@biggerhammer and @sigm4 - you two are right on! This is our first "system" of components for the Scorpion (everything else that we make just drops in). So we tried to take our time and be diligent about the install guide... and 30 min install video... Took the approach of too much info being better than not enough.
So regarding the charging handles, our handguards are compatible with every charging handle on the retail market today (oem, hbi, shooters element, manticore). We checked, using multiple third party testers with each variation of aftermarket charging handle, all with nothing to gain/loose in their feedback. The reason that we mention the aluminum on aluminum friction/drag is that two of our testers noticed it, but recognized that they probably pull up/down on the charging handles as they pull back. With everyone else it was a non issue. So, we are just trying to but 100% transparent up front with the form/function because we did have it come up in testing. It is fairly common in firearms (ie:mp5, even the tavors) where the charging handle operates off center-line with the bolt, a small amount of lubrication completely resolves the issue. Or, in the case of the scorpion, simply using the polymer charge handle. Again, this is only for users that also pull dramatically down/up as they pull back.