I've seen alchemers used effectively.

Yes, they are. They have a kind of moderate bullet speed (which means you have to leadshoot a lot in order to hit strikers dashing towards you) and less distance than Valiance, which by the way works as a decent 1v1/kiting gun.

As far as Alchemers go for PVP, I'd rank them like this:

-Storm Driver
-Nova Driver/Umbra Driver
-Hail Driver/Magma Driver

Storm driver is valuable because of its shock status, which means many lethal things that I won't get into now. It's my preferred gun, as a recon.

Nova/Umbra driver are stronger elemental or shadow damage, and should mainly be used to counterpick your opponents' armor (ex: just talking about wolver armors, Nova is good for everything except Vog, Umbra is good for everything except Snarbolax).

Hail/Magma driver have their utilities, which can be useful depending on what class mod you play and what you're trying to do with their statuses:

Magma/fire status is okay for fighting closer to your base and also dragging people around while you run away. (decent for striker class mod)
Hail/freeze status is okay for situations where you're outnumbered.

Anyway, the main thing to know about Alchemers is that their bullets ricochet left off of walls. If you're fighting near a wall, you can use this to sort of lay traps so your 'missed' bullet has a chance of bouncing back near you and interrupting your opponent while they (probably) dash near you.

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Most of that is shenanigans from either what I do or have seen others do (ex: Hail Driver is the one Alchemer line I haven't built yet). As far as a sidearm for a non-gunner goes, status effect might be more important for you if you choose to go with Alchemers, so Shock/Freeze may be easier to manage. Fire is a little tough to be reliable though (it can definitely make people run away, if you're trying to do delay tactics).

Eh, as far as the mentioned guns' ranges go:

Valiance/Polaris/Antigua lines have the highest ranges (8.5-9 tiles)
Alchemers have the least range (7.5 - 8 tiles max reach)

and for the non-mentioned guns,
Biohazard/blitz have higher range (8.5-9.5 tiles)

Randomly, while the magnus lines are imo terrible for PVP, the blast from the shot lets it reach a little further than the alchemer.

Anyway, I brought that up to say that Valiance is a pretty long ranged gun.

The best gun for T3 lockdown is obviously the Barbarous Thorn Blade.

if your planning on using it as a sidearm, get a polaris. end of story, however if you want damage get an umbra driver, no one ever has shadow defence, and if they do shoot them regardless

I am a striker who carries a:
flourish of finality
faust of... granny??
liance of van
laris of po

prolaris (nickname by Thrillhaus)( haha pro-laris) is great for clogging up narrow corridors, and is also fun to wreck bombers with. with enough people spamming it, you can block off an entire big entrance while you're capping.

valiance is the pro way to annoy people. use 2 shots every time, (not 3) so you don't have to reload, and yet you have a higher chance of hitting.
its great for delaying a person while your boost is out, or maybe just to snipe some guardian's shield.

Well that's pretty much what his/her name says.

Most guns have their applications in lockdown. The key is experimenting with them to find out what they're good for. The only guns I wouldn't recommend are the Catalyzers, and the Supernova as both of these are outclassed by the Polaris.