I make no pretenses of expertise with mortars. With all the great discussion & reading... then researching some of the key elements... Leads me to the following questions:
With conventional cannon... small arms too... the projectile seats on the charge. A reduced charge merely seats the projectile deeper. Wioth a chambered piece, the projectile can go no deeper than the shoulder or reduced diamerter of the chamber. This sounds exactly like a bore obstruction. Why doesn't the piece blow up?
I was reading some translations of the French commands for firing a mortar... and one was to add a bag of earth on top of the charge. This sounds like an inert filler, to fill the excess chamber space.
edit:
"1. Loading the propelling powder... ...It was possible to complement the powder charge with earth when firing a small charge..."
http://www.napoleon-series.org/military/organization/c_mortars.html
PS This source has interesting discussion of chambers.
The old airspace will blow up my gun discussion. Yes you can burst a gun with an air space, but all the credible reports that I have seen over the years testing this phenomenon two things were consistent. First the air space was substantial and the projectile was a stuck or wedged ball. Greener in one of his books mentions this issue in relationship to cartridge guns and he recommend that charge occupy no less the 70% of the available space. That is 30% airspace.
Seem fairly simple, it's not. This topic of airspace/wads/fillers-bursting barrel/ringed chambers in probably the most contentious subject in our shooting sport. What is most interesting is no one provides substantive proof to support their position on the yea side or the nay side--just lots of conjectures and passing old wives tales. One man and only one man, that I am aware of has done any substantive work on the subject in the past 20-25 years and that is the late Charlie Dell.
I haven't heard any recent reports of bursting barrels related to this problem. Lots of second hand, passed down the net reports lacking any sort credibility. I have never been able to locate the actual "victim" for a first hand report.
I have heard a number of reports on ringed chambers, actual real reports of genuine chamber ringing. But getting the details of what really happened is tough, to many people saying this is what happened who have no connection to the incident to be credible. Even finding the actual person doesn't bring many answers because they really don't know what happened..
Interesting huh? Well it is all is irrelevant to most of our cannons...Because we use windage in our cannons, although not the intention, windage bleeds off pressure. The purpose of windage is minimize the chance the ball won't wedge in the bore and act like an obstruction.
The small amount of airspace left in a powder chamber using a reduced charge in mortar is insignificant.
Now that bag of earth the French used is not for a filler but to add to the mass of the projectile and increase pressure and in theroy increase range with a smaller charge.
I would not be concerned with air space in a golf ball mortar with a reduce change..shoot it!
