I hesitate to differ with some very knowledgeable people here, but unoccupied air space in a cartridge is not a desirable thing and is most assuredly
not intentionally incorporated as a safety feature. You want as little air space as possible, ideally none.
This issue has been examined to death by military and civilian interior ballistics laboratories, including the H. P. White Laboratory, for quite a few decades. They have reached the conclusion which I adopt, above. Many lengthy papers have been written on this.
Military rifle ammunition is loaded to fill the case as far as possible without compressing the powder charge during bullet seating. They want the least empty air space possible.
As you reduce powder charge in a given case, accuracy begins to drop off; first a little, then a lot. The degree varies with powder type. But there comes a point where significant pressure variations are measured, i.e., high pressure spikes. Reduce the charge just a bit more, and you can blow up the gun. This is not a theoretical problem, and there are very specific warnings with certain cartridges and powder types. These are published in all reloading manuals. So more air space creates more danger, not less. And this is not about the risk of stuck bullets in revolvers; that's a separate issue.
The very design of modern cartridge cases focuses on creating just enough internal volume for a given weight of a specific type of powder to burn and generate pressure, within a narrow range, to drive a bullet of a certain weight at a certain velocity. It's quite a balancing act, and it does not include allowing for extra sir space...except, as I mentioned, to avoid compressed powder charges. But even that is sometimes O.K., with certain powders and within known limitations.
A recent example of such major design effort was when our military switched from the .30-06 round to .308 Winchester, aka 7.62mm NATO. The intent was to create a round which would be shorter, would feed better in automatic weapons, and would give almost the same external ballistics as its parent. And minimal unoccupied internal space was part of the target criteria. They got all of it right...as they later did with the 5.56mm service round; minimum or near zero air space.
This air space is an interesting thing to me just now, as I have been battling it for the last month; trying to make my .45 Long Colt cartridges shoot as accurately as those of two of our major ammo manufacturers, who are very secretive about the powders they use. This case was designed around black powder, and it performs very poorly with most loads of most types of smokeless powder, using any bullet weight at all. Many have tried to beat this and few succeed. Working on it today, in fact, once I stop playing with this computer.
Oh yes, there is a powder called "Trail Boss", I believe, which was specifically developed to fill up black powder legacy cases such as .45LC and .44-40 (.44WCF) to get rid of all the problems created by unwanted internal air space resulting from the use of higher density/lower volume smokeless powders. Still trying to find a local source.
I just wanted to cover this so any of you who reload would not be trying to get more air space. But don't be worried if reduced loads give you some. Powders including Bullseye, Red Dot, Unique, and others used for Luger ammo are not so critical with air space; you will absolutely not get those high pressure spikes with reduced loads. Such powders as Alliant 2400 are another story, if you back off too far in such things as .44 Magnum.
But 2400 is not a Luger powder.