Heater shield is outclassed by Omega Ward (but is a good non event shield for ele/freeze)

CIV is a safe weapon for taking out Howlitzers and for dealing good damage to deadnaughts. It is outclassed by a UVed Leviathan blade, and FOV. Thanks to Max damage armor sets why get any weapon that has a natural boost?

Drivers are dead to Autotarget players, if you turn off auto target you can deal incredible charge attack damage. (aim it right and all 4 ricochet bullets spawn inside the enemy, inflicting status and damage)
Thanks to ricochets the Hail driver breaks its own freeze.

Volcanic pepperbox is currently the best autogun for dealing damage to Slime and Construct monsters and like the other two lines it has great charge flinch rate (enough to make a Lumber flinch) and the Charge deals knockback which is useful.

Shards are random but have an excellent flinch rate and a great charge time.

Irontech is a mix of defensive and offensive bomb, the damage and status it deals is greater than staggerstorm but it has a lower chance of dealing that status. Plus the 4star bomb has a larger radius than its brother blast bombs.

The Cobalt and Skelly lines are underappreciated, Cobalt gear can help your through the first two token bosses and Dead skelly is the only non shadow lair shadow/freeze resist armor.

As somebody who knows all about underused weapons (I'm a guildmaster of Shard Squad, after all), the reason people overlook these things is because there is always something that is seemingly better. However, that doesn't keep these weapons and armors from being any less great in their respective uses.

CIV- I don't have any experience with this one, but generally, most people who do stick with calibur go for leviathan for the extra bit of damage. CIV is seen more as an estranged younger brother. However, this weapon does boast a lovely undead damage bonus that can be pretty useful.

The Alchemers- Really, I'm not sure why you mentioned these, I don't see these as underused at all. Most gunners have at least one of these weapons, if not more, due to their useful charges and the ever-present ability to switch-shoot. Great set of guns, although magma and nova don't seem to be usd quite as often.

Pepperbox- I cannot give this gun enough praise. While it does not boast as much damage as blitz or plague, it has universal normal damage, the very helpful fire status, and that one thing blitz and plague lack: knockback. This gun is useful in so many places and against so many different things. As I always say: Nothing provides a better wall of gtfo than a volcanic pepperbox.

Shard Bombs- While not the glorious bullet bombs that they once were, they now have a line for each damage type, which is very unique in the world of bombs-and any other weapon type as well. They do have their problems (dem invincitinks tho)but can be pretty darn effective is used properly-and in a party with other sharder's.

ITD- Oh, how I love this bomb. The stun, the damage, the radius! It's all so delectable. Only problem here is the speed, which is slow, but I wouldn't say it's too slow. It's the middle child of the blast bombs, with the nitronome being little brother.

Cobalt &Skelly- Nice armors, each has their niche, and are fairly simple to get, especially the cobalt line.

Now for some things you didn't mention....

BAB-The ol' Big Angry. Big brother of the blast bombs, this bomb packs a serious punch, and while I have not used it, a serious lack of speed. It does boast a massive radius though, and dat sweet knockback! Definitely a great bomb if you can get past the low speed.

The Troikas- Like swinging around a massive chunk of stone. Slow, but packs a punch. Stun or freeze, it's your preference. I personally chose freeze. Not the best in every situation, but then again, nothing is. These swords, though, are spectacular if you can use them correctly, and can be effective mostly everywhere.

Iron slug- Okay, It's pretty hard to defend this one, but I have to say, I do love my iron slug. It interrupts fairly well... and that really is about it. At least it'll be getting a buff in the gunner update, which will make it a much better weapon.

The Cutters/Winmillion- Great swords, but pretty hard to use. You don't just use them, you dance with them. Being the fastest swords in the game is pretty nice, but the charge is kinda funky on all 3 of the weapons. Nevertheless, if you can dance, you can make it work.

Aaand that's all I can think of.

• Heater shield
  Honestly not a bad shield, but:
  • Only one status resistance compared to the usual two of most other shields.
  • People are not as afraid of freeze as they are of fire and shock, both covered simultaneously by Gray Owlite and Crest of Almire. Therefore, people will get these shields first.
  • Unless you plan on getting a shield for every possible situations you could end up in, and that's a very expensive thing to try doing, you quickly learn to use any 5* shield even in completely wrong environments (i.e. Gray Owlite vs poison slimes) and make do with it. As such, once you have Gray Owlite, it becomes difficult to justify getting another elemental shield.
  • People would rather see silversaps nerfed than having to craft another shield because shields are boring.
• Cold iron Vanquisher
  Weak, takes forever to kill anything with it. Takes even longer to kill zombies than with other enemies because of their massive amounts of health. For this reason, it's better to use something like Combuster. Now, Combuster is not good against gremlins and beasts, but are you really going to use CIV against those enemies?
• Driver lines
  Not actually underused? Cryo Driver gets a bit overlooked, but these are generally the most popular guns. They're mobile and powerful, don't have the flow-destroying knockback of pulsars, aren't as weak and boring as antiguas and blaster, aren't as cumbersome and risky to use as catalyzers, magnus and autoguns.

  I have a Magma Driver and sometimes I use it to oneshot multiple lumbers with a single charge attack.

• Volcanic pepperbox
  This is actually a good gun, and I once made a knight that beat every mission using only pepperbox, cyclops cap, and chaos cloak, but it suffers from a much larger damage penalty than staus weapons typically do. For this reason, even things that are resistant to piercing but weak to normal such as Tortodrones and Vanaduke's mask receive more damage from Blitz than from Pepperbox.
• Shard bombs
  Broken hit limit puts a harsh punishment on skillful play. Erratic knockback puts a harsh punishment on unskillful play. Low damage puts a harsh punishment on trying to use the normal damage variant. Randomly disappearing shards puts a harsh punishment on trying to use these things near walls or large enemies.

  You need a master's degree in Putting Up With Annoying Bullcrap to use these things, and the payoff isn't worth it.

  They're fun, and I enjoy using them as a group. That's why I made a guild dedicated to them.

• Irontech destroyer
  Slight chance of minor stun. You really can't count on it for anything, and when it does inflict, it just messes up your timing. For instance, try keeping a greaver perpetually interrupted with Irontech. I can do it easily with Nitro, not so much when they sometimes get stunned and change their timing around.

  Irontech's charge time is almost as long as BAB's, and you move just as slowly. The damage is less, and the knockback is severely less. Not as fun as BAB, and nowhere near as reliable as Nitro.

  I enjoy my Irontech, I just wouldn't bring it anywhere I cared about anyone's safety. I made a thread about it once, but some of the information found in it is wrong. For instance, I though the Irontech charged at Nitro speed, but that was due to me being severely unscientific and assuming that my slower planting times with it was due to an error in measurement. I've since made closer examinations and found that Irontech's charge time is closer to BAB's than Nitro's.

  Due to the radiant situation, 4* Irontech is not a bad choice these days.

"Underused" implies that it should be used more often than it is. A lot of weapons are, as people have mentioned casually above without much analysis, not worth crafting or messing around with. Quite a few others are actually underused.

Like pepperbox. Pepperbox is like the brandish of normal damage. Its what normal damage is supposed to be, if normal damage wasn't just underwhelming everywhere that isn't tied to specialized damage. Pepperbox deals less damage and will never be best, but it still murders turrets, counters mechaknights, wolvers, all of the bosses, tortodrones, pretty much all of the non-extremist (danger missions/shadow lairs/xmas purgatory mountain) content in such a way as to come close to being best.