Okay, so I’ve been reading this thread for a while now… I’m not aiming to piss you all off, but I probably will.
We are peering into some serious science here.
1st- NO OPINIONS on this subject, PERIOD! Practicing an opinion here can get you killed.
Way back when our government actually did stuff for the benefit of the people they established SAAMI (Sporting Arms and Ammunition Manufacturers' Institute) later “Inc.” and invited all our countries gun and ammo manufactures to join and create standards for the industry. These guys have made careers out of pushing the envelope so you don’t have to.
www.saami.org
In the old days it was trial and error test methodology which required some poor dolt to pull the trigger and couple of saps to clean up the mess, while the smarter guy observed from a distance. The army figured it was better to keep the dolts for collecting enemy bullets rather than blowing them up in testing so they developed a method for estimating chamber pressures for launching projectiles which became known as C.U.P. courtesy Wikipedia- Copper units of pressure or CUP, and the related lead units of pressure or LUP, are terms applied to pressure measurements used in the field of internal ballistics for the estimation of chamber pressures in firearms. The key here is “estimation” though accurate it was still an educated guess.
In the late 50’s and early 60’s the advent of the strain gauge and transducers allowed measurement and recording of actual instantaneous pressures through the entire firing cycle. But enough history, the full read,
http://en.wikipedia.org/wiki/Copper_units_of_pressure
So to answer Joe’s original question, No the “air pressure” increase when seating a bullet in such a small volume is inconsequential in the big picture. Air as an inert gas neither adds nor detracts significantly to the overall reaction ratios during ignition. Unfortunately the thread has grown to include “Powder compression” and “fillers” known as suspension or balancing agents.
Now we are talking some serious science, head exploding science. There are powders designed for compression and there are some that have been tested to be used under light to moderate compression. These will be listed in your loading manual as designated or permissible. If not then “DON’T DO IT”! If you have changed bullet types or weight and are encountering this problem I suggest contacting the bullet manufacturer or saami.
Fillers- This is bad Juju… this practice is a follow on from the black powder days in an effort to insure ignition of a load that may be kept in a gun over an extended period with a piss poor ignition system. With the invention of the modern internal primer cartridge it is unnecessary. It is now used as attempt to control burn rates in hot short barreled loads (Pistols) primarily and is not considered a good practice. IE; if a manufacturer can’t get there normal powder or is trying to cheapen things up.
Hope I didn’t piss you off.