10.3 vs 10.5 makes no difference. I would bet if you put the same Flash Hider on both they wind up the same length. Twist rates and bullet weight are whats important at 10". And it looks like you're going to be shooting 5.56 out of a 30cal suppressor so I wouldn't worry in the least. Surefire makes a "SBR" suppressor. All they did was make OD of the baffles larger.