Air space with what powder? 
How much air space, and how it affects the charge is dependent on whether it's blackpowder, or smokeless. And then with smokeless it depends on how fast it burns, and not just the pressures, but how those pressures ramp up.
I use a ballistic reloading program that shows the chamber pressures for smokeless, but also shows the curve of how each powder reaches that pressure.
Of course there's plenty of debate on fillers, wads, double base or single base powders, etc., so much goes into how air space affects a cartridge and the gun.

Marlin Guy's offered some good words of advice.
I suppose if you stay within known publisher load data guidelines your safe. Maybe...

Reduced powder charges can be problematic from what I read for a number of reasons.
S.E.E. or Secondary Explosion Effect, from what I have read, is responsible for a number of destroyed guns.
Search 'Ringed Chambers' as well.
In addition to what Marlin guy said, 
Age of the gun.. Steel, action strength, primers, cast vs. jacketed, crimp vs. no crimp, seating depth, etc.

I am a novice shooter but I've being reading published information on the subject (internal ballistics) for 50 years. 

A good read would be Franklin W. Mann's, 
THE BULLETS FLIGHT,
THE BALLISTICS OF SMALL ARMS 

Jeff_Schultz wrote on Sep 16^th, 2022 at 8:43pm:

S.E.E has been adequately explained, with lab data to prove it, as a phenomenon of slightly reduced charges of very slow (and hard-to-ignite) powders used in large high pressure cartridges, with strong jacketed bullets.  What happens is that the bullet is driven into the throat by the primer, where it stops. A few milliseconds later the powder starts to burn, but that bullet is now a bore obstruction.  Boom!

S.E.E is impossible with our cast bullets. Any claim of such in our loads is much better explained by a double charge.  Many of our loads do not even fill half of the available space, so it's all too easy to do and have it go unnoticed.  

Ringed chambers are a matter of loading a wad on top of a small charge of powder.  Again there is extensive experimental evidence published by the late Charlie Dell proving this.  If you must use a filler to keep a position-sensitive charge close to the primer, a bit of kapok or toilet tissue is a much better bet.  I fiddled with this in years past, but never found it very effective.

Apologies in advance for showing ignorance... I couldn't tell if the OP was referring to smokeless or black?  I understood, and practice loading black with zero airspace to prevent the ring in front of the chamber.  It never dawned on me that smokeless powder could cause the same issue using the logic that most smokeless cartridges have airspace in them?  Difference in barrel metallurgy?

PeteG. wrote on Oct 28^th, 2022 at 11:37pm:


Even for black, it is an urban myth...  This after shooting lots of 577-450 in original rifles with airspace in the cartridges.  A load without airspace would be about 100-110 grs....    Wads and some fillers are a higher risk.

I have used the the open celled floral foam in some of my smokeless loads for my 45-70 and 38-55 to keep the powder down near the primer. This stuff is very fragile and I think it would crumble to a powder upon ignition. Never saw any real improvement in accuracy so I quit using it.But I'm not a bench rest target shooter so perhaps the gain in accuracy was underappreciated. More interested in minute of whitetail and gong shooting. 

John