I don't think an impact ability can affect the shooter, though I haven't tried it. ..........
( it's at least worth me looking into later if I get around to it first, (would be quite useful if it somehow worked) )
........ The second method you described sounds like it would create a trail of explosions toward the target and would hit anything along that path. It sounds cool but it wouldn't look like artillery.
The second one was specifically for Miros's seismic cannon (I was falling asleep writing that, and might be confused about what a seismic cannon is in WH40K, (could have sworn tabletop seismic weapons used a long narrow blast template and everyone under received more wounds the closer to source they are))
Moreartillery could another alternative be for the basilisk to have the weapons a delay with an animation ?(effectively placing a reload animation ahead of firing not afterwards,
I forgot the obliterator's stacking existed until seeing this thread but if the stacked weapons have this same shot delay it could mostly look ok couldn't it ? ))
Tested Slave/master weapon groups and they seem viable
try making longest range weapon Group master, only show an animation on the longer range weapon, make short range projectile invisible, close range enemy receives both damage, at long range only long weapon fires so far both seem like they will fire in unison, would need to test it though