Ruger AR556 to Grendel compatibility

[Drillboss]

You should be good to go. I did the same thing putting an Alexander Arms barrel on a Stag Model 3 upper. The Ruger looks like a standard M4 clone. As you noted, unless you get a carbine length gas system and you don't plan to use the Ruger front sight/gas block, you'll need a gas tube, gas block and handguard. Your Grendel bolt should swap out into the Ruger BCG.

Disclaimer: I don't have any first hand experience with the Ruger, I just spent a few minutes researching online.