The way a weapon is either activated by the gun or cannon trigger is really in the xdp for the gun. There are two types of "gun" weapons, one called bullet the other shell, with shell being a cannon - usually. So when you pick a gun type in the aircraft xdp it naturally calls this up. But if in the weapon xdp it is to be activated using the bullet or shell classification it really should not matter what you do in the aircraft xdp. If you look in the gun folder you will see such
AvHistory_gr_gun_Mg_17
avhistory_gr_gun_mg_17_cannon_trigger
both are mg 17s but activated two different ways. You will see this with many gun types in this folder. Now look in the weapon folder for the folder associated with the mg_17 cannon_trigger and xdp and you see
<?xml version="1.0"?><UnitData>
<General Allegience="" LongName="" ShortName="" ModelName="" BlastDamageScale="1.0" ImpactDamageScale="1.0" FireDamageScale="1.0" Priority="2" Type="weapon" EnteredService="01/01/1944" BlastDamageMod="0" ImpactDamageMod="0" FireDamageMod="0" AllowSpawn="n" Category="weapon" Country="Germany" Mass=".03033"/>
<Description String="7.92 x 57 x 10.0g"/>
<Weapon WeaponType="shell" ImpactDice="1" ImpactDieSize="1" ImpactOffset="4.03" BlastDice="1" BlastDieSize="1" BlastOffset="1" FireDice="1" FireDieSize="1" FireOffset="2" ExplodeEffect="fx_bltpln_s" GroundEffect="fx_bltgnd_s" AirEffect="fx_bltpln_s" WaterEffect="fx_bltwtr_s" TracerEffect="fx_tracer_blue" />
<Loadouts/>
<GunStations/>
<DamageBoxes/>
<Systems/>
</UnitData>
If you look for the normal mg 17 you see
<?xml version="1.0"?>
<UnitData>
<General Allegience="" LongName="" ShortName="" ModelName="" BlastDamageScale="1.0" ImpactDamageScale="1.0" FireDamageScale="1.0" Priority="2" Type="weapon" EnteredService="01/01/1944" BlastDamageMod="0" ImpactDamageMod="0" FireDamageMod="0" AllowSpawn="n" Category="weapon" Country="Germany" Mass=".03033"/>
<Description String="7.92 x 57 x 10.0g"/>
<Weapon WeaponType="bullet" ImpactDice="1" ImpactDieSize="1" ImpactOffset="4.03" BlastDice="1" BlastDieSize="1" BlastOffset="1" FireDice="1" FireDieSize="1" FireOffset="2" ExplodeEffect="fx_bltpln_s" GroundEffect="fx_bltgnd_s" AirEffect="fx_bltpln_s" WaterEffect="fx_bltwtr_s" TracerEffect="fx_tracer_blue" />
<Loadouts/>
<GunStations/>
<DamageBoxes/>
<Systems/>
</UnitData>
Not knowing this plane or how it was designed, I suspect that selecting the proper gun type could help. Hope this helps.