I have the clamshell from Brownells and it mostly works although I had to modify it for a friends rifle when we install a larue float tube. But was useless for my last billet build.
If I had to do it again, I would just get the reaction rod as you can use it for both barrel nuts and muzzle attachments. Barrel blocks for muzzle attachments are a pain in the ass, they either slip or mark up the barrel. Even if you can get to under the handguards, you still need to remove the gas tube.
Anyone try this reaction rod from gunthings.com? No idea why the lower cost version from Brownell's doesnt have flats milled into it, yeah I could use 1/2" bar stock in the end, but just more crap floating around the garage.
AR15 Barreling and Upper Receiver Support Tool. Steel, heavy duty for long term use. Made in USA. Put in vise and silde on receiver with barrel. Holds assembly while tightening barrel nut or can be used to support the upper while installing flash hiders, etc. $40.00