308 vs 338 Federal

[Robert101]

308 vs 338 Federal

I’m a big fan of the 308. In reading one of my books, “Cartridges of the World”, 12th Edition, it notes that the 338 Federal is a necked up 308 case to accept a .338 bullet. Ok, so then the pressure maximum of the 338 should be the same as the 308 brass at 62,000 psi and 52,000 CUP. Furthermore, the same book states, “Its (.338) muzzle velocity ranges to approximately 200 fps greater than its parent 308 Winchester with similar weight bullets”. Can anyone give an explanation as to why this occurs?

 

[rcmodel]

Because a .338" bullet is 10% bigger then a .308" bullet.

Same pressure, pushing on a bigger surface area.

Same reason you bore out a hot-rod car engine to give it more horse power.

Bigger pistons = more power.

The same holds true in any two calibers with the same bullet weight.
The bigger one always wins.

rc

 

[DM~]

Because a .338" bullet is 10% bigger then a .308" bullet.

Same pressure, pushing on a bigger surface area.

rc

True, but the .338 has a bigger expansion ratio, allowing a bigger powder charge with the same weight bullets as the .308... In this case, bigger powder charge = more velocity.

DM

 

[helotaxi]

The larger expansion ratio simply means that a faster powder is required to get the pressure up to the same level. With PSI the same, the bullet with the larger base has more pressure applied to it and will therefore accelerate faster. Using the same powder for each with the same weight bullet, yes, the .338 can use a larger charge without pressure issues, but the optimal burn rate is different for each cartridge so one or the other is getting shortchanged in that comparison.

 

[35 Whelen]

True, but the .338 has a bigger expansion ratio, allowing a bigger powder charge with the same weight bullets as the .308... In this case, bigger powder charge = more velocity

Exactly. It's the same principal that allows a 260 Remington to achieve more velocity with a 100 gr. bullet than the does the .243. Likewise, a 338-06 to achieve more velocity with a 200 gr. bullet than a 30-06, etc., etc.

35W