To add to what others have said (which is all correct), you might want to skim Release/all for a record of all major updates.

Regarding those weapons, excluding the bombs, there really isn't anything there for you to change.
From what I'm hearing:
AP and Sentenza have an upgraded damage bonus from MED to HIGH.
Callahan (Patch Notes) now pierces through multiple targets on charge, with no stagger / recovery period.
*Volcanic Pepper Box has received a buff to its (damage / status effect?), with more manageable charge and regular clips.

*Not sure if true or not...

But overall, there isn't anything you should replace, unless you decide to get the new Gilded Griffin, Winter Grave, or Grim Repeater for specialized purposes.

The main weapons I use are Ardent Peacemaker, Sentenza, Callahan, Volcanic Pepper Box, Ash of Agni, Shivermist Buster, and Voltaic Tempest. Are any of those out dated? Are there new items I should replace any of those with?

Before this big gunner update, my advice would have been to replace your Argent/Sentenza with Nova/Umbra Drivers. They are more fun to use, they do more damage in the hands of an expert, you can get statuses on the elemental one if you want, etc.

After this update, you should consider replacing Sentenza with Winter Grave or Grim Repeater or perhaps Permafroster (I'm not sure how well that last one works). And you should still replace Argent with something better: Nova Driver, Storm Driver, maybe Wildfire? Give them a try, anyway.

Don't forget the new armour ^_^
Regards
Comics

To say that it all comes down to luck is to say that it's random, which is true. But that doesn't mean that you have no idea what to expect. There is a central tendency or average, and the number of Radiants tends to fall somewhere around this average. By collecting more and more data, we can get a better and better idea of the average and how tight the clustering about that average is.

Lancer Knightz currently has an average around 17, but they don't have many data. Another poster (I forget who) reports running FSC many times this fall with an average of 18. So the true average is probably somewhere around there. Of course, you don't always achieve this average. Sometimes you get fewer Radiants, and sometimes you get more.

Captain-Teemo, I suspect that Arcade FSC and mission FSC are identical in their Radiant payouts. What's more important is that the drop rate of Radiants changes over time, most notably in release 2014-02-26. That's another reason why we must keep taking measurements.

Difficulty levels were introduced in the 2013-07-30 release --- the mega-update with battle sprites, forge, no more mist, etc.

You don't have to be on Elite difficulty to get rarities. But Three Rings claims that your chances are improved. I don't have data on it one way or the other.