General

Blunderbuster I think is the GMS name for it. The skill that does 450% x2.
It's your main mobbing skill in 3rd job though, so would recommend leaving your bullet bill skill unmaxed since the range, number of enemies, and the effect of the skill is the same at lvl1 as it is at max lvl. The only real difference between levels for that skill is damage, and even then it's kind of small and only use it to rush monsters.
I haven't really used the bullet bill myself much to be honest since easton kind of destroys everything to fast.

oh ok, also are there any pointless skills in 3rd job which I dont use in 4th, because if there are I could apply my 3rd job skills to to booster instead of puting it on the pointless 3rd job skill.

Honestly would say none of the above. If anything booster should not be maxed if maxed rapid blast.
Physical training= more dex/str and lose out if don't max.
A higher lvl of wings= more distance I think. Not sure on that.
Higher lvl recoil= more distance, so not maxing it means hindering your mobility.
You need triple fire to get rapid fire in 4th job. No exceptions, period.
And if don't max infinity blast just wasting more ammo.
If anything, don't max booster, all you lose out is duration of the buff and nothing more which honestly is extremely fixable.

@Zorqy: It's not bad enough to delete, but if want to doesn't matter. UA is an okay choice, nothing special and even remaking as a regular character is okay. Mean lvl50 only takes a few hours.

think this is bad enought to consider remaking my corsair? Also I'm working on making my cygnus to 120 and should have it to 120 soon. Right now my corsair is only lvl 51 so I could delet it and make a UA. Do UA's have more hp or less?