Welcome to my Graves guide. Bet you're all here, because a lot of Youtubers have said that he is a pick or ban champion. This is correct and you've come to the right place.
Why should I pick Graves?
Graves is an offtank carry with loads of outplay potential and mobility in his kit. But he is very vulnerable to CC, and doesn't possess any in his kit.
P.S. I apologize for my English and no editing, because I'm lazy. I also won't go over the Runes and Masteries, because nobody reads that. Feel free to comment, if you need something I am always active.

Pros:
- Coolest character in league
- Tons of outplay potential
- Mobility
- Insane burst
- Can 1v5 if fed
- Fits in most team comps
- Strong early game
- Scales well into the late game
Cons:
- Not the best ganker, due to your Q being an ability that is easy to dodge
- No CC in his kit
- Takes a lot of time to master
- Useless without experience

Now that we know his pros and cons, let's get into his skills/abilities:

Passive: This is what makes Graves the champion that he is.
Every basic attack against non-champions knocks them back, which makes him a viable jungler.
After two basic attacks he has to reload. By building attack speed, your reload speed increases and also the speed between the two shots.
His basic attacks fire four bullets in a cone. Shooting closer to your target increases your damage (works well with crits)

Q(End of the Line):
Graves shoots an explosive shell that detonates after 2 seconds(goes back), or instantly if it hits terrain. This deals absurd amounts of damage if the champion tanks the shot both forwards and backwards. However, you should know that it does damage where you put it to go. So if you cast it and move away from it, it won't follow you back.

W(Smokescreen):
Graves throws out a smoke bomb that blinds and slows enemies inside of it. Use it when you're catching someone.

E(Quickdraw):
Graves dashes in a location. During the dash, he reloads one bullet. Every time he uses it he gets a stack of True Grit. True Grit are those circles under Graves. Every stack of True Grit gives him 5-15 Armor (no more 5-15 MR :( but this is why it's important in the early game). He loses his stacks after being 5 seconds out of combat. (Hitting minions doesn't count as being in combat, but it works with jungle camps)

R(Collateral Damage):
Graves shoots a big explosive shell that knocks him back dealing massive damage. After hitting an enemy champion or reaching the end of its path, it explodes dealing damage in a cone. Using your Q and this will most likely delete any enemy squishy.

Those were his abilities, but here are some of his combos:
Q+R: Massive burst damage, but use it near terrain to reach its full potential.

AA+R+E: A combo that is not that known. You can easily mess it up, so be sure to use it in the practice tool. USE IT ALL AT THE SAME TIME! After using your AA, press R(first) and E at the same time, as it will cancel the knockback from your ult.
(Rest is up to you, make those sick plays!)

First things first, I recommend starting E over Q, because of the early stacks of your True Grit, and the extra bullet is always welcome. The best thing about Graves is that he doesn't need a leash and can clear the whole jungle, whilst staying relatively healthy.
This is the jungle path I recommend:
Blue Buff (use E as much as you can)
Gromp (smite it when you're about to kill it)
Wolves(be careful here, you want to keep your True Grit stacks when you are switching to the red side, so left click the main wolf and when it is at around 200 health, start moving towards the red side, and clear it somewhere in the middle of lane)
Raptors (when you kill the big wolf, use your E one more time to keep your stacks and be strong enough to kill them)
Red Buff (you will most likely have smite up when you're clearing it, so smite and use your E over the wall to get to the Krugs)
Krugs (just kill them)

Note: You will have around 800-900 gold after clearing and instantly look for a place to gank, to get the Warhammer in your first back.

I love this champion. You always have something to do with him. If everyone is winning lane, be sure to put the jungler behind. If they didn't clear their Raptors/Krugs, take them. More gold for you. Your items are cheap, so as soon as you're full build you will be unstoppable.
Keep dragons and barons warded.
And remember, always do something if you're a jungler. If you're feeding, clear the jungle or get dragons. Even if you're behind, you can 1v1 almost anyone in the game. Adapt to the game. If they have AP, get a Maw and Mercury Treads. Be smart. Be like me x
(A quick mention for the build:
The path is: Warrior, Cleaver, Phantom Dancer, Sterak's Gage, boots of choice, and you're last item depends on the game.)
Good luck and start getting those pentakills! ;)