Here is some info that may be helpful. http://sunfloweroutdoorsports.com/Accu-Strut-SOCOM-Barrel-strut-for-Ruger-Minis-ACCU-STRUT-SOCOM.htm
My mdl 5801 barrel measures .779 where it exits the gas block and .628 at the muzzle (my calipers are close maybe not exact).