What volume and pressure would be agreeable to you? Let's assume the bullet is at the end of the barrel but not out?
It is still thousands of PSI, and even higher for rifles.
I just ran QuickLOAD with 8.0 gr of AA#5 and a 230 gr FMJ.
With a 5 inch barrel length the pressure is over 3000 PSI when the bullet leaves the muzzle.
A .22-250 AI with 41 gr of IMR4895 and a 40 gr bullet shows over 14,000 PSI at the end of a 26 inch barrel (and right around 4,000 ft/s).
Once you ‘uncork’ the pressure vessel (bullet leaves barrel) that gas expands into the ambient air.
It was pushing the bullet before, now it has essentially no obstruction to its free expansion.
It is a few hours of work to go through all the analysis with the Explosive Engineering Handbook (Paul Cooper) and even attempt to compute the products of decomposition for nitrocellulose and then estimate the final state of the reaction products.
The reactions do NOT typically go to completion with simple balanced chemical equations. Welcome to the nasty world of organic chemistry.
This is not combining H2 with O2 to create water (and a bunch of heat).
Cellulose nitrate is C6H7(NO2)3O5 and it is then esterified to produce a stable material.
Everything from mono atomic nitrogen, hydrogen, water, ammonia, and various nitric oxides (among others) will be produced.
Add to the straight nitrocellulose products the deterrent coatings, left over solvents, graphite (carbon), primer compounds, etc., and temperatures well into the 4000F and up range and pressures from 20,000 PSI to over 60,000 PSI and the gases out the muzzle are a real witches brew.
These hot gases will further react with the oxygen and nitrogen in the atmosphere (producing more muzzle flash) until they reach a final state and cool off.