If its AR, then 18". Still would prefer my 16" 7.62x39 Vepr or Vepr Super in 308 (depends if environment is open field or in town/forest)
In scenario other than in car or pull from closet, longer barrel always better for me (fact that I am 6.4" does sway my opinion)
In scenario other than in car or pull from closet, longer barrel always better for me (fact that I am 6.4" does sway my opinion)