After finding this image I wanted to write up a magical auto-loader for a crossbow which makes firing each round possible. Crossbows are already interesting in Ars Magica because they inflict a lot of damage (with the optional rules) with not a lot of skill if already loaded (reduced botch dice), but they are darn slow.
With the two enchanted devices I think it is reasonable that a crossbow could be fired at least once per round, if not twice per round; as all the soldier is doing is pointing and pulling the trigger. Lords of Men indicates that shooting a preloaded crossbow is a quick action, meaning the user can fire and move or fire and do something else. With these enhancements they could fire twice – perhaps each enchanted device allows firing in one round, granting two shots per round if both are used.
And the bolts being fired will not be resisted by MR. Designing and placing these items on an arbalest would be brutal, and no harder magically.
Snipers Gleam (ReHe 20)
The gleam is matched pair of brass pins depicting crossed arrows, one is placed upon a crossbow and the other upon the container of ammunition. Each time the gleam is activated it moves a bolt from the container to the crossbow in a firing position. The power is activated by specific clicking tone of the crossbow being drawn back, or a command word.
Effect: Load the crossbow
Rego Herbam 10, R: Voice, D: Momentary, T: Individual
A bolt is moved from a container to the crossbow or bow, loaded into a suitable firing position.
(Base 3, +2 Voice, +1 accuracy in positioning)(+10 unlimited uses)
Snipers Hold (ReTe 20)
The hold is a buckled leather band which is strapped to the soldier’s arm. The effect moves the crossbow’s metal bow after each firing, so that the holder does not need to act to redraw the bow, which saves significant time.
Effect: Draw the Crossbow
Rego Terram 10, R: Voice, D: Momentary, T: Individual
The crossbow’s arm is drawn back and locked properly, ready for firing. Terram is added as a requisite to allow for different materials in the crossbow’s construction.
(Base 2, +2 for metals, +2 Voice) (+10 unlimited uses)
Note: updated with the feedback below.
The Iron-Bound Tome 
I did these as enchanted device effects myself a few years back here:
http://forum.atlas-games.com/viewtopic.php?f=4&t=21531&start=270
the enchantments (there are a lot on that page) were in the Item gaze of a Gaze of the Watchful Fatherm their names were: Pull with the Strength of Giants and Self Loading Crossbow
For loading the bow I did ReHe base 5 but looking at the guidelines again I can’t see why. I also used range special because it was a device and the crossbow didn’t make any noise.
For drawing the crossbow I did terram as my device had a metal bow. The terram spell is more difficult than an Herbam spell because you need a base level of 4 to effect metal. I’d probably raise the level of your Draw the Crossbow spell. I’d use the higher guideline and then add a magnitude for the requisite.
So a the “pull” ReTe Base 2, +2 mags for metals makes sense, and drop the Herbam altogether. Fair enough – I’ll edit.