What’s the best home defense 556 ammo and grain?

11.5” Barrel
16” Barrel

1:7 twist

You tryin' to start a war man? You can't just ASK a question about the shape, weight, and caliber of a projectile and expect civil discourse!

The answer is "12 Gauge 00 Buck", but if you MUST use an army-man gun: It's Hornady Critical Defense .223 REM, 73gr. FTX Slow n heavy. Barrel length is immaterial with those two lengths presented unless you're just talking about the tactical considerations of moving with 4.5" more inches hanging out the front.

Twist matters less because...it's your home. Any shot you take with a rifle caliber is basically point blank. Unless your home is 100yds long. In which case, you're rich...hire security, spread the wealth. The weapon you pick should be 1x red-dot or iron site equipped (NO magnification, for the love of all things holy!), with a flashlight, and zeroed in at 50 to 100 yards. I'm guessing your 11.5 has something attached to the back made by a company called "SB". You should jam a 2nd 30-bopper in that slot (feed lips facing your shoulder, pointy tips up, primer side down) and tighten the strap. Having 60 in a fight is better than 30. It should be suppressed, or you should be okay with being deaf for the rest of your life. I chose the 2nd one, because money. You should know ALL your backstops, because anything out of either of those barrels is not stopping at the siding. These projectiles will be either landscaping your front lawn, or saying 'hi' to the neighbors (depending on proximity to neighbors). There is no 556/223 round 'frangible' enough to change that fact.

In all honesty, out of those two, I'd pick the one that makes you feel the coolest and most deadly. Go by feel. Honestly. In a live-or-die situation, confidence can be the scale-tipper.

Now, will someone PLEASE tell me how wrong and stupid I am!?!

I've been on the same journey of what to use. I've got some different ones but have settled on rounds that use 55gr or 60 gr V-Max bullet. Freedom muni sells these. If you want brand name, you can go for Horday TAP. For me, this seems like a decent way to go to be able to stop a threat and still minimize over penetration. Can read reviews a lot of places (and see lots of different thoughts like Daniel says ;-)), but here is one place: https://www.shootingillustrated.com/content/6-hard-hitting-loads-for-your-home-defense-ar-15/

"Hornady TAP Urban
Designed for law enforcement, Hornady’s Tactical Application Police (TAP) Urban load features either a 55- or 60-grain bullet and is designed to limit overpenetration through barriers when collateral risk is a serious concern."

Cool 😎!

Underwood Ammo, while pricey, specializes in defensive ammo. I've had good experiences with them every time, though.