM. I think we have all gotten obsessed with speed and everyone thinks they can shoot out to 400 yards.

Just my opinion. George
Click to expand...

338 Federal can only use medium sectional density (medium ballistic coefficient) bullets at medium velocity so it's not that good at long range.
338-06 is much better and 338 Win Mag is probably the best common large game long range cartridge. The big downside is that both have significantly more recoil than 30-06.

270 WSM and 7 Rem Mag deliver better than 30-06 long range performance with a 30-06 recoil level. That is the key to 0.270" and 7mm high velocity cartridges popularity.

It's really tough to argue with a 140gr Accubond travelling at 3200fps ;)

Alex
 
AlexF said:
The key is that 338 Federal has a much lower expansion ratio than 7mm-08.
7mm-08 expansion ration: 3.5
308 Win expansion ratio: 3.0
338 Fed expansion ratio: 2.5

This means that for a 24" barrel (22" of barrel and 2" of chamber)
a 7mm-08 propellant will expand to 22" / 3.5 = 6.25 times
a 308 propellant will expand to 22" / 3.0 = 7.33 times
a 338 propellant will expand to 22" / 2.5 = 8.8 times
As in a car engine the expansion ratio (or the inverse compression ratio) determines the efficiency of the transformation compressed hot gas into a bullet's (piston's) kinetic energy.

As simpler way to put this is that all of these barrel length are as efficient:
a 20" 338 Fed (180gr 2770 fps)
a 23" 308 Win (180gr 2660 fps)
a 27" 7mm-08 Rem (175gr 2550 fps)
The 338 Fed get an extra 7" of "equivalent barrel" for free (7 x 25fps = 175fps).

Alex
Click to expand...


Alex, what do you mean by "expansion ratio"?

This is all very good info here. There is some dynamics to launching projectiles I was totally missing before.
 
cody c said:
Alex, what do you mean by "expansion ratio"?

This is all very good info here. There is some dynamics to launching projectiles I was totally missing before.
Click to expand...

Here is a good link on a cartridge's expansion ratio:
http://kwk.us/powders.html

The other information that you would need is related to efficiency of conversion of chemical energy into mechanical work. It's called thermodynamic and gets complicated quick!
To get around 30% of energy conversion efficiency you need a barrel which is roughly 8 or 9 time the expansion ratio in inches of barrel.
Here is the best link that I've found:
http://www.z-hat.com/Efficiency%20of%20the%20300%20Hawk.htm
In this page you have the efficiency of 308 (30%), 30-06 (27.5%) and 300 Win Mag (25%) compared for a 24" barrel.
For the graphs a 20" 308, a 24" 30-06 and a 28"+ 300 Win Mag all have the same efficiency of 27%.

Last comment, my favorite bolt-action is a 20" 243 Win which is probably less than 25% efficient but it is flat shooting and handles very well.

Alex
 
AlexF said:
338 Federal can only use medium sectional density (medium ballistic coefficient) bullets at medium velocity so it's not that good at long range.
338-06 is much better and 338 Win Mag is probably the best common large game long range cartridge. The big downside is that both have significantly more recoil than 30-06.

270 WSM and 7 Rem Mag deliver better than 30-06 long range performance with a 30-06 recoil level. That is the key to 0.270" and 7mm high velocity cartridges popularity.

It's really tough to argue with a 140gr Accubond travelling at 3200fps ;)

Alex
Click to expand...

Some would say it is hard to argue with a 250 gr bullet travelling at 2200 fps (358 win) i guess it would depend on the situation. I shoot a 338 win mag for moose and elk as most elk that I have seen are 200 yards+ if I hunted strictly in the bush I would have bought a 338 federal. My buds shoot 308 and 358 with open sights in the bush.

G
 
I've hunted with everything, now I shoot .338 winmag and will 'til I'm too old. lots of theories and ideas out there but if you can shoot confidently thru the kick, the 338WM will do it all. Mine is a $450 Savage I bought from P&D it is light, accurate,fugly and I love it. The kick is not so bad and I'm not so big or young, but it is entirely manageable. There aint no free lunch.
 
Thanks for the info Alex.

I now know a little more, and am starting to know that I dont really know all that much.
 
There are other ways of visualizing the expansion ratio, consider for example the bore volume and sectional density. Lets assume 3 bore of different calibers, a .224", a .284", and a .338". Let assume that each barrel has a 20" rifled section, the volume of each rifled section, discounted that lost for the lands is:

.224 = .788 cubic inches
.284 = 1.267 cubic inches
.338 = 1.790 cubic inches

The greater the bore volume per inch of barrel, the shorter the barrel can be yet to contain a given volume of propellant gases at any given pressure.

A bullet's sectional density reflects how efficiently the propellant gases can drive it. Lets consider a 7mm and a .338 bullet each with a SD of .231. The .338 bullet will weigh 185 grs while the 7mm bullets weighs 130 grs. We would expect those two bullets to have similar velocities when fired from the same case (in this example .308, but it would hold true with the .30/06 case or the magnum case) loaded to the same pressure, from equal length barrels.
 
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