Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Just a test build for myself. If people want to try it out and let me know how it works for them that would be cool too.
Laning until ~6-10: Pick up boots, a health potion and a mana potion and head to your lane, try to lane with a ranged if possible but if anything make sure they have some kind of disable or Exhaust. If you see a chance to go for a kill and bushes are clear go into the bush to charge up your Powerball once your laning partner opens on them then charge them and go for the kill play it safe though now is not the time to over extend and get killed yourself, only go for kills during laning phase if it is 100%.
First trip back: You want to pick up your Ninja Tabi or Mercury Treads depending on who you are against in your lane also pick up Ruby Crystal and either Cloth Armor or Null-Magic Mantle. Make your Aegis of the Legion if you managed to hold your lane for long enough to afford it.
Mid game second trip back: Get your Aegis of the Legion if you haven't by now and start working on your Sunfire Capes get whatever you can for them now and make a quick stop back to finish them off when needed. If you feel that one Sunfire Cape is enough then you can pick up a Atma's Impaler instead which should work out for maxing your AD early.
Late game: Time to pick up Thornmail and Guardian Angel staring with Guardian Angel first if you are being heavily focused.
Be aware this build is very LOW on Magic Resist I don't run into many problems with casters too often so I don't bother to put it in my build but if you do I would suggest changing the Health Seals to Magic Resist Seals and taking out one Sunfire Cape and replacing it with Force of Nature also Ninja Tabi can be replaced with Mercury Treads for heavy caster/disable teams.
I like Rammus for his simplistic play style some people hate him for it, but no one can argue that once you get even just decent with him you can tear up the field and tank/initiate like no one else.
Good luck have fun and please if you try this build out leave a comment here, I'm always changing my build and trying new things with Rammus so I can take constructive criticism well. Thanks.
P.S. Will update and make the guide cleaner when I get a chance to spend an hour or so here fine tuning.