Barrel Nut, 6940/M5

This barrel nut requires a special wrench that is not readily available.
Unfortunately - there is no way to build an "inexpensive" wrench for the 6940 nut.
The Colt M5 also uses this same nut. 

SA originally purchased a wrench ($100+) designed by Phil Arrington.  (15+ yrs ago.)
And we destroyed it on the third barrel that we removed.

Yes, we have engineered and built precision tool-steel wrenches for several govt and industry clients.
Seven versions:
6940 - Original design (2009) - 8 Tooth
6940 - SA Krieger Precision 18 inch Barrel (1 shop wrench)
6940 - Heavy Barrel (SCAR A)
Colt IAR - Heavy Barrel (+)
Colt IAR - Torque wrench version
Colt M5 - (2025) - 8 Tooth
Colt M5 HB - (2025) Heavy Barrel - 7 Tooth

These SA wrenches are very expensive to produce:
$100+ material, hours of machine time and fabrication, TIG welding, multi-part heat-treatment, finishing, and phosphate plating.
We do not stock or actively market industrial tools, fixtures, and gages.  

SA does currently have a very limited number of M5 wrenches in inventory, if needed. 
$750 Delivered to you.