Charles Daly, CMMG, Colt, LMT, Noveske and Sabre Defence all use 1:7 as a standard twist rate and Armalite, Bustmaster, Rock River Arms all offer it as an option.
What weight bullets have you been using? Twist rates relate to projectile length, a 1:7 rate is better for longer, therefore heavier projectiles, say 75 or 77grs. Slower twist rates like 1:9 are better suited for lighter 45 to 62gr weights. Like any rifled system your results may vary depending on propellant type, gas system length, projectile construction, bore condition, throat geometry, etc, etc.
Someone noted it should be stamped on the barrel and they are correct.