So you call Magcargo bad?Prove it in another way bar stating "its typing is bad". That typing allows it to maul major fire types such as Talonflame,Volcarona,fairy types and ice types without 4* SE coverage (which you can predict since the opponent wouldn't switch it in otherwise).Ferrothorn,Skarmory and Forretress can't do much to it,whilst Magcargo incinerates them in return.
But...Magcargo's typing really is awful, lol. It gets a handful of resistances, but it also gets several common weaknesses as well (including two 4x weaknesses and a Stealth Rock weakness) that prevent it from actually switching in and walling much. Even with the resistances it does have, its many flaws keep it from effectively taking advantage of them. It can't even really beat much of the stuff you listed.
Most Fire-type Pokemon don't really care all that much about Magcargo. Volcarona almost always carries one of Giga Drain, Roost, or a super effective Hidden Power in its last slot, in which case it outright wins. If it sets up a QD on the switch and a second one before Magcargo gets a chance to attack, then it reduces Ancient Power to a 3HKO (and that's just the offensive variants, not even counting bulky Volcarona). Volcarona with Roost just sets up all over that same Magcargo, those with Giga Drain easily heal off the damage and keep sweeping, and those with a super effective Hidden Power just OHKO (unless you're running a specially defensive set). Mega Charizard X generally carries one of Earthquake or Roost. Earthquake destroys you while Roost lets it set up all over Magcargo (Ancient Power can't 2HKO). Mega Charizard Y can 2HKO with Solarbeam, KO with Fire Blast + Solarbeam with Stealth Rock, or outright OHKO with Focus Blast (and Ancient Power can't OHKO). Heatran demolishes you with Earth Power, while Infernape obliterates you with Close Combat. You beat Talonflame I guess. Yay?
Most common Fairies don't really care what you do. Mega Mawile easily 2HKOs with Iron Head (or OHKOs after SD), and Stealth Rock gives it a shot at KOing with Play Rough + Iron Head. Clefable (most have Magic Guard) just laughs at you and sets up Calm Minds all over Magcargo or just poisons it and stalls it out. Azumarill...lol. If Togekiss doesn't carry Nasty Plot or Aura Sphere, you can sorta beat it provided you get around the paraflinch. Even then, you just beat it through PP stalling, which I wouldn't exactly call "mauling." Sylveon, again, can only be beaten through PP stalling (unless it carries Calm Mind or Toxic, both of which are pretty common). Gardevoir beats you with Focus Blast. You beat Klefki outside of bad Swagger hax, but Klefki gets more irrelevant as you get into higher play.
There are only 2 relevant Ice-types in OU, Mamoswine and Cloyster, and both commonly carry moves that destroy Magcargo completely. So yeah, you don't even beat the common Ice-types.
Okay, so you can beat Skarmory, Ferrothorn, and Forretress. That really doesn't mean much seeing as how they're each specially frail and/or 4x Fire-weak. Even Life Orb Scizor with HP Fire can beat these three depending on the set. It doesn't make it any good.
Aside from its bad typing, Magcargo has other qualities that make it terrible. Its special bulk is very low, and it's special power isn't even that great with a measly base 80 SpA stat. Its physical bulk isn't even as good as its high Def stat would make it out to be; in fact, a physically defensive Magcargo is taking physical hits worse than an equally invested Terrakion. Above all else is the fact that there's literally nothing that Magcargo can do that something else can't do better, especially Heatran. It can spread burns, set up Stealth Rock, and beat Talonflame with Ancientpower just like Magcargo can, only with far better typing, way more power, better bulk on both sides, and no Stealth Rock weakness. The only thing Magcargo has over it is reliable recovery, which doesn't mean much when you have so many common weaknesses and so many opponents 1-2HKO you so easily. It's just so mediocre in so many areas with too few redeeming qualities to set it apart.
Magcargo is bad. There are no and's, if's, or but's about it. It is bad, and the best thing to do at this point is to just find the best ways to utilize it in order to make something out of its pitiful existence. Stockpile is not the way to do it. In practice, Stockpile Magcargo will usually not be walling much even with those boosts because it'll constantly run into problems with critical hits, super effective Earthquakes/Surfs/etc., phazing, status, etc., and that's ignoring the fact that no good player is going to let you get more than one, maybe two boosts to begin with. The best way for Magcargo to actually work in this metagame is probably with a specially defensive set like this:
Magcargo @ Leftovers
Trait: Flame Body
EVs: 248 HP / 8 Def / 252 SDef
Sassy Nature
-Lava Plume
-Stone Edge
-Toxic / Stealth Rock
-Recover
Stone Edge > Ancient Power because it hits harder on average even when taking into account Magcargo's low Atk stat, and you need it to actually net the OHKO against Mega Charizard Y and 4/0 Volcarona. The set isn't that good, but with maximum special bulk, Magcargo can survive Mega Charizard Y's Focus Blast from full health before OHKOing with Stone Edge, and it can actually beat most Volcarona variants not carrying HP Ground (or Life Orb + HP Rock, which still can't OHKO a full health Magcargo at +1), each of which it was unable to do before. You still handle Talonflame just fine since even Adamant CB Brave Bird can't 2HKO after Stealth Rock. With Toxic, you can also beat Togekiss, Sylveon, and Unaware Clefable more easily (Toxic's PP outlasts Heal Bell/Aromatherapy's). It's not much, but at least it lets Magcargo beat
something besides a handful of Fire-weak support Pokemon. The only thing I can think of off the top of my head that you really miss out on beating with a specially defensive set is Bisharp, but even then, Bisharp's +2 LO Knock Off OHKOs physically defensive Magcargo 50% of the time (and all its attacks are guaranteed OHKOs after Stealth Rock).
