Lazrbeam-
I fiddled a bit with the jbm stability calculator on this problem. The shorter solid copper bullets from Barnes, Cutting Edge, and Lehigh were not positively stable at 4100 fps with a 14"-twist.
As suggested above, a trial with a shorter lead-core bullet constructed to hold together at high velocity may be worthwhile. Nosler's manual #8 shows the 40-grain Ballistic Tip at 4200 fps in the 220 Swift and 4400 fps in the 223 WSSM, Hornady's manual #10 lists the 40-grain Vmax at 4300 fps in the Swift. Some forum members having experience with these bullets may be able to confirm whether they hold together at these velocities. It would be interesting to see whether they can stand still higher speeds from your rifle.
Alternatively, it may be possible to shorten your Barnes bullets with filing or other technique. Removing length starting at the nose would give the greatest change in length for the least change in weight. This certainly wouldn't help ballistic coefficient and may detract from accuracy, but you might obtain a bullet form that wouldn't keyhole.
--Bob
