Well, BTS is 5*, dark chaingun is 3*. If you mean the equivalent 3* shei;d in the bistling buckler line, that would depend on your current gear. If you are using swords for most of your damage or having trouble with beasts or slimes (like Royal Jelly Palace), then you need the shield first.

BTS and Owlite comparison is like comparing apples and oranges I think, they protect against different types of damage. Owlite protects against elemental damage, which is perhaps the most common type of damage, and helps greatly against the ranged gun puppies, which deal elemental. It also protects and fire and shock, a couple of the most dangerous statuses, and makes if good for both the Roarmulus Twins (shock) and for FSC (fire).

BTS is for pierce enemies, or if you carry swords and can reliably dodge attacks, anywhere at all that you can take it and not die. You will probably want both a Grey Owlite and a BTS eventually though.

Also, Aegis is crap, don't ever get it. I'm thinking of getting a Heater Shield, which a 4* Mighty Defender shield can be made into instead of an Aegis, but other than that, don't upgrade the defender line of shield. About same stats as the Bristling Buckler line, but no bonuses at all (so if you EVER carry a sword it makes Aegis a worse shield) and pierce enemies are the easiest to deal with.

Well, Blitz for FSC is pretty much standard. Last I heard Plague Needle's poison doesn't make up for the less damage. If you are still mostly in tier 2, you can wait on it though, if you need a gun that will help more for the tier 2 bosses, or if you just need something else entirely.

EDIT: Sent you a friend request in Steam, I'm Telchar44 there. Also sent you a friend request already in-game as you requested. :D

I'm afraid that I can't really give so much advice there, I don't have much experience with guns in general, and I don't have any of the grim repeater line. I have a blitz, but hardly ever use it since I typically shiver for vana, use DBB on wolvers, and a pierce sword, pulsar, or voltedge charge on fiends. Then for RJP I already have pretty OP weapons (obsidian edge and DR) so I don't really need to get a grim repeater.

TL;DR: Don't have gunner experience, can't advise. :D

You can try out your autogun in tier 1 and see how you like the mechanics of it first. Can you get wolvers with it? Destroy a gun puppy with it without getting hit back? A lumber? Groups of slimes?

I'd get Combuster before Glacius, most are of the opinion that Glacius is the worst elemental Brandish since the freeze keeps enemies from riding the full wave of the charge attack, taking less damage, and that frozen enemies interfere with kiting. Combuster on the other hand is useful even in fire strata, since enemies always ride all the explosions.

Gravitron is good, but requires high CTR to use well, and MSI helps as well. It's not as useful as the electron vortex, and if you don't have a Voltedge either enemies will tend to scatter at the final explosion. A tundrus charge attack works well in non-freeze strata to hold enemies in and do damage too, but only on shadow weak or neutral enemies. I'd check out Bopp's sword guide for all your sword questions. FYI, Wild Hunting Blade isn't very good from what I've heard, although the poison variant has some use.

Is BTS better then Grey Owlite??? Or perhaps Aegis???

You were already told that BTS was a good piercing shield for sworders by multiple players, for example here, here, here, and here.

In fact, here and here you were specifically advised about Defender/Aegis, Grey Owlite, and BTS.

BTW if I'm a gunner will Dark Chaingun be good for JK?

You were already told that Dark Chaingun is good for Royal Jelly Palace here. It's not a crime to solicit other opinions, but you seem not to understand the basic idea of damage type, even though it has been pointed out to you repeatedly.

Goodbye.