Is there a simple rule of thumb for Case Capacity vs Barrel Length vs Bore Diameter?

quote:

Originally posted by Rhodes:
Is there a simple rule of thumb for Case Capacity vs Barrel Length vs Bore Diameter?

If there isn't there should be! roger

I think that there should be two different rules of thumb; one for case volume versus bore diameter that tells you when your cartridge is so big that it will wear out your barrel before you can develop a load (see the "Crazy Wildcat" thread). And a second rule that is case capacity to barrel volume, (i.e. how much pressure is still pushing the bullet when it exits the muzzle) that you then figure out your useable barrel length depending on bore diameter.
Of course there will never be rules of thumb for these as none of us ever agree on either of them - how many shots is too few to wear out a barrel or is it worth carrying that extra few ounces of metal for the little bit of extra velocity that last inch of barrel is adding.

Yep.....ok,

Not trying to re-invent the wheel here. I remember reading posts comparing bore size vs case capacity ratio between one cartridge and another that is accepted as being efficient. Just as a guide.

Also read a post but can't find it for the life of me, discussing suitable barrel length for a given case capacity. Hence the question.

I have QL but don't profess to be an expert in it's use. Guess I'll play with it some more and get myself into trouble :-)

Rhodes
DRSS

quote:

I have QL but don't profess to be an expert in it's use. Guess I'll play with it some more and get myself into trouble :-)

What QL will normally show, unless you have a VERY SMALL case capacity, is adding barrel length will add velocity. Question will be in the real world how much barrel do you want to carry.
what I kind of lean too is 7x57 size 22" 06 size 22-24" normal mags like the 7mm and 300wmag 24-26 real overbores like the 257Wby, utlras etc 26" minimum.

I would really love to have about a dozen 1.5" cylindrical barrels about 36" long to fit to my rail gun.

I think I could very profitably pass the entire winter just measuring velocity with half a dozen cartridges as the barrel is shortened in 2" increments from 36" to 16".

Several years ago I had the opportunity to test, with a Savage 12-BV-SS, single shot bolt rifle in 308 Win. I fired about 300 rounds with different bullet weights, then rechambered to 300 Win Mag and fired about 300 more, then out to 300Wbee and the same procedure.

A friend suggested that the only fair way to find case volume was to drill the primer pocket out to 1/4", seat the bullet(s) to just chamber, and then fill them with powder (IIRC WCC-846 for it's easy flow measuring characteristics), and then empty them in to the pan on the scale. We did each once-fired, neck-sized, dummy bulleted case ten throws to find actual loaded round powder capacity.

I need to dig those notebooks out, and try to post a small report.

Rich
healing

PS: what Paul says here is a very realistic yardstick.

Thanks ramrod. That looks like a pretty good yard stick.

We all know adding barrel length increases velocity but if you plotted barrel length vs increase velocity for a given load I'm guessing you would get a curve where, once you are past optimum barrel length, you get into ever decreasing returns. I guess I could run this through QL to get a rough guide.

Rhodes
DRSS

A very useful formula for determining efficiency of a cartridge is the case capacity in cubic inches ÷ cross-sectional area of the bullet. Here are some examples:

30-30 Win. = 2.4
308 Win. = 3.0
30-06 Spfd. = 3.6
300 Win. Mag. = 4.8
300 RUM = 6.1

From the 30 caliber examples we can see that anything over about 4.0 starts to require a longer barrel than 22 inches, while anything less than 2.5 is quite efficient and loses little velocity with shorter barrels.

quote:

We all know adding barrel length increases velocity but if you plotted barrel length vs increase velocity for a given load I'm guessing you would get a curve where, once you are past optimum barrel length, you get into ever decreasing returns.

No doubt you will get a change in the curve at least in actual measurement. Hopefully QL would show the same. Speed of powder will have a huge effect on the curve and the drop off point as well. Looking a a 300wmag using a max load of RL22 from 20 to 28" . From 20-22 the increase is 84fps from 26-28 it is 58. The drop in the increase appears pretty smooth. Doing the same test with H4831 the increase from 20-22 is 80 from 26-28 is 57. There does appeare to be a slight change in the curve after 26.

QL will give you a good set of calculated numbers. My guess you will find the more over bore the slower the powder the longer and smoother the curve. Faster powder smaller cases will show more obvious change.

After all those calculations and plotting speaking from my limited playing with cutting a barrel back I wouldn't bet you a that your calculations would match real numbers.

Off the subject but you can do pretty mcuh the same thing with case volume and velocity for a give caliber. Take all the cases for a given caliber take max velocities adjusted for barrel length and you will get a pretty smooth curve until it breaks over. I've used that for years designing wildcats looking for the most efficent point (case volume) for a give caliber.

Rhodes, potential cartridge performance boils down to three variables: 1) expansion ratio, 2) peak pressure, and 3) ratio of charge mass to bullet mass. The Powley computer is based on this, and while it's not spot-on accurate, it's a good starting point.

In these 3 parameters, the expansion ratio compares the barrel length and bore area to the case capacity. The mass ratio effectively compares the bullet weight to the net case capacity.

Powder selection is a different problem and is also addressed in the Powley computer; you might read more here. Powder "speed" is determined by 1) peak pressure, 2) the ratio of net case capacity to bore area, and 3) the bullet sectional density (ie. the ratio of bullet weight to bore area).

I should add the above applies to cases filled with powder, which is what you use for top performance. For reduced loads, loading density enters the equation.

Karl