I was musing on the virtues of a 6mm/221 Fireball with a quick enough twist (1:8 maybe?) to spin up the 107 gr Matchking. Loaded as long as possible it should still fit in a magazine intended for the 5.56x45. The 107 ought to tip quickly on impact; sort of a 300 Whisper with lighter bullets and flatter trajectory.

Then I came across an article by David Fortier on the 6x35 TWSC cartridge Hornady is loading for the Knight Armament PDW project. Its 65 @ 2400 seems to behave a little like a rimless 256 Winchester, delivering more than twice the punch of the HK and FN PDW cartridges.

Anyway, can anyone tell me what sort of velocity a fella could expect from a 107 MK in a 6mmx221 Fireball out of a 16 inch barrel?

My guess is whatever energy a 221 gives, a 6/221 even in a 16", it will give similar or possibly a tad more energy, and then correlate fps.