Cartridge airspace

  • When we keep a live round between our fingers and we shake it, we can feel the propellant from inside moving. This is not the situation for a .303 British round [7.7X56R] loaded with cordite or shotshells. So there is some amount of empty room inside of a loaded cartridge, inside space full with air. —> QUESTION: Does that airspace influence in any way the propellant burning and the type of propellant used [slow or fast burning propellant] in its performance ??? The blank cartridges have much more airspace than regular rounds with bullets. Thanks in advance for any reply, Liviu 11/06/08

Hi, target punchers use cotton wool or similar to hold powder in uniform positions, thats bout all I know, cheers


It’s called loading density. The percentage of the available powder space that is used and actually occupied by the powder.

Yes, it is important and does affect the internal ballistics up to a certain point. The ideal is 100% loading density and most accuracy shooters (Benchrest etc) will strive for that ideal. We do it by selecting case size, bullet weight, and powder type to achieve what we call a balanced load. Only a very few factory cartridges can be found that approach 100% density and, in real life, any loading density of 80% or more is considered good.

As new powders are developed it is possible to design smaller and smaller cases that will give the same or better ballistics than much larger cases. The 30-06 vs. 308W is a good example. A more extreme example would be something like the tiny 22 cal Benchrest cartridges that will equal the velocities of older cartridges like the 22-250 with 1/2 the powder charge.

The US military is famous for spending our tax dollars to investigate things like loading density. One such test completed in the 1940s showed that the position of the powder in the 30-06 cartridge affected the velocity by only about 1 1/2 %. Not enough to worry about for a military cartridge and well within the normal range of velocity extremes that you would expect from any factory loaded ammunition.

Many pistol cartridges are notorious for having very low loading densities but the powders used are very fast burning and are little effected. The same for special rifle loads such as blanks, gallery cartridges, etc.

Internal ballistics is a fascinating subject. Most Benchrest shooters are ameteur ballisticians. They have to be if they expect to hear their name called when the trophies are given out.


…mmmhh Ray… maybe it would be better to shorten the 6 mm Pivi magnum body of about 5 mm to reach a good loading density…

My italian friends are passing the hat for build a rifle so chambered,maybe a rechambered Sabatti Rover 870 :)


Or use a very slow burning powder. Some of my shooting friends like the big cases similar to the Pivi Magnum. They load them with 50BMG powders and usually get 100% + density. The slow powders also add considerably to barrel life. 50 years ago the Pivi Magnum was considered very much over bore capacity simply because of the powders that were available.

Your best approach would be to chamber a rifle for it, fire some test loads, then shorten the chamber, fire some more, shorten it again, etc. Then you’ll know what works best. Developing a new cartridge is not cheap.


OK, all of you wildcat cartridge collectors out there who are now rushing to look up the 6mm Pivi Magnum - save your time. There are only a very few known to exist and they are in the collection of Mr Pividori. (OK, I have one also) ;) ;)


…very scarce cartridge… :)

from left: 6 mm Norma,6 mm PPC, 243 Win,6,5-284 Norma,6,5 x 55,6 x 62 Freres and three 6 mm Pivi magnum samples with different bullets

Ray thats a good answer, its a deep and complicated subject and you made it very readable.

Liviu…Many of the earlier military blank cartridges are loaded with “EC Blank” powder…VERY POWERFUL STUFF…so it doesn’t take much ( 5 to 10 grains) to make a big bang. This powder is/was specifically designed to be used only to propel a wad or paper bullet down the barrel; used to propel a metal bullet, it can be disastrous !! It is usually off-white in color, but other colors like pink are also known. There are stories of guys breaking down blank cartridges and then using the powder ( full charge) to reload bulleted rounds…one round fired and the firearm was toast. Just as an aside, early military blanks like .30 Army also have about 5 grains of EC Blank powder in the nose of the paper bullet; makes a bigger noise and blasts the paper to smithereens upon muzzle exit. Hopefully, this post does not get to far into the field of “reloading” !


It goes deep into the burning characteristics of different powders and how the fire travels through the charge during the two thousands of a second or so that it all happens.

Some charges, particularly in shotgun cartridges, need to be compressed and a force of 50lbs is typical during the loading of shotgun cartridges.

Its an amazing subject and growing more amazing as new powders come on line all the time. Much of it goes on without us even being aware it is there.


As the others have said, ballistics is a complicated science and there are few simple answers. I was purposely vague in my answer otherwise I’d end up with a two page response and still not get to the meat of an answer.

But, just to add a little more - the reason that a higher loading densisty is better is because the more empty air space in a case the harder it is to ensure uniform ignition and, therefore, uniform exterior ballistics. For example, if the case is only half full, the powder could be up against the primer, lying evenly in the case, or all up against the base of the bullet. When ignited, you will get slightly different burn times and pressures. This is complicated by the particular powder that is used because different powders react differently. Some perform best under higher pressure or temperatures, some perform better if they are compressed (shotgun, black powder, etc) and some, such as ball or spherical are hard to ignite even under the best conditions. (That’s why you seldom see Benchrest cartridges loaded with ball powders).

Factory cartridges are actually a compromise. A particular caliber may be loaded with 4 or 5 different bullets at different velocities so the factories will use different powders to get the best density they can. But factories have an advantage in that they can mix and blend powders to meet their needs and then have the powder manufacturer make them a ton of it.

And just when you think you have everything figured out, a new powder comes along with new or improved burning characteristics and you start over. But, that’s been going on since the very first firearms were invented and is not likely to change.


Another part / factor of this air space is that when testing weapons / cartridges or proofing weapons the operator often takes care to handle the cartridge carefully and to perform a smooth, somewhat slow vertical, then horziontal movement first to evenly distribute the charge before chambering. Thus insuring a repeatable distrubution of the charge in the available air space. This method, or variations of it, is often noted in manuals for test operators. Sometime it is even seen printed on / in proof boxes.

I’ve no doubt some (most?) bench rest shooters also practice a carefull, repeatable cartridge handleing in obtaining the amazing scores some of them achive!

To add data to the words: a shooter I know has performed speed tests with .357 - and .44 Magnum revolvers, loaded with (safe) half powered cartridges fired under two different circumstances:

  1. The revolver pointed to the target from a downward position (so the powder was in the front part of the cartridge against the bullet)
  2. The revolver pointed to the target from an upward position (so the powder was in the rear part of the cartridge against the primer).

He measured an average speed difference of 45 m/s (148 ft/s) in the advantage of the second circumstance for the .357 Magnum and about 40 m/s (131 ft/s) for the .44 Magnum. So even the location of the powder in the case has a substantial influence on the speed of the bullet!

Powder position can even have a dramatic effect in full power service loads in .38 Special. I remember chronographing some loads for my Colt Detective Special prior to a state IDPA match. The first shot from the cylinder was always dramatically lower in velocity than succeeding shots.

I imagine Cowboy Action Shooters have it even worse given that some try to use the lightest loads they can without leaving a bullet stuck in the barrel. Unless they’ve changed the rules regarding caliber and power factors, it is only a matter of time before some enterprising CAS shooter converts a Ruger Bearcat or Single Six to .25 ACP. There are now extra bulky powders like Trail Boss available to fill up large cases like the .45 Colt when using light loads with lead bullets.

While not limited to strictly internal ballistics, I highly recommend “Hatcher’s Notebook” by Maj. Gen Julian S. Hatcher, a career U.S. Army Ordnance Corps officer and later a big player in the NRA technical branch. This volume gets into all aspects of weapons and ammunition covering tirvia and questions most people never thought to ask. Lots of history, as well as technical stuff. Out of print but copies turn up fairly often.

Hatcher is also author of several other books on similar topics. All considered very authoritative (for their day) and very readable.

I admit that I would not be eager to read most books devoted exclusively to internal ballistics, although insomniacs may welcome the opportunity.