10" handguard will tuck a KX5 nicely on an 8.5" barrel , but you will not be able to tuck your can inside any slimline handguard. You will be forced to go with a fat free float quad rail or a Seekins MCSR. (large diameter) to tuck any 30 cal can.

If you want to butt the can directly up to the end of the hand guard for a seamless look you need 3/4" offset for your barrel length i.e your 8.5" barrel would need a 7.75" or shorter hand guard for any QD silencer mount to work, otherwise your can will bottom out and come into contact with the handguard.

We use a 7.25" rail on our 8" blackout barrels and the silencer ends up with about 1/8th" clearance when mounted which is perfect

Hope that helps

My above reference was for QD mounts. Direct thread is different and I still use that rule of thumb just to be safe. Either way if you only have a half inch difference your threads may  be slightly buried. With .75 difference the threads are just barely fully accessible. I.E Daniel Defense MK18 RIS hand guards are 9.55" exact top rail length to mate with a 10.3" 5.56 barrel and get the can tight to the hand guard with a QD mount..


Here is a 10.5" barrel with 9.7" handguard. Threads are barely exposed