First, 20 Jelly Gems is not really very much. It means running RJP seven times. Most players do RJP much more than that, because it's a pretty good place to earn crowns in the middle of the game. Also, you don't need shadow weapons for RJP. A 3-star Calibur is more than enough to handle the whole thing.

Now about your question, I'll say what I always do.

Antigua-line guns are easy to use. They have good range and can interrupt soft monsters continually. But they are ultimately boring. They reward repetitive clicking rather than any kind of skill. They don't have interesting mechanics (e.g. ricochets). They don't offer many statuses (just poison). They don't do very much damage, compared to guns like alchemers.

Magnuses are harder to use; you have to make every shot count. I haven't crunched the new numbers, but the regular attacks seem not very strong still. But the charge attacks are quite nice. I'd definitely go for Magnuses over Antiguas.

Well, there's also the Needle Shot, which has for a long time been one of the most highly valued Pierce weapons in the game. It requires a bit more brainpower to use than an Antigua, but really only just a wee bit, and it tears Fiends up nicely. It's a wee bit more expensive because it's crafted from the 2-star Autogun, but not as much as it might seem; you don't actually need Orbs of Alchemy to get your paws on a basic Autogun; you can just buy it from Ricasso without having to alchemise it, and I think 2-star items are 7500 Crowns. (I'm not sure if the price might have changed recently, but I do remember buying one from Ricasso since the last update and s/he didn't gouge me for it...) That's not a lot of extra cost, the Autogun is really useful in all of Tier 2, and if you don't like Shadow weapons, the Needler is one of the best guns to stop Gremlins before they thwack you on the head. (I don't know whether you're planning on specialising in guns, or if swords are also on the table.)

Antigua recipes are Basil-only, so as long as we're talking about that, the Pierce version of the Blaster should also be on the table. If you can hit Fiends with an Antigua well enough to make it a decent weapon against them, you can also hit them with a Blaster or a Magnus. The Blaster is generally better for this than Antigua, and as a bonus it has knockback. The Pierce blaster also crafts from a 2-star precursor which you can buy from Ricasso for the same 7500 Crowns, but unlike Autogun, you get a Blaster for free by the time you earn Tier 2 clearance. I highly recommend going to the bother of looking for the new Blaster upgrades instead of using the Normal recipes you get for free, because the Blaster was already a workable weapon for Fiends, and the new Pierce version should be even more so. In my opinion, Blasters are much easier to use than Antiguas, since they're more deliberate rather than spammy, but without slowing you down as much as a Magnus. Quality over quantity; fewer but more damaging and carefully-aimed attacks are going to be much more efficient and give you a higher portion of your weapon's theoretical DPS.

Really, I should recommend that you don't take our word for it; try a variety of weapons while you're still in Tier 2, since the experimentation is almost free when compared to the time it takes to upgrade Tier 3 weapons. If you can't decide, get all 4 of the 3-star Pierce guns and compare them at similar heat levels, since everything you learn about their mechanics will be applicable to some of your options for other damage types. If that seems expensive, you'd better steel yourself for Tier 3!

Well I suppose it was a matter of time until we had this thread as opposed to "piercing handguns discussion". But here goes- You can't truly compare Callahan and Peter Griffin, because like in the first of these threads it's apples to oranges.

Callahan is used to deal damage over an area, to pierce through and deal a lot of damage to a lot of monsters. You'll probably want the gun at some point so that you can use it to deal with huge numbers of beasts and fiends that are in the same area, as well as to punch grievers with it at a distance. There are not many monsters that do this at the moment (only Dustbuns, really), so getting it is more to future-proof and entertain yourself.

The Gilded Peter is used to deal damage to a single target while strafing, as well as toggle switches and all of that business. This makes the Griffin Guild far better suited for use alongside another weapon that deals higher damage, that has higher risk. Since you can sneakily sneak shots in, the gun is perfect for times when you're low on health and compliments higher risk higher reward weapons quite well. Overall, I'd be surprised if the piercing blaster variant "Raft Walker" didn't simply outclass the Gilded Griffin entirely. A single bullet from Sift Bopper will out damage a single bullet from Gilded Griffin, making it better for sneaking in damage and all of the play suggested above.

If you're a pure gunner, you'll almost always prefer Callahan. If you're not, then you'd probably prefer to use Gilded Griffin- it gets an extra damage bonus that you won't have to think about and will make good use of any damage vs fiend buffs that you do feel like using (I don't believe there are any false fiend monsters, aside from Vanaduke who obviously prefers blitz). Callahan on the other hand is a very resource heavy handgun- you'll want to max out your stats on it, giving it as much damage, charge time reduction, and even ASI as you can. After you've done that, you'll be AWESOME, but you really can't just not or you'll be at a large disadvantage.

Yes.