22 rds for a 155 seems okay. But it depends on the type of
mission. The old M113 SP mortar carried 55-60 rds like the old 21
SA mortarmen told me. But mortar fire as compared to 155 howitzer
is more inaccurate, so they depend on volume of fire to bracket a
target...its a cost-effective area weapon IMHO.
155 shots tend to be more accurate, thanks to technological
developments over the years...weather ballons, air density,
humidity, etc etc....all that Firefinder (24 SA) jazz that no one
understands.... 
With the typical
missions that the Primus would face, 22 rds would be sufficient,
especially when ammo trains follow them around anyway.
Arty missions vary. So it depends on whether the mission requires
sustained fire support, and if so, there has to be ammo carriers
nearby. (btw, they do use BV206/Bronco for ammo and POL). But most
missions that these SP guns/mortars usually do, are the typical
shoot and scoot types...that's what those 21 SA guys told me...so
the max would be about 2-3 shots before having to move again;
that's their SOP. And they moved around ALOT from one location to
another after firing a shot or two; it gets dizzying after awhile
for them, racing up and down Area D. For them, their vehicle is
their biggest chance of survival in a counterbattery attack, so
basically they do everything within the confines of the vehicle
during operational mode.
Apparently the person who suffers the most in the SP vehicle is the
layer, or simply known as the No. 2 man, since he has to stay with
the gun/mortar 24/7 in case of fire missions...lol ...I think I'll go
nuts staying in such a confined space for an extended period of
time...gimme the freedom of a scrambler anyday!!!
