Hello. I've just joined the AR forums. Great stuff on here.
I have a ballistics question. Why is it that a higher MV can be obtained with a larger dia bullet using the same case and bullet weight? Ex: .35 Whelen with 220 gr bullets will move faster than 220 gr bullets in a 30-06. This is based on reloading manual info.
Thanks for looking.

Expansion ratio.

IE, the 35Whelan has a lager bore capacity in relation to its case capacity than the 30/06 does. This will allow you to use a faster burning powder - for a given bullet weight - in the Whelan than the 30/06.

When you think of cartridge "efficency" this is what you should be thinking about. Similarly, compare for eg a 243 and 7/08.

The larger diameter bullet has a larger surface area on the rear of the bullet for the expanding gases to push on.

quote:

Originally posted by woods:
The larger diameter bullet has a larger surface area on the rear of the bullet for the expanding gases to push on.

Bingo......at constant pressure the larger diameter bullet gets more force.

Pounds per square inch pressure is applied to larger square inches of larger diameter bullet and this means a larger force is acting on it. This generates more acceleration and yes.....a faster burning powder is required to keep up with the greatly increasing volume left by the escaping bullet.

In the same way a 1" diameter hydraulic shaft at 5,000 PSI will put out more force than 7/8" shaft at the same pressure.

quote:

Originally posted by Claret_Dabbler:
Expansion ratio.

IE, the 35Whelan has a lager bore capacity in relation to its case capacity than the 30/06 does. This will allow you to use a faster burning powder - for a given bullet weight - in the Whelan than the 30/06.

When you think of cartridge "efficency" this is what you should be thinking about. Similarly, compare for eg a 243 and 7/08.

One should first say "when all else is equal" the larger bore will produce a higher velocity. Were any part of this equation to change (case volume, bullet weight, pressure) one should expect different results.

Thanks. The hydraulic shaft comparison makes this easier to understand. The incresing volume part is interesting. That would have never crossed my mind and I've been reloading a long time. LOL.

I agree with most of this.

One exception is that slow powders at starting loads can give such low pressures in larger calibers that velocity is slower than the same load in a larger caliber.

The phenomenon you mention is normal once pressures have reached a critical minimum. I am not sure what the exact maths & theory is.

JM$0.02W

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quote:

Originally posted by ALF:
Sectional density ! ...

I can always count of alf being good for a laugh.

Could you "dumb it down" for the rest of us alf???

quote:

Originally posted by Hot Core:

quote:

Originally posted by ALF:
Sectional density ! ...

I can always count of alf being good for a laugh.

Could you "dumb it down" for the rest of us alf???

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"forces of retardation" is that why it took me three trys to get out of the sixth grade?

quote:

The larger diameter bullet has a larger surface area on the rear of the bullet for the expanding gases to push on.

I would also think that bullets of the same weight but of different caliber have different amounts of bearing surface. Example: a 150 gr 7mm bullet has more length than a 150 grain .30 caliber bullet, thus more friction with the bore.

quote:

have different amounts of bearing surface

IMO, while it seems a logical assumption, I believe this factor is quite small once 65,000 PSI is applied. F=MA is the over riding equation. (Force = Mass X Acceleration.)

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quote:

Originally posted by ALF:
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AGAIN .. goodness, alf has more periods than a professional hockey team!

but SD is actually not a "far out" answer as it may seem.. and the 30-06 or 308 case is a good example...

if you pick an SD .. sad, .25, which has been considered the "perfect" north american SD .. and then look at the vels of the various 30-06 and 308 cases and calibers .. there's a pretty close coorelation, IF bullet construction and shape are roughtly the same ...