Spirits are not 100% immune to non-magical attacks, actually. Assuming the player's bound fire spirits were of Force 6, if a ganger gets a single net hit, that AK-97 will deal 11P/-2, enough to pierce the spirit's Hardened Armor: the Fire spirit will roll 7+(12-2) = 17 dice, and get (12-2)/2 = 5 free bonus hits, for an average of 0.94P damage if they hit the spirit with a single net hit. It's very little, but it's not 0. If you give the gangers APDS, it's suddenly 11P/-6, so 7+(12-6) = 13 dice and (12-6)/2 = 3 free hits, for an average of 3.68P damage if they hit the spirit with a single net hit. If the gangers have good dice pools and/or work together to reduce the spirit's dodge pool, they can combine burst fire (-5 on the dodge test) with a Vitals Called Shot (-4 on the attack roll, +2 DV).
Second, binding spirits is pretty expensive. If we assume a player with 12 summoning and binding dice and 3 Edge, spent on Second Chance for both tests, they'll get ~4.7 Services on average from the summoning test, and ~1.82 extra Services on average from the Binding ritual (with a 2.30% of the summoning failing, not counting KO chance, and a 17.99% chance of the binding failing, not counting KO chance). Added together, that's ~6.5 Services in exchange for 150 drams, or 3,000¥. At 450-500¥ per service, even if you use all the services on combats, that's significantly more expenses than the gun users will have.