Let us also keep in mind that a "Mil-Spec" M4 is a 14.5" barrel "select fire" (semi-auto/burst or semi-auto/full auto) carbine. . . . ."NOT" a 16" barrel semi-auto carbine. . . . .that's a AR15 carbine. Calling a AR15 carbine a M4 is like calling a AR15 20" rifle a M16. . . . .not the same, not at all!
Also, you'll be VERY hard pressed to shoot 1/2 MOA groups with either a M4, M16, AR15 carbine or AR15 rifle if they have a chrome lined bore. . . . .which ALL "Mil-Spec" M4 and M16 variants will have.
The chrome line bore helps with ease of maintenance, durability and bore life but has a real negative effect on accuracy. Anyone that was in the military or that has really world experience with chrome line bore M4, M16, AR15 carbines and AR15 rifles would know this.
NYH1.