I just got copy of nosler #5 and barnes # 3
reloading manual.I was looking at 300rum&338rum,
why 338rum has more muzzle velocity then 300rum
same weight bullet and same barrel 26"
3oorum brass is longer then 338rum ??
can any body tell me???

Well, I cannot speak for the RUM cartridges specifically, but as a general rule, one can get more velocity out of a bullet of a particular weight by making it bigger around, so there is more cross-sectional area of bullet base for the propellant gasses to act upon(ie., lower sectional density). For example, a 200-grain .338" bullet can be driven faster than a 200-grain .308" bullet, at the same peak pressure. This rule is independent of cartridge case design. But, the .338" bullet will also lose velocity faster, if the form factor of the two bullets is identical.

Here is a short table on Swift A-Frame bullets. You will notice that .338 bullets have a greater BC. Is it possible that the greater BC helps the bullet maintain its velocity for a longer time?

CAL WT BC SD
.308 165 .367 .249
.308 180 .400 .271
.308 200 .444 .301

.323 200 .357 .274
.323 220 .393 .301

.338 225 .384 .281
.338 250 .427 .313
.338 275 .469 .344

I do know that of two similarly constructed bullets shot from the same barrel, the heavier one maintains its speed for a longer period of time. The lighter one may be faster at the beginning, but somewhere around 200 yards it starts shedding enough velocity that the heavier one finally passes it in flight. It may depend on the type of bullet being used, since some have greater BC than others.

Take a look at the ballistics tables from factory ammo at the back pages of Gun & Ammo Annual and other "Annual" magazines.

[ 05-26-2002, 23:38: Message edited by: Ray, Alaska ]

The .300 RUM and the .338 RUM has the same parent case,necking up with the .338 RUM you will loose some "lengh" capacity case. The same bullet both rifles,we are talking about different diameter bullet; the more the diameter of the bullet will increase energy ,velocity and the psi will make the difference. The .338 RUM will kick harder than the .300 RUM with the same bullet...

actually the 338 has a slightly shorter case than the 300 or the 375 RUM.