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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?

CWPINST
09-18-2008, 09:50 PM
I don't know how this applies to small bore rounds, but my rule of thumb is that if it takes powder slower than 4350 to get max velocity out of medium weight for caliber bullets it is usually overbore.

Montana Pete
09-19-2008, 01:24 PM
cwpinst--

Could you name a few powders slower than 4350? It's my impression that this powder is roughly comparable in burn rate to 4381 -- I have used both.

Roger T
09-19-2008, 01:25 PM
COTW some years ago had a 50BMG necked to 30 cal with 240 smk and 270-280grs of H-5010 I belive.Muzzel vel. out of a 36" barrel was only 3700fps or so.Thats overbore.:eek:

Larry Elliott
09-19-2008, 02:20 PM
Some of the .17 and .20 cal wildcats use a powder charge that's as heavy or heavier than the bullet weight. I don't have any loading data on them but something like a .20-250 or .20 Swift is pretty well going to have a powder charge that's right up close to the bullet weight unless a 50 gr bullet is used. The same with .17's in the .17-222 class on up unless the heaviest .17 bullets are used. Doubling the powder charge over that of a smaller case for another 100-200 fps sure seems like a case of overbore to me.

Con Cross
09-19-2008, 06:46 PM
cwpinst--

Could you name a few powders slower than 4350? It's my impression that this powder is roughly comparable in burn rate to 4381 -- I have used both.


Mr. Montana Pete would you believe, that ADI Smokeless Powders Handloader's Guide's has under the Powder Equivalents the burning rate info you're asking for.

Con

Tony C
09-20-2008, 12:37 PM
Pete

There is a nice article in Ackley's Handbook I about bore capacity. The 220 Swift is an example of what he calls a low expansion ratio cartridge in which we see high velocity with shorter barrel life. That being said, I would have no concern about owning and using a swift. It would not be my first choice for plinking and target practice, obviously, but I estimate one would do me for many years with occcasional shots at varmints and such.




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.

I will quote:

"By increasing the case capacity to a point where the powder charge exceeds the weight of the bullet, barrel life drops to a very few rounds with no appreciable gain in velocity...

It is generally understood that, if we increase the "boiler-room" behind a certain caliber of bullet and use it, that the resulting velocity and energy will keep step with the powder used. This is true only up to a certain point, beyond which if more powder is added, very little increase in velocity is attained; in some cases it is actually reduced. Pressures under these circumstances are very likely to be enormous."

In another place he mentioned that in his opinion, the .270 caliber should be about ideal for the .308 case capacity.

Tony

Big Al
09-20-2008, 01:02 PM
From Varmit Al.

http://www.varmintal.com/pburn.htm

You will note that the last listed (two) is H-870 and above that is AA-8700.

On this list you will find a lot of other powders slower than 4350.

http://www.reloadbench.com/burn.html

Louis Boyd
09-22-2008, 12:40 PM
My definition of overbore is any combination of cartridge bullet and load which gives shorter barrel life than it takes to develop a useful load and use the gun for it's intended purpose.

For commercial cartridges the 223 WSSM, 264 WIn Mag, 7mm Rem Ultra, and 30-378 Weatherby are pushing into the area of what I consider being "overbore". Still, they give acceptable barrel life for hunting and similar uses.

Barels are replaceable. They're just part of the cost of high muzzle velocty.

Con Cross
09-22-2008, 06:01 PM
Pete

about bore capacity. The 220 Swift is an example of what he calls a low expansion ratio cartridge in which we see high velocity with shorter barrel life.

"By increasing the case capacity to a point where the powder charge exceeds the weight of the bullet, barrel life drops to a very few rounds with no appreciable gain in velocity...
Pressures under these circumstances are very likely to be enormous."

Tony


Gentleman, would you believe, that the Expansion Ratio and the powder Burning Rate has something to do with this.

Gentleman, would you believe, that if the barrel is long enough and the powder is slow enough the pressure is not going to be an issue and than the true definition of overbore is perverted.

Gentleman, would you believe, that even with the somewhat perverted definition of the overbore Mr. Louis Boyd is about right with his personal definition of an overbore.

Con