Barrel nut shims are not the first thing thing - or even the 7th thing you think of when installing a new quad-rail or keymod handguard onto your rifle, but they go a long way to help time things and make them line up with your receiver.

I try to keep a couple handy so I can tweak and customize my rifle, or the rifles of buddies and keep the spacing and everything lined up.

Maybe you'll be lucky, and that barrel nut for your new handguard will line up perfectly without being timed?
I wasn't so lucky, but these fixed me right up. Good, quality shims.

I bought 2 sizes of these so I would prepared for my new handguard install. Works as they should, no issues.