[Lazy_Perfectionist]

Spells with symbolic precision have a value of 100.

I'm thinking the precision is actual.
If you cast darkness (75% hit to precision), a value of 40 (30 plus base 10) becomes 10.

On the other hand, if it was symbolic, with prec. 100,
the actual precision value becomes a bit more than 25. Considering that precision over ten counts double, thats darned accurate even under conditions of darkness.

Practically speaking, a commander with precision ten plus a precision 30 weapon will rarely deviate.
Lets see...
30+10=40. Since precision over 10 counts double:
10+30x2= 70 precision for calculations.

If the range from attacker to target is greater than precision/2-2, then the missile will deviate.

That means it will shoot 33 squares before deviating. I'll do the math for that in a bit.
Longbow of Accuracy - range 45
Maximum Deviation = distance x 1.25 / precision
45x1.25/70 = 0.8
Longbow of Accuracy will never miss its target square. The defender can still dodge, though- and the longbow is of little use against MA Agartha's very well shielded infantry.

I do wonder if it's considered a magical weapon for calculating hits?

Piercer has less range and accuracy, I'll leave you to calculate that on page 77 of the manual.