Defense doesn't really make that big of a difference in LD. As long as you have the offensive bonuses you want, I wouldn't worry about it.

I would suggest against taking the Callahan. You do want a gun with fast bullet speed, but it's better to pick something that doesn't root you in place. Valiance is the standard choice for this, but something from the Antigua line will work too.

There are very few people who choose not to use heart trinkets. Might want to reconsider.

Storm's a solid choice, although I prefer Hail for crowd control.

General gun opinions:
Storm driver/shock - If you understand invincibility frames, this is the most dangerous gun. During the shock animation, there aren't any invincibility frames, so they become damage sponges. Great when you have aggressive teammates nearby, but the shock pulses aren't reliable in timing, so it's more of an offensive than defensive gun, really.
Hail driver/freeze - When you're alone or not near crazy teammates, freeze can save you. Freeze gives you time to charge for massive damage if you're in a 1v1, and buys you some breathing room when you're outnumbered. You can also gamble on a re-freeze if you use hail again.
Magma driver/fire - When you're gonna die or are having trouble hitting your targets, fire is the status to go with. The slow damage also bites through guardian shield. Nice for kiting or scaring people off points (sending them to go heal).
Callahan - Terrible. You stand still while you shoot, making it unviable when you're outnumbered. You can't turn during the shooting animation, meaning you're committed to the direction you click towards, AND it does piercing damage, which is only the most common non-normal defense in lockdown. The charge is fun for sniping guardians though.
Pulsars - Useful. The large bullets at their mid-long range points make a nice wall that you can run with, whether you're striker/recon. People seem to have trouble dodging them.

What class mod do you use? (Striker/Recon/Guardian). It can really affect your gear too. Strikers are always visible and have low health, so heart pendants are almost a must. Recon/Guardian have a ton of health so they're better off with damage/speed/ctr trinkets.

Striker
Striker playstyle is more of a kiting playstyle, where max gun damage and hard-hitting guns complement the style better. The shoot-pause technique is good here, so ASI is more of a "how soon can I dash to gain distance?" deal. That said, Polaris is a great defensive gun, especially if you swap-shoot: You can lay out a polaris bullet to 'guard' your movement while you aim for damage with a faster/longer-ranged gun. Valiance is good for striker/kiting battles, as the 2shot-pause playstyle lets you shoot quickly before you have to reload, and the knockback interrupts striker dash. Magma is viable due to the kiting playstyle.

Recon
Recon playstyle is more of assassination and annoyance. The one thing you can count on as recon is the ability to get the first hit (assuming you have good enough aim), so you generally want to open with a status gun (storm/hail are my favorites for this) and act assuming that it causes the status. Valiance is useful again as an anti-striker counter-weapon or as a no-shield weapon, but here's the amazing part with recon: If you can cause deathmark, it removes their armor's defenses, meaning the normal defense goes away and Valiance becomes a portable nuke.

If you use Justifier (I do), you get to max ASI and VH CTR (level 10 gun + med ctr), and it makes storm driver and swap (or in my case, double-switch) shooting really dangerous. Normally, you'd have to slow down your shooting to appease the invincibility frame delay, but when they're shocked, it removes the limiters and lets you go berserk with your attack speed. It's my preferred playstyle, really lol.

Anyway, I'm a recon gunner. If you want to see any of this stuff, find me in-game `_`; It all sounds like some crazy theorycraft otherwise.

Just wanted to add one more thing. While status drivers, polaris, and valiance are nice to have and fun to play with, you don't need a full complement of weapons to succeed as a gunner. My highest damage game was played with Umbra only.