Auto loading crossbow via RPM

[Varyon]

Originally Posted by starslayer

My thought is that this would give it the same RoF as a repeating crossbow, but maintain the damage of whatever crossbow it is being used on. (assuming that no effective ST increasing technologies like a windlass have been used)

That sounds about right. Realistically, a repeating crossbow should probably either have lower ST than the user or require two seconds to draw, but a single second to draw at full ST is hardly game-breaking. For a crossbow equipped with an ST-enhancing device, said device would work in half its normal time, round up (so a x2 ST windlass requires 8 seconds to draw with this spell).
EDIT: A closer look reveals the repeating crossbow from LT is effectively at a lower ST - it does a lot less damage and has much shorter range than another crossbow of the same ST. It's up to you what default speed you want to go with, but if the device retains its power and range - as well it should - it should probably have the same draw time requirement as before. In that case, an ST-enhancing device works in it's normal time - that x2 ST windlass still requires the normal 16 seconds to draw.

Originally Posted by starslayer

What if the caster wanted to increase the effective RoF?

Use a crossbow with a lower ST (or keep it the same but use Bestows a Bonus to boost the spell's ST). Effective RoF is 0.5*(ST of spell)^2/(ST of crossbow)^2. So, with an ST 16 crossbow and Path of Matter 16, effective RoF is 0.5 - 1 second to Ready, 1 second to shoot. Using that same crossbow with Path of Matter 18 and Bestows a Bonus (Narrow, ST to draw this crossbow) +5, effective RoF is 0.5*529/256=1.03, or 1 - the character can fire a bolt every round.
EDIT: If changing RoF to match with my suggestions above, change the multiplier from 0.5 to 1/3 - an ST 16 crossbow with Path of Matter 16 would have effective RoF of 1/3, for 2 seconds to Ready and 1 to shoot.