Montana Pete

09-18-2008, 06:02 PM

In the course of my reloading this summer, I have come onto a number of instances where powder weight is well over half the weight of the bullet.

For example, a load for the 22-250 with 36.5 gr. of powder behind a 52 gr bullet. And a number of loads for the .270 that use the 90 or 100 gr. bullets require over 60 gr of powder for the max. safe load -- it varies up to nearly 65 grains depending on the specific powder.

What are some of the implications?

Recoil can be calculated exactly in ft/lb using a simple formula. However, bear in mind that the ejecta coming out of the muzzle will include the bullet weight plus the propellant weight, plus a small additional weight for the igniter material in the primer.

So the 270 rd. calculation would involve the 90 gr. Sierra HP bullet plus 60+ gr. of powder plus a few additional grains for the primer igniter compound.

Could this ratio of bullet weight to total ejecta weight be a basis to identify so-called "overbore" cartridges?

Some of the overbore cartridges were developed between the two world wars, but initially -- with a much narrower choice of powders -- it was almost impossible to make full use of their case capacity. They were highly inefficient with the powders of their day and hence picked up the term "overbore." This situation may also explain the tendency of highly overbore rounds like the 220 Swift to burn out barrels.

I am sure some of the extreme Weatherby rds. are very overbore also, and you could get "wild" ratios between powder and bullet weights if you searched your reloading manuals for these cartridges.

Where this situation of overbore eventually breaks down-- my reloading manuals show that when the overbore condition gets extreme, even with today's efficient powders, you start burning significantly MORE powder but obtain very little if any additional velocity.

Some of these overbores have profited greatly from improved barrel metallurgy as well as the much wider range of today's slow-burning powders, and chemistry has corrected what physics set asunder, so to speak.

Anyway, what are the most extreme examples of the propellant/ projectile weight ratio that you can find?

The most extreme example I could find is a load for the Swift with the 45 gr. bullet. The max load is 46.1 gr. of N204 powder. Add slight add'l weight for the primer pellet and here's a case with more weight coming out the muzzle in the form of incandescent gas than is represented by the bullet.

Not surprisingly, the Swift is a prime example of an overbore cartridge -- to a tee.

Are there any engineers out there who love math who could do something with this?

For example, a load for the 22-250 with 36.5 gr. of powder behind a 52 gr bullet. And a number of loads for the .270 that use the 90 or 100 gr. bullets require over 60 gr of powder for the max. safe load -- it varies up to nearly 65 grains depending on the specific powder.

What are some of the implications?

Recoil can be calculated exactly in ft/lb using a simple formula. However, bear in mind that the ejecta coming out of the muzzle will include the bullet weight plus the propellant weight, plus a small additional weight for the igniter material in the primer.

So the 270 rd. calculation would involve the 90 gr. Sierra HP bullet plus 60+ gr. of powder plus a few additional grains for the primer igniter compound.

Could this ratio of bullet weight to total ejecta weight be a basis to identify so-called "overbore" cartridges?

Some of the overbore cartridges were developed between the two world wars, but initially -- with a much narrower choice of powders -- it was almost impossible to make full use of their case capacity. They were highly inefficient with the powders of their day and hence picked up the term "overbore." This situation may also explain the tendency of highly overbore rounds like the 220 Swift to burn out barrels.

I am sure some of the extreme Weatherby rds. are very overbore also, and you could get "wild" ratios between powder and bullet weights if you searched your reloading manuals for these cartridges.

Where this situation of overbore eventually breaks down-- my reloading manuals show that when the overbore condition gets extreme, even with today's efficient powders, you start burning significantly MORE powder but obtain very little if any additional velocity.

Some of these overbores have profited greatly from improved barrel metallurgy as well as the much wider range of today's slow-burning powders, and chemistry has corrected what physics set asunder, so to speak.

Anyway, what are the most extreme examples of the propellant/ projectile weight ratio that you can find?

The most extreme example I could find is a load for the Swift with the 45 gr. bullet. The max load is 46.1 gr. of N204 powder. Add slight add'l weight for the primer pellet and here's a case with more weight coming out the muzzle in the form of incandescent gas than is represented by the bullet.

Not surprisingly, the Swift is a prime example of an overbore cartridge -- to a tee.

Are there any engineers out there who love math who could do something with this?