I'm going to re-barrel my Ross Mk III

[Bruce_in_Oz]

.30-.303?

Been around for ages. At least one US gun magazine did an article back in the 1970s.

I have been running one on No4 action for about twenty years. Mine used a pre-loved Omark target rifle barrel trimmed at the chamber end and re-profiled. Only catch is that it has 1:12" twist rifling. The chamber was cut with a standard .303 reamer with a .299" pilot fitted for the job. It WILL chamber and fire Mk7 ammo, but accuracy is not good because the Mk7 bullets are too long to properly stabilize in a 1:12" twist at sane .303 velocities. If you are intent on using longer heavier bullets and have a spare 1:10" twist .300 / .308" barrel jying around, go for it.

Sierra 155 Palmas work well in my 1:12" .30-.303, as do the Australian-made BJD "HBC" bullets, even with the generous throating for the Mk7. Most "off the shelf" .303 reamers are cut to "traditional" dimendions; meaning that the throat (Leade) will be long enough to easily accommodate 220grain round-nosed .312" bullets If you are going for maximum performance get the chamber cut with a throat SPCIFICALLY for 155 Sierras or such. It then certainly will have problems with longer, heavier bullets, but, does that really matter?

.308 bullets in the 168-180gn range (HP or SP, but NOT some of the super-slinky "VLD" style) may well work in the 1:12" twist, IF you keep velocities up. For any given bullet and rifling combination, stability is determined by spin "velocity" / "rate". Spin rate is determined by muzzle velocity from that twist rate. See .300 "Whisper" / Blackout for an extreme modern demonstration.