Ok, your scope is pretty good for 300 yd groups but you can only shoot as good as you can see. Your rest sounds good but I would suggest putting some type of sand bag on it. I have seen problems with groups shooting off concrete with a bipod. I guess it had to do with vibration.
My main concern is the targets you are shooting. Too shoot good groups at long range or short range for that matter, the target needs to be as small as possible. I prefer to shoot at dots. If you have a large target with no pinpoint spot to shoot at you can wobble around the target and not be able to tell how much you are moving. Still, you should be able to do better than 6 inches!
I use 20x to 25x optics for load and long range testing. They can see a 1/4" dot at 200. What you need is a aiming point that allows you to see how much movement you have. Also, at 300, you have to take in consideration the wind. It will start effecting at 300 much more than at shorter distances. If the wind is gusting it will move your bullets around. Try to shoot when the wind is at the same speed as you feel it to your face.
Another tip would be use a 1-inch square and turn it so you are looking at a diamond. This allows you to align the crosshairs better.
All in all, your shooting skills have the greatest impact on what groups look like. The longer your range is the better they have to be. Hope this gives you something to think about in an effort to reduce the group sizes. If they still open up to 6 inches, the load needs to be considered.
