I think the question is more about choosing between Iron Slug/Callahan and Winter Grave.
Iron Slug and Callahan fill pretty similar roles, except the Iron Slug is good against Lumbers, Lichen Colonies, and is still generally powerful. Callahan would be better for Fiends though. But as far as how to use these weapons, Iron Slug and Callahan are identical (deal damage, knockback, and stun). I do not know if Callahan knocks enemies back more or less than the Iron Slug.

Disclaimer: I do not have the Winter Grave. This is mostly speculation based off of my experience with the Triglav and the fact that the Winter Grave has a good chance of moderate freeze (similar to brandishes with their good chance of strong status).
The Winter Grave is a very different weapon with Ice instead of Stun; just like how the Triglav is a very different weapon from the Sudaruska (however it seems like the Winter Grave vs Iron Slug will be much more different than Triglav vs Sudaruska). If the Winter Grave acts as the Triglav does (freezes enemies in place before knockback), you have to be careful about close-range charge shots, and awareness to not get yourself backed into a corner. The freeze will nullify knockback, causing you to not deal much more than a few hits per target (maybe it's different for large enemies). The only time where your # of hits will be similar to the Iron Slug is at near maximum range, where you'll probably get 3 hits on the charge attack. Maybe it's different for large enemies, but I don't see the Winter Grave getting the ohko Lumber Iron Slug charge on Elite with 2 people at near maximum # of hit potential dealt with med damage bonus.
The Winter Grave will probably be more of a mobile Shivermist Buster.

I have the Tundrus, and I think it knocks back at least once before freezing. That said, it has a really good chance of freeze, so if you shoot at a group of enemies they're about as likely to get frozen as if you were to drop a shivermist in there. It has great CC. Similar to the Glacius it has slightly less damage potential because it won't necessarily deal the full number of multihits (Which by the way is some insane number like 7), so that's the trade-off.

i'm a huge proponent of the iron slug being best bang for buck, and then filling your other weapon slots with specialized damage. It's like carrying a neutral damage brandish and having greater versatility in other slots instead of carrying two brandishes that you have to switch back and forth between.

After using iron slug in SL Snarbolax post-update, my opinion of the pierce Callahan is that wolvers are going to teleport around too much to pull optimum use out of the gun charge, and the normal attack may have knockdown but lacks DPS and that same knockdown power in 4 man elite parties. Callahan charge is very good for destroying silkwings and Pit Bosses through crowds of enemies... But other fiends will destroy you while you're slightly immobilized. To me, callahan's usefulness now generally falls behind Slug and Grave, with either having more value per slot.

i still need to test Grave's usefullness versus slimes and gremlins though. Might be worth the knockdown and freeze versus gremlin mortars and thwackers who get up in your grill, as well as sniping pesky healers through crowds, with freeze setting them up for easy pickings if you don't get the OHKO.

The speed nerf of the charge projectile is most apparent against Fiends and Beasts because both are highly mobile in nature and will be more likely to run around randomly. They still get hit by regular attacks as per the usual.

The piercing effect of the charge attack is apparently most useful against larger and/or slower moving enemies, such as, but not limited to: Lumbers, Giant Lichens, Gremlin Schorchers, Chromalisks.

If you only want one of the guns, I say Iron Slug. But there's no reason not to get them all!

Thanks for your well-reasoned comments, Orbwanter. I'll take them into consideration.

It seems that fundamentally Callahan is still a weak gun, even after this update. Boo.

Magnuses after the patch are like Autoguns before: Good charge, bad normal attack. Sadly Autoguns now have both...

Callahan charge wrecks chromas and can work on wolvers too if you are patient enough to wait till they pop up behind you and commit to a bite.

As I mention in post #8, I'm more concerned with Callahan's performance against fiends than against beasts. So what is your opinion there?

(And keep in mind that I've owned and used Callahan for about three years. I just want to know what other people have figured out about how the new Callahan works.)

I've figured out what my real question is: Which is better against slimes and gremlins --- Iron Slug or Winter Grave?

They seem to have the same baseline damage. Winter Grave has the advantage of damage type. One stuns and the other freezes. In general I prefer stun to freeze, but that's not my main concern here. My main concern is that due to freeze the Winter Grave charge attack misses some opportunities for damage, as pointed out already by Qwez, Klipik-Forum, etc.

So I wouldn't be surprised if Winter Grave's charge did less damage to slimes and gremlins than Iron Slug's charge. Anyone have anything to say about that, specifically?

To clarify, I'm specifically interested in a charge-oriented playing style, where I run around releasing charges that kill crowds of enemies in one shot. This style is popular with Brandishes. After the update, some players are saying that the Magnuses are the new Brandishes. So I'm trying to evaluate the extent to which that is true.

Basically, is Callahan + Winter Grave the new Combuster + Acheron? Or does Winter Grave's freeze mess that up? I guess it's more like Glacius + Acheron.