Long story cut short, I'm allergic to melee combat, and I've always had a small problem with greaver swarms. How do different guns and bombs measure up in their effectiveness against greaver swarms, especially in confined spaces?
(I'm still in T2, btw.)
I was going to suggest Callahan, but then maybe you'd be better with Argent Peacemaker? Assuming ANY hit will interrupt greavers, you need the clip size.
For bombs, The best I can think of is Scintillating Sun Shards, or maybe Dark Briar Barrage/Nitronome. Not sure which is actually more useful. Vortex and Big Angry Bomb could work IN THEORY if they weren't so slow to charge.
By the way, all of these, save Big Angry Bomb, have tier 2 versions that have the same function. In fact, Antigua may be even better than AP in tier 2 since it's piercing.
Batabii is correct in stating that any amount of damage will interrupt Greavers. I used a Proto Gun once before...
The issue with bombing Greavers is that you often don't have the time to finish charging, if they can take you surprise. Any gun with reasonable mobility and firing rate will work well against Greavers; Callahan will do okay, but oversize butterflies are liable to come in swarms and it isn't always possible to fire fast enough to take out all of them. With bombs, your best option is to be prepared and charge beforehand; any bomb with decent damage and some knockback should do a reasonable job at killing them with some CTR slapped on.
The Antigua line was what I was thinking of as well, but the lack of piercing damage beyond 3* made me wonder if the Callahan would be a better option. I sadly do not have either; I'll work towards the Antigua first =) I have a needle shot but the immobility associated with it makes it extremely difficult to use in tiny Greaver rooms. My current choice is an alchemer, but the small clip size doesn't help much. It's not too shabby against other fiends though (i.e. Devilites), though I guess the Antigua would be great as well.
Bombs wise, I'll definitely practise with my DBB and Nitronome (T2 versions). I usually have a demo helm and a gunslinger armour on; hope that's enough charge time reduction to pull off bombing. I used to rely on haze bombs (fire, unless it's a fire stratum, the I'll freeze them in place and shoot instead). Didn't work =(
I always found alch-switching effective against greavers. Constant shots interrupt them, the damage is sufficient, the ricochets catch other greavers nearby as well as your target, the statuses mess them up, and the guns let you keep on the move while shooting fast so you can backpeddle and funnel them.
Easy peasy and notably safe, albeit not the fastest method of killing them.
At risk of tooting my own horn I've actually written a Steam guide on how to kill Greavers (among other things) that you might want to look at.