This is not simply a thread about Callahan's piercing damage vs. Iron Slug's normal damage. I want to know what exactly are the differences between these two guns. I want to know about the details.
Right now I have a level 3 Mega Magnus. I'll heat it to level 10 but I won't upgrade it until 5* fire crystals are back. I have a Chaos set and I usually use Swiftstrike buckler.

So how are they different?
I don't think they have different knockback (because they simply can't knock back). Range (not likely)?
The main question is WHEN do they INTERRUPT monster's attacks. Does it depend on the party size? On the difficulty? On the monster type (elite-Mecha Knight, knockback-resistant -Howlitzer)? Monster's family (example: Cal interrups devilits while IS doesn't, but IS interrupts Constructs)? As far as I know, IS interrups more that Cal and Cal interrups less monsters, but WHICH exactly are these monsters?

I need a "heavy", underpowered gun like Magnuses, I want to turn off damage numbers and snipe monsters. I'd take IS but I need information about its interruption rate.
I play a lot of LD too, as a Recon and I know that Magnuses are very useful in PvP. I want to use one too. But if I take IS... it'll be fun to use it in PvE but in PvP everyone has Normal defence and when we add Strikers' life-saving Pendants I'll do no damage. I'll probably get IS but I may get Cal just for PvP.
If I get them both and switch shoot them (talking about PvE), will that increase my DPS?