I wanted to attempt a RG variation on the now popular GW Hardened Scales builds. Been a long time affinity player, but admittedly I don't play much anymore and have only recently adjusted to the Mox Opal ban. I will never get over that ban, and in my opinion other decks ruined it for all of us. I'll also miss Cranial Plating but maybe it could still fit in somewhere perhaps even the sideboard. Sadly, this nerfed Inkmoth Nexus but you can still win via infect if you have a Ravager hanging around. Anyways, about ten years ago I bought into affinity because it was a very old, consistent and rarely changing deck. I really like how this deck has more inevitability than traditional affinity, and honestly I like how the deck has evolved.
Naturally, your big hitters are going to be Hangarback Walker, Arcbound Ravager and Walking Ballista. All of which make the deck intensely involved with complicated math, which with a good pilot simply makes other player struggle to keep up. Many don't know how to handle the deck aside from sideboard artifact hate. Obviously the namesake Hardened Scales should be in your opening hand, and works so well that the deck handles mulligans relatively well.
Many of the cards here are simply an easy slot in and are the core of the deck, but I'll talk about some personal thoughts. Grove of the Burnwillows should not be played in an aggro deck, but the art makes it my favorite land... so here we are. Others may prefer Karpulsian Forest but you should play what you like, don't get hyper focused on being competitive. Speaking of which, Welding Jar is significantly better than Spellskite but it is a really cool card if you like artifacts. You should replace it with Jar. I am not entirely sold on Animation Module, but when it works... it works and has an emphasis on artifact creatures that synergies especially well with Hangarback Walker. Which is why we play this deck. That said, it is resilient, inevitable and surprisingly grinds.
We can bury the opponent with card advantage via Ancient Stirrings and Urza's Saga and seek out toolbox cards like Shadowspear and Welding Jar. I was typically jealous that other decks played planes wakers, and have therefor been testing Karn, Scion of Urza. In grinds games I. was very happy with him, but it is not for everyone.
Since I don't play much anymore, I don't know much about the current meta and because of that most should disregard my sideboard.
This description isn't at all very helpful, but I hope you enjoyed skimming through the deck. I definitely recommend that you watch some walk-throughs on YouTube.