I suppose Hunting shotgun + choke mod + slugs would give more accuracy.
Trouble is, Cass starts off with 47 in Guns while Hunting shotgun requires 75 to use to full effect, so she's still going to miss a lot, I guess. Although slower ROF means less ammo wasted before she gets in range for good shots.
Snowy- Yes, indoors is the best for shotguns- but it's outdoors that you REALLY need that DT reduction from Shotgun Surgeon. Not a lot of Deathclaws inside Vaults...
Back on topic, I tend to fight higher level creatures from the start, because I'm sooo macho.
Which leads to leveling quickly, by the time I hit the Mojave outpost area in a generally linear fashion using guns I may be as high as level 12.
Even if not I find I get Cass as a companion level 20+ and often have 7 - 9 charisma with her, so I may just be seeing her at her best tbh.
Raul was the same though with a riot shotgun player level 25 charisma 5.
Back off topic, yes LadyCrimson unfortunatly I tend to be a clutter bug, and dump junk guns and ammo into NPC's to keep my weight down to 90 ish.
I just tried to replicate the ctd but no luck, however just removing most of the options from companions solved the two times I got it.