First up since I have time now: I let the player who was playing the myrmidarch use it in full attacks because it seemed stupid if you couldn't especially because the Myrmidarch's ranged spellstrike even mentions full attacks.
But if memory serves for spellstrike wording it says that the attack deals normal damage and the effect of the spell. Where as many shot says you fire 2 arrows with 1 attack duplicating composite bow strength bonus, and things like rangers favoured enemy bonus as bonus damage on each.
The spellstrike effect works on the attack roll, not the damage from how I see it.
Second:
Nefreet wrote:
Oh, although, I just realized, I suppose a Myrmidarch with a Double Barrel Pistol could still get off two rays with a Spellstrike.
My reading of a double-barreled pistol is that when you use that ability you make 2 attack rolls rather than 1 for both, so the spell would discharge on whichever you roll first if you tried that.