I want to talk a bit about momentum. On IRC, we had a pretty sizable discussion about Steel / Flying. While it initially seemed appealing to me (Ground immunity + Fighting neutrality), it really doesn't do anything against key revenge killers. Furthermore, I want to note something very key here:
being weak to Volt Switch and/or U-turn sucks for this concept. One of the biggest issues with Belly Drum is that it can result in a considerable amount of momentum loss. If we create a Pokemon that is forced out by either of these moves, we've created a giant momentum burner, which won't be a popular option for competitive players. While things like Stealth Rock neutrality and immunities are important, I think focusing in on how we interact with these two moves is going to be key. In fact, that's why I've been researching Ground-type so furiously for this CAP; that Electric-type immunity is critical here. Unfortunately, Ground-type has a lot of other weaknesses that make it undesirable for this concept, so let me propose something else. NinjaStars brought it up a few posts earlier, but I'd like to dig in-depth on why I think it's such a great typing for CAP 6.
Dragon/Steel
2x: Fighting, Ground
1x: Fire, Ice, Dragon
1/2x: Normal, Water, Electric, Flying, Psychic, Bug, Rock, Ghost, Dark, Steel
1/4x: Grass
0x: Poison
This is the result of me wrestling with the things I didn't like about Poison / Steel. As I listed above, I am largely wary of Volt Switch, and I didn't like how Poison / Steel doesn't do anything to discourage its use. Furthermore, it doesn't resist Water, which is absolutely rampant in this metagame as well. Finally, I think Dragon-type gives us the
perfect offensive typing to fulfill this concept. While this new typing does create some new weaknesses, those weakness are desirable.
Why exactly are we against making CAP 6 weak to Fighting-type? I asked this in #cap, and I was greeted with a chorus of "Mach Punch". But which Pokemon in OU are using Mach Punch? Breloom can easily be picked off by neutral priority. Conkeldurr and Infernape might give CAP 6 a run for its money, but its being weak to those Pokemon necessarily a bad thing? They're not really common in OU at the moment anyways... To finish off priority weaknesses, Dragonite and Scizor are both resisted by this typing, so no dice.
So what about scarfers? My case is that popular scarfers in OU are either a) stopped with neutral priority or b) stopped by having all of their attacks resisted. Terrakion and Keldeo are a good example of the former. They're not necessarily bulky, and won't stand up well to a neutral priority attack from a +6 Atk behemoth. The ones that are more bulky (Latias and Jirachi) don't typically run moves that are Super Effective against Dragon / Steel. This creates a fantastic situation for us:
CAP 6 is seldom stopped once it sets up Belly Drum. Note the 'seldom' in there. It makes it so that CAP 6 can be stopped and isn't completely broken in OU. If Jirachi is forced to run Earthquake on its Choice Scarf set, that's a predictable move that a competitive player can utilize to get momentum. Defensively, Dragon / Steel is sound. It resists switching moves like U-turn and Volt Switch, while creating momentum in that it's only stopped by a few, select, super-effective attacks.
I want to make one more comparison between this and Poison / Steel. One of the things that I struggled with in Korski's suggestion is that Poison / Steel doesn't have any good STAB options. While there are advantages to that maneuver, I do think it opens CAP 6 up to being a lot more weak than it needs to be. With Poison / Steel, I could see CAP 6 being forced to run a set of STAB / Coverage 1 / Coverage 2 / Belly Drum. As I discussed in Concept Assessment 2, this is not optimal. If we can free up a moveslot for self-reliant move like Wish (for healing) and Confuse Ray (for forcing switches), then we're in much better shape. Therefore, I'd like to see CAP 6 with strong, offensive, neutral coverage. We can easily limit this coverage option with either typing, but I believe that Dragon / Steel does it better because it has a strong STAB option that is its default maneuver. Dragon-type pairs up with a few other typings (my favorite being Fighting) to create an unresisted core of moves that allows for two free moveslots.
Furthermore,
the STAB, physical moves for Dragon-type fit this concept to a tee. We can utilize the typing to force CAP 6 to use Belly Drum in order to be viable. Dragon Claw is a great in that it's largely unusable without some sort of boost. Its interaction with Outrage is curious in the case of Belly Drum. Almost all Dragon-types opt for Outrage (even on boosting sets) simply for the sake of more power. At +6 Atk, you have all the power you need. While Outrage would be a solid option, I could envision Dragon Claw being the preferred move due to its non-locking properties. Finally, relying on Dragon-type STAB pushes CAP 6 away from being tempted to use an All-Out Attacking set, or even a Choice Band set. These are both horribly outclassed by Kyurem-B, Dragonite, and Garchomp without Belly Drum. CAP 6's viability would come through what it has to offer over other Dragons: Belly Drum.