General Ballistics Question

[danacobb]

First thing; I'm new to the site and long range shooting, but enjoy hering from all the very knowledgable people on this site.

My question is general, but I don't understand why you get more or close to the same velecity out of larger 30 caliber (150grn) bullet, than a 270 caliber (130grn) bullet using the same case.

Example: 300WSM 150grn @ 3300fps
270WSM 130grn @ 3275fps

These are factory ammo specs. Are they using more powder with the 300? I don't currently hand load, but am curious about the physics behind it.

Thanks,
Cobber

 

[Michael Eichele]

W/o going into mass details, the general idea is that the 308 cal bore is bigger than the .270 bore, therefore there is more Cubic inches of space for the powder to burn. Think of it as having a longer barrel. This is one reason why I personally dont use a 7-08 or a 243 win. These are both necked down from the 308 win case. You can push heavier bullets with higher BC's faster with the same barrel length in 308 vs a 243 or 7-08. I am not saying that either cartridge is useless, everything has its place. I admit I am bias when it comes to the 308 bullet vs the 7mm bullet. 7mm rifles need much tighter twists to stabilize the bullets which generate alot of torque upon recoil and they are typically barrel burners, they recoil hard and they are hard to get accurate (typically) Then we go back to heavier bullets, more energy, higher BC w/more velocity with 308 cal vs 7mm. Same barrel legnth and all. The 308 and 338 cals make sense to me

[ 12-13-2003: Message edited by: meichele ]

 

[danacobb]

That makes sense, more surface area for a more efficient burn with the same case, and different bullet diameter. Thanks for the info.