[Daizee]

I learned that I can shoot .357 bullets with my rifle if I get a .45 barrel. which got me thinking about charge efficiency in different bore barrels...

Hopefully someone with actual experience can provide some real info.

Following my intuition, it seems that for a given sub-caliber bullet (in a sabot sleeve) and matching powder charges, the smaller diameter bore will use a sub-maximum charge more efficiently (i.e. extract more velocity) due to the decreased bore volume following the bullet.

For instance:
.50 barrel with .429 bullet and 80gr FFG.
vs.
.45 barrel with .429 bullet and 80gr FFG.

Furthermore, it sounds reasonable to me that a longer barrel in the smaller caliber (for the same bullet) will make be able to stretch the burning powder out longer as well.

Is it true, empirically from anyone's experience, that:

1) the same charge/bullet out of a narrower bore is faster
2) adding another (say) 6" of barrel to both bores, the narrower bore gains more?

Obviously with the .50 bore you could shoot .452 bullets at the same 240gr of the .429 bullets, and that might be a more effective combination within a given target distance. Conversely, for the same 240gr you'd get more range and penetration from the .429 bullet with better aerodynamics.

it's the intersection of all these ballistic trade-offs that interest me.

Does anyone have actual numerical comparisons?

I'm theorizing about the best loads for my 22" .50 carbine, and if getting an extra .45cal 28" barrel makes ballistic sense. Clearly I *want* one, but I'm wondering if the numbers actually justify the expense. In my state .40 is the minimum legal projectile diameter for deer. The .358 bullets in the .45 barrel would just be for cheap, convenient paper-punching.

No, I still haven't even shot the dang thing yet, but such is my obsessive mind. :-)

-Daizee

[arcticap]

MMP and Harvester are now making sabots that shoot 10mm/.399-.40 caliber bullets in .45 & .50 caliber.

http://www.mmpsabots.com/

These new sabots are helping folks to obtain better accuracy and downrange performance from their fast twist .45 barrels by offering more bullet options where they were previously lacking. They have only become available recently.

Here's a small Traditions trajectory table:

http://www.traditionsfirearms.com/files/ballistics.pdf

[Daizee]

That pdf is intereting.
Most of the information doens't reveal too much of note, but there are a pair of lines where the claim is that identical bullets/sabots/charges in two different bores (.54 and .50) produce the same velocity. This seems weird.

More interesting is the retained velocty difference between .452 and .429 bullets of the same weight out of the same bore over the same powder charge - but this isn't news, of course.

What I'd like to know is if I can get more velocity "for free" by shrinking the bore to somewhat closer to the bullet diameter, and what kind of increase that would be. And how does barrel length affect velocity for given powder charges? I'm much more familiar with the smokeless trade-offs.

Normally the trade-off is to switch bullets and powders, but I'm interested in the effect of swapping barrels for specific loads. I like the .429's for long-range ballistics, and those bullets would shoot in either a .45 or .50.

Why are the sabots so expensive, anyway? each one costs as much as one of my bulk-purchased cast bullets.

-Daizee

[arcticap]

Why are the sabots so expensive, anyway? each one costs as much as one of my bulk-purchased cast bullets.

Manufacturers charge the full list price for those hard to find sabots but it does includes shipping. But many sabots can be purchased at Midsouth and other online discounters for a lower price.

http://www.midsouthshooterssupply.com/deptsearch3x.asp?dept=%4D%55%5A%5A%4C%45%4C%4F%41%44%49%4E%47&dept2=%43%4F%4D%50%4F%4E%45%4E%54%53&dept3=%53%41%42%4F%54%53%20%4F%4E%4C%59

The pdf link was intended to show the higher velocity of the .45 and also that .40 caliber bullets are listed but not any .357's.
Plus it was handy.
If you go to the MMP sabot home page listed above and click on the different color sabots, there is a page of velocity data for more bullet weights.
The separate pages do make the velocities harder to compare, but at least there's more data to sort through, including some for .357 bullets.

http://www.mmpsabots.com/blue.html

http://www.mmpsabots.com/tan.html

http://www.mmpsabots.com/green.html


About your ballistic performance questions. There is a formula relating to powder efficiency versus the diameter of a barrel per inch. I don't know how to find the formula, figure it out or how much that really matters.
That's because the formula is not going to provide a real life measurement of downrange accuracy or performance.
Notice that the MMP data doesn't list loads over 80 grains of powder for .40 or .357 caliber bullets in a .45 rifle.
There isn't any sabot listed that will shoot a .429 bullet out of the .45 either.
There's basically 2 perspectives. One is to choose faster lighter bullets for flatter shooting, or heavier, larger diameter bullets for more knockdown power.
Whether the .45 is actually more accurate or not depends on the rifle and variables like the effect of the wind on lighter bullets.
The .45 may indeed be more accurate, but will it have as much knockdown power at 150 - 200 yards as the .50?
A longer barrel is a plus, but people make their choices based on gut instinct, preferences and what rifle model is available that they like and can afford.
The .45 would recoil less using lighter bullets, but the barrel is also heavier per inch verses the .50 which then needs to be carried around in the woods before it's even aimed and fired.

What if you were to discover that a certain caliber provided the best powder efficiency for any given barrel length but that it's only meaningful if shooting light weight bullets?
Maybe it's better to first choose the range of bullet weights that are best suited for the intended purpose.
The .45 bores work fine for many, the .50 bores work fine for most, and the .54 can basically take down any game in North America.
Yet the .50's are the most popular BP caliber in America for many reasons. :)

[Daizee]

I've poked thru the individual sabot numbers a bit. Fortunately they used the same barrel length for the various tests. Some of it is interesting, but it would be a great project to graph out a set of numbers like that and find the sweet spots.

There's a good comparison between .50 and .54 shooting .430 bullets of the same weight. At 90gr equiv charge, the .50 shoots faster, which is what I expected would happen for medium charges. The same holds for .452 bullets in the same bores - about 100fps gain to the smaller bore in either case.

Clearly practicalities will intrude on all the theory. It's nice to know what's being traded off, tho, as it informs accessory choices down the line.

I'll chrony some loads the first chance I get. here's to hoping the plastic sabots don't destroy the chrony. Any idea how quickly they leave the bullet? Heh. Could get messy.

-Daizee