I had a little peek to refresh my memory - it seems to be this projectile: 'exploding_monster_bombard_shot' according to the EDU entry (Did yo know that we put the Krupp symbol on the canons barrel?)
In descr_flaming_projectiles that leads to 'ground_impact_explosion_bertha_set' as well as descr_area_effects with 'ae_monster_shot_large'
Code:
projectile exploding_monster_bombard_shot ; Dicke Bertha
flaming monster_bombard_shot ---> descr_projectiles
effect huge_cannon_ball_set ---> descr_arrow_trail_effects
end_effect ground_impact_explosion_bertha_set
end_man_effect ground_impact_explosion_bertha_set
end_package_effect ground_impact_explosion_bertha_set
end_shatter_effect ground_impact_explosion_bertha_set
end_shatter_man_effect ground_impact_explosion_bertha_set
end_shatter_package_effect ground_impact_explosion_bertha_set
area_effect ae_monster_shot_large ---> descr_area_effects
The detailed 'ground_impact_explosion_bertha_set' is in descr_effects_impact which has a ton of effect lines, those references you can find all over the same file, two are default afaik, the rest is custom:
Code:
{
play_time 2
lod 1000
{
ground_dirt_explosion_bertha
fiery_bertha_explosion_light
fiery_bertha_explosion_light_02
fiery_bertha_explosion_light_03
ground_dust_wave_bertha
ground_explosion_rising_bertha
explosive_fiery_cloud_bertha
ground_fiery_explosion_bertha_flash
ground_fiery_explosion_bertha_flash_02
ground_fiery_explosion_bertha_flash_03
ground_fiery_explosion_bertha_flash_04
shockwave
ground_dirt_smoke_1648_bertha
cannon_hit_ground_sound
bertha_smoke_linger_huge
bertha_smoke_linger_huge
bertha_smoke_linger_huge
}
}
All that's left is to follow up on the lines marked with blue in the same manner. Somehow I would think further 'set' entries will possibly lead you to descr_artillery_effect as well as descr_muzzle_flash. And then there is descr_sound_weapons to give the unit just the right teeth shattering sound effects when firing.
Designing a complex custom projectile is one of the more intricate modding procedures in M2TW.