Today I saw two Ragna Barrets. Both of them made me cry. Now here is advice for him (from me playing Barrett for some time now).
1. Carts/Parts
Cart wise choice should be pretty simple. Grab his capacity carts, At least one internal (I feel low on ammo, plus his internal weapon is simply fantastic) or you can ignore his internals, and a movement cart. Your choice if you want to use MB. Guard carts should be obvious, Beam/Blast, your choice. Broad Radar I highly recommend (I can't play without Bored Radar Ok?) if you can fit it in your build.
His overall parts isn't that bad, just I exchanged the head and legs. His arms is wonderful +4 tec/ +1 tgh, with Anti-Beam canceler attached (screw you wing sluts trying to alpha me). His BS is the same way with +4 fly/ +1 tgh. Greatest thing though that comes with Barrett is his slotted Amjs. You barely notice them and they give you a extra slot.
Last thing is you don't need to worry about slots for Barrett, since he has a locked hdj part, and comes with those two amjs I mentioned.
For switching out parts for him, I went mainly towards getting parts for fly. Wlk based is viable too, I think not sure since I made him fly instead.
2. Tuning
I highly recommend at least getting high movement of some sort. Hungry melees everywhere in this server, and airs like to tunnel you too. Overall you can tune how you want with Barrett. With Barrett having so many slots, you can't possibly go wrong (unless you use great tunes).
Once again Calc is still there peeps, if you need to make a build with him, so stop making bad builds plox.
3. Weapons
If you put on a Box missile while trying to alpha with him, I'll kick you.
Not much for this department, standard weapons like LL Baz or usual L ART weapons can be used on Barrett, just remember to switch out his parts to make it fit. Again if you went Alpha ART try to keep his Ams since they are so great.
Greatest thing about Barrett is his weapon that he got all to himself. It has a railgun pushback, but comes with 400 speed, and CHARGE DELAY. Oh its manual aim btw too, so AIM plox. His core also has a backdash ability, which is great I want to say but, has melee tracking. :/ If a target is near you, it will backdash sort of weird, and might land you in a corner.
Going ahead and saying plox don't rely on his main weapon though, too little ammo on it to actually be good enough to carry his cost. So if you are a fagget thinking his weapon will carry him, well its not going to so get used to it (stop double pompoming shit).
Edit: TL;DR
0/10 Guide since this is based on my playstyle etc.
Again he is a decent ART bot, just try not to ruin him plox. :/