Free: Contests & Raffles.
High powered scope wont make you a 400 shooter...I know guys that with enough practice are surgical at distance with fixed 4x to 6x scopes.. practice makes you a 400yd shooter..Stick with what you got, practice with it and build up your distance comfort level..a 3x9 is plenty for 400 if you know how to use it. If you desire a new scope focus on clarity, not power. A really clear 3x9 or 4x12 is perfect for blacktail. 44 or bigger objective lense for light gathering and your good...then lots of trigger time..step angles, different positions, different wind...etc.
I don't see anything saying you're going to practice a lot.I don't see anything saying you've tried either of your rifles at 400.I don't see anything saying you've ever shot at 400.I do see a statement that you haven't rifle hunted in 10 years.Missed opportunities because of distance are part of hunting.
I hunt with and love my 243, have killed several deer and lots of coyotes with it, it is NOT a 400 yard gun the bullet weight and energy is just not there. Can it be done? YES. go with the 06 /put on a great scope and practice,practice,practice. Carl
The trick is being able to accurately range to that 400 yards. It's pretty tough to judge distance in the field some times and the difference between a 300 yard shot and a 375 yard shot is enough to cause problems. If you're going to shoot critters past 250 yards, you really need a rangefinder.