Pro's:
Making squishies your shield fodder
Extremely hard nuke
His offense feeds his defense!
You are the Master of Metal!
One of the best farmer's in the game.
Nigh unkillable when fed/farmed in enemy minions.
Calling yourself Mordecaster is such a pleasant play on words!

Con's:
Stuns hurt... alot.
Ranged harass can hurt you rather well without minion's.
CC does bring you down rather painfully.

(This build assumes you are knowledgeable enough to know how you would like to buy potions, wards etc throughout the game)
Now firstly, you may ask, why Regrowth pendant when there is a perfectly good doran's shield!? My answer is this: With the healing seals and this you will have a lot of hp5 which is great for the first few levels of game play which allows you to spam and keep your shield up. That leads to the second item, if the farm is doing well, and you can, buy the revolver right away, this will make your attacks, which cost health, to heal you and feed your shield, as long as you aren't taking heavy harass when your shield is down, this makes it so you can stay in lane indefinitely. Though if you are forced to back, get basic boots with a tome if you can afford such luxuries! =P

On the boots, I prefer boots of lucidity because I like the ability to spam my attacks more, you can go sorc shoes, merc treads, whatever suits your play style or how the enemy team is like.

The next two items should be farmed for or killed for asap once you have rank 2 boots and your gun. Rabadon's will give you a heck of a lot of AP which will once again, feed your shield and your ult. The next item Lich bane, this is normally where I sell my regrowth pendant, it makes your Mace of Spades hit like all hell, end of game you can easily have your mace of Spades do a single target hit for 1.3-1.5k damage. Did I mention its CD is under 3 minutes? This is great for picking off squishies, those with no magic res can usually be 2 shot by Mace and your Ult, though a siphon somewhere in there doesn't hurt.... you.

The last 2 items are rather special, special in the sense of they change a lot depending on the game, sometimes it can be Rylai's and a Void which is great if the enemy team cant bring you down, or you can go with Zhonya's and an Abyssal scepter, which are both great items for AP and resistances. It leads to many different road's, sometimes a GA is in order, but as I mentioned, all depends on the game.

Now though this may sound completely awesome so far, why turn the revolver into a gun blade? I have a few answers for this:
-I feel it has more damage output then a will of the Ancients.
-I like shooting people
-Its a good slow if someone is just out of range and you can get em
-See second point
-Because shooting Caitlyn back once or twice is the least you can do for tickling your shield with all those peacemakers and head shots.

But once again, all depends on play style and how the game is, I have been know to buy a Will instead if my team is caster heavy.

I hope this item build is helpful and informative.

The order of skills is just a rough outline I have added, though depending on who you are laning against, you can take Mace of spades at level 2, I do that if my shield is doing fine and not being harrased much, this will have a chance of one or two of your maces to fly off and hit the enemy champs.
But all in all, the priority for skills should be:
1-Children of the Grave
2-Siphon of Destruction
3-Creeping Blades
4-Mace of Spades

Spam, spam, spam and more spam, enemy minions are your friends, fodder, and gold mines. Siphon minions and you can hit champs too! Harass, farm, and shield! 3 bonuses for just clicking E! Oh the wonder, Oh the excitement! Oh the rofls!
The best defense sometimes is a very, very angry offense, Which is especially good against those who harass by nickle and diming, such as Malzahar's Damage over time, keeping your shield up will make him lose mana and leave you unharmed.
Though once you get into team fights and late game, you are not a good opener, you need a bit to get your shield up and have the stuns out of the way, once your tank or silly silly shaco has absorbed all of the enemy stuns for you, its like shooting fish in a barrel, a really big barrel with a giant mace!

One thing that should be kept in mind, Your ult late game can do a base amount of 55% of enemy's health in damage, if as I said, mace of Spades them, a siphon, ult them and ignite, there are very few champions who would survive, giving you a minion who powers up your shield.

Okay, thats about all I can think about now, please feel free to message me with any thing which you wish for me to elaborate on, such as runes, or masteries etc. As well as any mistakes I may have made (And if you say I didnt build tank, I will cry, and there will be lots of it! not a pretty sight, promise) I will try to add/change/fix whatever I can in a timely manner. Thanks for checking out this build!