Originally Posted by ar4711
Bipod (bayonet lug) I am shooting hornady 75 grain bthp rounds and yes a Nikon 9-14 40 mm
So, as I was saying, barrel harmonics can play an important part in repeatable accuracy by allowing the barrel to vibrate the same for each shot. One of the worst effects on accuracy can come from putting pressure on the end of the barrel, and having that pressure change with each shot. A bipod at thatched to the bayonet lug will not be great for accuracy. If a shooter wants to use a bipod or a sling for stability it is best to have those attached to a free floating handguard. This allows the barrel to move freely and consistently for each shot.
Also on heavy bullets and twist rate: most Bushmaster barrels will have a 1:9 twist. This means it will make the bullet make a complete rotation as it travels forward over a distance of 9 inches. This twist rate will usually stabilize .223 cal bullets up to a weight of 68gr. Some 1:9 twists will stabilize some manufacturers bullets up to a weight of 75 gr but it may be pushing the edge of stabilization. For bullets of 75 gr and heavier a 1:8 or 1:7 twist is better.
The 16" barrel is capable of accuracy out to 500-600 yds, but you will not have optimal velocity for keeping heavy bullets stable. Also with the lower velocity you will experience more drop at 500 yds.
F you want long range accuracy I would take the 200 bucks and save it. Then apply it to a separate upper half that is made with long range precision in mind. This way you have your carbine upper for quicker handling and shorter range shooting, and a precision upper for your longer range shots. A good 20" 1:8 twist match grade barreled upper with a floated handguard will make a big difference.