Well here is a beginner question.

When reloading cartridges like the 300 WM with loads close to what is specified in reloading manuals, it seems more often than not there is plenty of room for more powder. You shake the reloaded case and it sounds like a muffled rattle. Others like the 7x57 oftem have the case full and sometimes the loads are compressed.

In the Nosler manual they list load capacity % and it's often well below 100%.

So what is the extra space for? If the case could be made smaller so that there was very little extra capacity what would that do?
Is this what is meant by "over bore"?

Thanks for any info,

David

Not all powders listed in the manual are optimum. The most accurate loads tend to happen around 100% fill, but many reloaders will load a charge that is 90%, just because they have that powder on hand, and it will work satisfactorily.

"Overbore" is short for "over bore capacity". As case capacity goes up, for any bullet diameter, a point is reached where the energy conversion from chemical to bullet kinetic energy becomes less and less efficient. In the 30-06, 7x57, and many other instances, case capacity is well matched to bullet diamter. In other cases, such as 22-250, the efficiency is lower. You're burning much more powder to get a little more speed. Such rifles tend to be more picky about what they will shoot well, and barrel life is shorter.

When I work up loads for a new round, I look fora powder that will get me close to 100% density & still give me the vel. I want. The reuslts are often more uniform loads & improved accuracy. Example, I recently strated using H4350 under a 210grNP in my .338-06. The load is slightly compressed & I get 2750fps w/ SD in the single digits, accuracy is better than any powder/bullet combo I have tried yet.

Expansion of propellant gases mostly.

Consider the following simplification:

1) Take a 22 long rifle. Assume that all the powder burns instantaneously, before the bullet starts to move. You have a case with a volume of gas at a certain pressure. Now when the bullet moves a short distance, approximately equal to the length of the case, the pressure drops in half, because the volume occupied by the same amount of gas doubles. By the time the bullet gets to the end of the barrel, the pressure is much lower than the peak pressure.

Now take a hypothetical 50 bmg round necked down to .22 caliber. It takes more powder to do so, but suppose you get this case to the same pressure as the 22 lr. In this case, you can see the the pressure will not drop nearly as much by the time the bullet reaches the muzzle.

So, just because you can't fill that extra case capacity, doesn't mean you don't get a benefit from it.

Consider two cases of similar capacity like the 30-06 and the 25-06. The maximum amount of powder you can put in a case is determined by the amount of pressure in the barrel. Pressure is affected by powder burn rate and area of the bore. If you try to cram the same amount of gas into a smaller bore you get higher pressure. The 30-06 can shoot higher charges of similar powders because it has a larger bore (and lower pressure). To get the most ouit of an overbore cartridge like the 25-06 you have to use less powder (leaving more psace in the case, and lower load density) or a slower buring powder that does not create pressure as quickly. You also need more barrel to completely burn the slower powder.

Some cases are so large compared to the area of the bore that you can cram more and more powder into it for very little increase in velocity. Look at the 7mm Weatherby and the 7mm RUM.

The most efficient cartridges are the ones where the case capacity matches the bore area fairly closely. In an efficient case you can achieve the maximum velocity with a standard bullet weight for the caliber, in a reasonable barrel length.

Most reloaders will say there is never enough room in the case.....some loads do not fill the case for sure but many others will fill the case with the "right" powder and compress it.....and want more.

quote:

Originally posted by DKim:
Well here is a beginner question.

Hey David, Looks like "3" to me.

quote:

So what is the extra space for?

It is for use with a Slower Powder than what is shown in your Manual and/or to hold the Pressure down with the Powders shown.

quote:

If the case could be made smaller so that there was very little extra capacity what would that do?

That would "limit" being able to use Slower Powders and Increase the Pressure with those existing powders. This is fairly easily seen in a comparison of a 7mm-08 and a 7x57mm

quote:

Is this what is meant by "over bore"?...

This is a very old term created back when there were not an abundance of Slow Powders. Some of the folks answering above are fairly accurate in their descriptions and I see one in particular that obviously doesn't know Powder from Beans.

When the term was coined it meant, the volume of the case was too large for the Powders available.
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The following example is not valid today, but would have been when the term initially came into being.

Lets say you had a 250-3000 and a Wildcat 25 caliber based on a 300H&H way back then. With the Powders available at that time, you could get fine performance with an 87gr bullet in your 250-3000 and run out of case room to keep adding Powder. Of course you used the excellent CHE and PRE Pressure Indication Methods to determine when to STOP adding Powder(and those Methods are still excellent today).

Next you begin loading that same 87gr bullet in your 25-300H&H Wildcat using the slowest Powder available at that time. As your Pressure began to reach a SAFE MAX you STOP adding the Powder (which is a lot more Powder than you used in the 250-3000), but there is still a whole lot of extra room inside the Case. You also happen to notice your trajectory drops off quicker with the Wildcat than the 250-3000 out at 300yds.

That meant of course that the bullet from the Wildcat was actually going "slower" than the same bullet from the 250-3000. So, that Wildcat exhibited "Overbore Capacity", for the Powders available at that time.
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Today we have a vast abundance of extremely Slow Burning Powders, including some Military Pull-Downs. Due to this, you see people trying to "re-define" Overbore Capacity to mean "efficiency" in amount of powder used, etc., but that was not the original definition and never will be.

Thanks for the responses. Does this correct?

So if two cartridges of different sizes using the same bullet are filled with the same amount of the same powder, the larger case (with extra space) has reduced pressure and velocity. The extra space is almost like extra barrel length, but it is before the bullet.

David

quote:

Originally posted by DKim:
...So if two cartridges of different sizes using the same bullet are filled with the same amount of the same powder, the larger case (with extra space) has reduced pressure and velocity. ...

Hey David, Normally yes, "if" that Load is SAFE in the smaller cartridge.

I can't think of any "exception" at the moment, but one of the other folks might.