It's been talked to death and there are many people who've ranted and raved about these guns. Especially now that it's clear that the dev team means to fix them

so that they're more useful, and so that more people use them aside from the few who like the Sentenza and the many who use AP just for Vana (and FSC by association, since they're carrying it anyways.)

Sorry to those who want a DL/DR, but I'm not going to put a summary at the top, and I can be verbose. XD
I'll do my best to space and pace so that it's easier to skim, and doesn't get too boring for those who don't care for background.
If all you want is my suggestion/opinion without the information, just skip to the bottom. :P

As it stands now, as most people know, they do split piercing and elemental/shadow damage respectively, and as a result the damage numbers that we get when using them are a bit funny, to say the least.
From all appearances, they are somewhat strong against two monster types, somewhat weak against two, and quite weak against two - which is a great bit worse than weapons that do special or normal/special damage, and a fair bit worse than straight up Normal weapons.

I'm not going to get into strategy and uses, nor am I suggesting that they're BAD weapons. I'm fully aware that they have their uses and can be quite effective, offering fast, low damage without sacrificing mobility (etc, etc.) My goal here is JUST to look at the damage numbers and make some suggestions for ideas that might be worth trying to make them more vialble than they are now.

Some months ago, I posted in the wiki contributors forum section offering to record damage values for weapons with missing tables, and I have been doing that, albeit slowly. Recenly, with the use of an elevator pass, I've been working to fill out my damage tables for the Silversix and Blackhawk so that we'll be able to fill out their entries in the wiki before they are changed. They may be obsolete soon (the numbers, not the guns :P,) but at least we'll have numbers to compare if we want to! (I've nearly finished my chart, and I'll post it in there very soon to be entered into the wiki.)

In addition to getting damage numbers against the ordinary monster types with the normal and charge attacks at the top and bottom of each stratum, I've been making a point of getting this information for Drones as well, since I saw it mentioned in the wiki section that they appear to do neutral damage to drones. This does appear to be true, but it's possible that drones are slightly weak against all damage. For my purposes, I am making the assumption that they are indeed taking neutral damage. (Confirmed - Drones take neutral damage from everything.)
What I've found isn't necessarily exciting or alarming, but it certainly doesn't make the SS/BH look good.

The bonus that they get against the monster types against which they are strong, is BARELY more damage than they do against drones. In the case of the highest stratums (tier 1,) they do the same damage to drones as their strongest monster types, but weaker damage to everything else.
It isn't until tier 2 that the Silversix does better than one (1!) damage point more against undead than shankles, but meanwhile does quite a but less damage against everything else, except for fiends.
As you go farther into the clockworks, this never really improves very much. At the end of tier 2, it is doing 4 more damage per shot at its best, vs something that is totally neutral against its attacks.

Basically, what this means is that a silversix is doing VERY SLIGHTLY more damage against fiends and undead than it would be if it was entirely normal damage, and less against everything else (the same can be said of the Blackhawk and beasts/gremlins.)

From what we've seen previously on the forum (and from the test server,) the current idea is to make them full elemental or shadow damage respectively, with a buff against their favored enemy. While this might make them more viable, it doesn't really make them more interesting, nor unique, and many players seem to really dislike the proposed changes.
We already have plenty of options for elemental guns, and a few very good options for shadow guns. The main concern that people have, seems to be that these guns will cease to be unique, or useful for what people are using them for now.
The flipside of this is that the danger of changing them in a way that keeps them too similar to what they are now, but making them more useful is that they will become overpowered, and other weapons are ignored in favor of them.

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I've put some thought into what might be done to make them both more useful, and more unique/interesting as weapons, and also to maintain what seems to be the intent and concept behind these weapons: cartridge revolvers that when upgraded are infused with energy that is more effective against certain monsters in the Clockworks.

My proposed change is that instead of simply doing split piercing/elemental and piercing/shadow damage that they still do both damage type, but A: Not Split evenly, and B: that both damage types are not done simultaneously.
To explain better, the change would be to have them do less piercing damage than they currently do, then on a successful hit do one of two things after a brief instant: either a second hit that does a bit more elemental or shadow damage than it does now which would be treated like a seperate hit, or to then cause a small blast or explosion on the point of impact that does either elemental or shadow damage to anything close enough to be hit. Obviously the second one here is a bit like the pulsar, but hopefully not too much - not including knockback (or very little) would probably keep them from being too similar.

In theory this would have several repercussions on how the weapon(s) work:
1) The weapons would be more unique, and would have something special about them unto themselves, without being greatly overpowered (as many people would say about the Pulsar line, for example - which is one of the other most unique weapons in the game currently, but arguably too powerful.)
2) Players would be able to better see how much damage they are doing (if they have damage numbers enabled) and will get a better sense of when they are doing effective damage. Instead of normal sized numbers against things that the weapons are strong against, you'd reliably get the "big yellow numbers" against enemies that are weak against them. Against enemies that they are less effective against, the damage numbers should similarly indicate how well they are doing. This will make people more confident about when to use and when not to use these weapons.
3) The weapons would make some sense from the perspective of what they're supposed to be, something akin to an explosive round or an elementally charged round fired from a cartridge revolver.

Well, sorry my post ended up being a bit of an essay, but hopefully these thoughts are of some use and consideration.
In my testing of these weapons (and delaying upgrading them for the sake of filling out wiki entries >:-P) I've become pretty fond of how much fun they are to use, even when they can be quite useless. I for one am hoping to be pleasantly surprised one way or another if/when they do get hit with a balance patch.