Being somewhat green at reloading and not always the sharpest knife in the drawer, I saw something that raised a question for me the other day. Looking at the 30-06 case and all of its younger offspring I noticed that for a given bullet weight (100 grains) , each time you go to a larger caliber the speed goes up for the same weight bullet. Never minding bc, sd, or intended purpose, I am wondering why this is so. Is this due to having a larger surface area for the pressure to work against, or less bearing/surface area dragging on the bore, or a combination of the two. Or none of the above and I'm lost.

Both, probably more due to the first than the second. You're catching on.... there's more physics involved in reloading and fewer mysterious incantantions than some people would have you believe.....

Oh and third, the larger-caliber bullet of the same weight will take up less room in the case, allowing a slight increase in powder capacity if seated to the same overall length.

Hmmmm Little wheels just a spinning here. I didn't try the math yet but I can see where a larger caliber bullet would be shorter but being larger around the total surface may not change much at all or at least of no major concern. Might this be , though, where the relatively recent bullets with groves be coming from?? Trying to reduce surface area for a given caliber? And if that were true and did in fact produce measurable results wouldn't those grooves disrupt air flow over the bullet ,hence lowering the bc and reducing any gains from reduced friction somewhat??

I reload for the 375 Whelen, basically the 30-06 case necked up to 38 caliber, big brother to the 35 Whelen.
I use 35 Whelen data for my caliber of the same bullet weight and go up. There's more space for the same volumn of gases to expand into, less pressure.
Now if I reverse the process, take 375 Whelen and try to use that data for the 35 Whelen or 30-06 things would get real interesting real fast.
Jim

Being somewhat green at reloading and not always the sharpest knife in the drawer, I saw something that raised a question for me the other day. Looking at the 30-06 case and all of its younger offspring I noticed that for a given bullet weight (100 grains) , each time you go to a larger caliber the speed goes up for the same weight bullet. Never minding bc, sd, or intended purpose, I am wondering why this is so. Is this due to having a larger surface area for the pressure to work against, or less bearing/surface area dragging on the bore, or a combination of the two. Or none of the above and I'm lost.

The large the bore for a given weight bullet, the less of that bullet as bearing area....they get shorter, and have less contact area with the barrel.

Couple of rules of thumb (ust rules of thumb...not exact, and not "laws"):

Given the same case volume, pressure, and bullet weight...velocity goes up by about 1/4 the percentage of bore area increase (how fast can a .243 shoot a 100gr. bullet? How fast can a 7mm-08?)

Given the same case, pressure, and bore area...velocity increases by about 1/2 the percentage of bullet wieght (how fast can a .30-06 shoot a 110gr. bullet? a 220gr. bullet?).

Given the same pressure, bore area, and bullet weight...vel. increased by about 1/2 the percentage of case volume increase (how fast can a .308 shoot a 180gr. bullet? How fast can a .30-06?).

Looking at the 30-06 case and all of its younger offspring I noticed that for a given bullet weight (100 grains) , each time you go to a larger caliber the speed goes up for the same weight bullet. Never minding bc, sd, or intended purpose, I am wondering why this is so.

Expansion ratio, i.e. as a VERY general rule of thumb, the bigger the bore the more efficient the powder burn becomes. If you look at a Powley ballistics computer or any of its' descendants, you'll find that bore and barrel length is part of the calculations.

If you want to see it in action, chronograph a .22 centerfire and a .458 centerfire. Then cut four inches off each barrel and compare how much velocity each lost.

True...none of the facotrs work in isolation. Using the same bullet weight in a 25-06 and then again in the 30-06 also changes the case volume (the short squat 110gr. .308" bullet doesn't seat as deeply, so the .30-06 would also have a small volume increase).

Seldm is the best powder for one round also the best powder for the other. Int eh 25-06 cs. 30-06 example, the best powder for pushing a 110gr. .25cal. bullet at max. vel. is not the best choice for pushing a 110gr. 30cal. bullet its fastest.

One poster, in a previous discussion, used the anaology of water in a pipe or hose. Connected to the same fire hydrant (which would simulate the same pressure) the bigger the line, the more water it moves.

Thanks everyone. That gives me the basic idea.

And you're right about the grooves reducing BC. Lyman's loading manual has shadowgraphs showing how the grooves in cast bullets create turbulance and produce secondary shock waves. All that increases air drag.

-91

How does this thought apply to the new Barnes X Triple Shock. On their web site, the folks there claim "higher velocity" from those grooves. Is it the Copper vs Lead??? Any thoughts??

Cris

How does this thought apply to the new Barnes X Triple Shock. On their web site, the folks there claim "higher velocity" from those grooves. Is it the Copper vs Lead??? Any thoughts??

Cris

Can probably get higher vel. than you could it they hadn't lathed those grooves in it...does cut down on the amount of bullet metal the rfiling has to engrave.

Way bach, Nosler had a "Zippedo" bullet with a large solid shank...they lathed grooves in the shank to reduce engravemnt force. The oringinal Noslers had a rebated secion (like a wide groove) over the solid partition. They decided to drop that feature either becasue (1) it wasn't needed as there isn't that much solid section or (2) the changed to swaging rather than lathing the bullet jackets.