Other systems may favor building characters that work great after a few more adventures, but Shadowrun is a system where you build your character to work right now. You will never again have as much funds and as much karma, especially since at character generation, cost of skills is linear, it later gets more expensive with each level. So basic chargen rule for Shadowrun is: get it now and max it out.
There is one way to build your gunslinger: pick the gun you want. Seriously, just pick one. Then look at the Availability rating. If it's higher than allowed (12 normally) then take the positive quality Restricted Gear (Run&Gun, 10 Karma) to make it work anyway. As a gunslinger has no magic requirements, so you should have enough money to easily buy it, no matter what it costs. Don't forget smartgun and if it's an automatic weapon, addons for recoil compensation. But if you want something that splatters people, it won't have automatic fire anyway.
Then, if your pick wasn't an assault cannon, buy APDS ammunition. A ton of it. It's dirt cheap and hands down the best way to kill.
You probably cannot licence the gun. If it's cool, you cannot. So you need to decide on a way to pass regular checkpoints. Either use a regular, non-cool gun. Sporting rifle, pistol etc. But you didn't want to play that character, so you need a way to be able to hide your gun. So invest heavily (that means the max of 5-6 points at character generation) in
- the skill required for the weapon (rifles? heavy weapons?)
- a skill required for hiding the weapon and yourself (infiltration, impersonation maybe?, maybe sleight-of-hand, ask your GM, there are no hard and fast rules)
Then, find something that works as a backup for when you have to go legal or when you have to go against your personal gun. For example: You have Pistols 6 for your favorite bad-ass revolver? Pack a harmless taser for fancier places. You have long arms 6 for your sniper? Pack a shotgun in case somebody gets up close. Don't be a one-trick-pony. Building a character around a weapon is easy. Making that character work in the 95% of other situations is more difficult.
That's it. Blast away.