It'll fit, yes, but the muzzle would be below flush of the handguard. Your options, as they currently stand, are to roll with the tri-rail handguard, or, cut your standard barrel back to 12.5" to keep the basic handguard.
I want a shorter basic handguard too. It's still early in the game yet, time will tell.
Thanks DSM. So you are anticipating a basic handguard for 10.5 barrels to be produced later? What about cutting the handguard? Do you already have an SBR for your ACR? I wonder if barrel cutting is something thats easy for a gunsmith to do, or is that a specialized procedure.
I haven't heard anything about a shorter handguard but I've seen that more people than just me want one as well. I wouldn't cut the handguard, don't think that would turn out very good. I reckon it might be an option later on. Maybe the Shot Show will have some more news hopefully.
As for cutting a barrel I'd look no further than Adco Firearms. They do great work and are pretty fast. Some folks get 7 day door-to-door service on their barrel work. For a non-NFA barrel job (cut back to 14.5" and pin a flash hider) will run right about $100. To cut back and thread a barrel (NFA) would be $65 plus shipping. Almost anyone can do it but to do it right takes experience.
Haven't submitted my paperwork to SBR the ACR yet, have to pay off some other stuff first but soon, maybe in a couple weeks or so.