The barrel is treaded.

I would point out that this is a .308 not a .223 and the brake is fully functional.

You could have a custom barrel nut manufactured for your rifle or just take the stock one and cut it down.

Personally I wouldn't concider the looks of my rifle for a nanosecond to two. They are weapons and performance is everything. Dropping shots onto the target is way way way (way) more important than what the end of my weapon looks like. A functional muzzle brake on a rifle is there not just to keep your shoulder from taking a whacking but to keep the weapon on target for quicker follow up shots.

Back during the ban people would argue against the looks of the barrel feature - to leave it unthreaded or to hang an non-GI brake onto the thing. I have plenty of AR's and tried A1, A2, Phantom, and AK brakes and compared them to bare muzzles. Even with the whimpy-ass .223 round the muzzle flips off target. With a more manly 7.62 the kick is amplified.

I've retained my flash suppresor. It's a simple thing to replace the barrel treatment - you do need a barrel nut pliers.