Projecting a sling stone at the target a sling stone, vs a tennis ball, vs a bowling ball, vs beach ball sized projectile - fun discussion. I got to thinking that the V's Improved Sling spell could optionally project much larger ordinance without any change in level; which in turn should see a much larger base damage for the same spell power.
If the magus then wishes to optimise the build of the spell, they'll get as large a projectile they can using the same base cost in magnitude. I'd guess that a sling bullet is probably not the largest projectile created by the base cost for Terram, but is close in terms of not paying extra magnitudes. Wikipedia indicates that slings could be anywhere from 50g to 500g (2 oz to 1 lbs) in weight (en.wikipedia.org/wiki/Sling_%28weapon%29). So lets say 1 lb as a base, as its easier on the math (I'm an Aussie and working in imperial measurements hurts my brain sometimes).
There is a difference between baked clay, stone, lead and such as sling ammunition was improved (slinging.org/wiki2/index.php/Main/PROJECTILES), and also in the use of ovoid bullets which were improvements over stone balls and rocks.
It seems that the concept of a sling bullet properly shaped is well within the understanding of the 12C, so safe to assume that the magus would opt for shaped ammo vs just using spheres. But its getting too meta-detailed to stress when the damage is a combination of the speed and size, rather than size alone, given that a bullet can get close to 90 meters per second from a standard sling.
So a 0.5 to 1 lbs sling bullet inflicts +5 damage, and can be improved by 2 Mags, to a rough maximum of +15.
- What is the base damage for a 15 lb projectile with the same velocity?
Base target for stone is 1 cubic pace, which is much bigger than a sling (far far larger!), weighing around 150 lbs.
Given that the base magnitude is not altered downward for less mass (150 lbs vs 1 lb) then the magi considers changing Target to allow many projectiles, without too much increase in Mags. They have to pay for that increase, by changing the target from Individual (as a single projectile) to +1 mag for a group of up to 10 projectiles.
That all means each projectile can still be 15 lbs in weight, which is a very dynamic increase in projectile size. I tried to dig for an indication of what a 15 lb or 150 lb projectile would inflict, but can't find an accurate reference. A 150 lb projectile sounds far more like a catapult's ammunition.
Even recreating the spell with +1 mag for 10 objects gives x10 targeting rolls, but also the potential to inflict a lot of small wounds at d+5. Even if only an average of 4 hit, the spell has increased in effect dramatically for a small cost. Using the same damage scaling in HoH:S that could be x10 d+15 impacts; with a spell level of ReTe30. Sure that is high, but certainly possible to create.
The drawbacks I can see is having enough ammunition available to actually use that many stones, and also carrying that ammo around.
Which ends my thoughts with an invested staff which casts this spell ReTe30 effect, drawing ammo from a backpack worn by a grog. You could even enchant the backpack (or even a smaller device) to affect the backpack to lessen the weight while it is within the pack.
Then go all out and give the invested effect a +1 mag for complexity, which allows the user to chose single fire (x1), burst (x3) or full auto (x10) rounds to be fired.
Have I broken the 12C mindset yet? 
