Using the 5.56 end cap on the Surge, what would be the min barrel length suggested for the 5.56 round?
The end cap will not have an affect on the barrel length and the 7" min barrel length requirement will apply with the 5.56 end cap. Prior to attaching the suppressor, you will want to make sure your bullets are stabilizing and not key holing to help prevent end cap strikes.
Will this end cap fit the Rugged Razor? Is it designed for use with a .223 rifle?
Yes it will fit both the Surge and Razor.