New Army "Caliber Configuration Study"

[stanc]

The tracer does not need to be a matching 130gr projectile. I 115-120gr tracer can be loaded to ~match the ballistic trajectory.

Yes, tracer bullets are often lighter than ball projectiles. For example, Swedish 6.5x55 Ball is 140gr, but 6.5x55 Tracer is only 120gr.

However, I did not mean equal weight. Perhaps "companion Tracer" would've been a better choice of words.

In any case, the point is that a lead-free Tracer bullet would almost certainly be longer than a lead-free 130gr Ball round, which itself would be quite long. What rifling twist would be needed to stabilize such a long projectile? Would it be stable in arctic temperatures? Would it be suited to the M855A1/M80A1-type construction?