Purely based upon stats, I favor the Gallant over all other weapons. Take a peek here:
docs.google.com
You'll see in cell 8U on the spreadsheet that the Gallant does 66.98 damage on a single burst (and it does this all the way out to 70m). This number is unusually high because the Gallan't burstfire is 5 very fast shots compared to the burstfire of other weapons, which are always 3 shots. This is enough damage to fire through the torso of heavy armor and kill an opponent with just one burst even while wearing a suppressor, and since the rifle has virtually no recoil, those shots usually all land on target. If you watch the Star Marine vids, most of the top competitors use the Gallant for this reason, and because it shoots across very great distance without losing power. It's common to kill at 70m with this weapon, so if you are outside mining, it is hard to imagine a better weapon. If you haven't tried the suppressor I encourage you to. It makes a huge difference because firing the weapon will not cause NPCs to aggro your position, and it's much harder for players to locate you since it also functions as a flash hider. Anyone running the 890 Jump missions should strongly consider adding a suppressor.
Perhaps the reason the Gallant is the first choice of the military and Star Marine competitors, is this flat shooting, long distance, low reload, low recoil, burstfire kill combination. BTW, the spreadsheet still says each Gallant battery is good for 120 shots. This is incorrect. It's good for 60 shots, which is 12 burstfires per battery. That means you're reloading much less often (which is when most deaths seem to occur in Star Marine.) The TTK with this weapon is longer than with other, harder recoiling weapons, but targets can't shoot you between the 5 shots they recieve from a burst, since they're busy being thrown about. So TTK is really not as important as people think. Low recoil so all the shots hit is much more important. Even though the Karna does more damge in less time with three shots than the Gallant does with 5, it recoils so much the third shot usually does not hit. Star Marine champions generally don't use Karnas. (I'm pretty sure this is all meant to emulate the difference between the American M4, and the Russian AK47.)
One thing to keep in mind when suiting up is your weight and movement rate. The faster you move, the harder you are to hit. NPCs have a terrible time hitting PCs moving at .85 and above. PCs struggle to hit targets moving at .9 and above. So IMHO, keeping your weight below 35 kg when armored is important. If you only carry the one weapon, and go medium arms and legs, this is very easy to do. You can still carry a multi-tool, 4 grenades, etc. You do not need a pistol according to the competitors. Lose what you can and choose speed and stamina.
Star Marine champions generally run all the time so they're hard to hit. Their running is limited by stamina so they dress light. If they're running slowly because of armor, they have other players run up from behind and shoot them. If they're running faster than other players they shoot others from behind. You can learn to run and fire from the hip if you round a corner and an opponent is suddenly there, and not bring the rifle to your shoulder unless you need the scope. Handy tips from the vids. Slow is smooth. Smooth is fast. Fast is deadly.