6.8 SPC Barrel Twist Rate?

[rsilvers]

None of those will fit in a mag

I think you use the 110 Accubond.

As for the others, I don't have drawings for them, but I checked some other bullets.

Sierra 150 GK, 1.258 long.
Deepest one can seat this bullet: .651
Amount of bullet in case when loaded to 2.283 OAL: 0.657

Berger 150 VLD, 1.310 long.
Deepest one can seat this bullet: .694
Amount of bullet in case when loaded to 2.283 OAL: .683 (0.011 off but close enough).

On the other hand, these don't seem to be a match (when loaded to 2.283" but some may work if you are able to load longer):

Berger 130 VLD, 1.181 long.
Deepest one can seat this bullet: .560
Amount of bullet in case when loaded to 2.283 OAL: .603

Berger 140 VLD, 1.245 long.
Deepest one can seat this bullet: .627
Amount of bullet in case when loaded to 2.283 OAL: .643

Sierra 135 MK, 1.250 long.
Deepest one can seat this bullet: .524
Amount of bullet in case when loaded to 2.283 OAL: .707

Nosler 140 Partition, 1.195 long.
Deepest one can seat this bullet: .573
Amount of bullet in case when loaded to 2.283 OAL: .597

Hornady 130 Interbond, 1.262 long.
Deepest one can seat this bullet: .544
Amount of bullet in case when loaded to 2.283 OAL: .671

Hornady 140 BTSP Interlock, 1.215 long.
Deepest one can seat this bullet: .606
Amount of bullet in case when loaded to 2.283 OAL: .625

Hornady 150 SP Interlock, 1.220 long.
Deepest one can seat this bullet: .656
Amount of bullet in case when loaded to 2.283 OAL: .782

So going back to twist... If you wanted to be able to shoot the 150 VLD at subsonic speeds at very cold temps (worst case), then a 1:8 twist should do it with a 1.3 stability factor.

If you wanted to shoot that bullet at around 1800 fps at cold temps, say in an LWRC PSD, then 1:9 twist would give a 1.4 stability factor.

So probably with an 8-9 inch PDW type of setup, 1:9 would have been better then 1:10 if you wanted to be able to shoot any supersonic ammo with a good margin of safety. Or 1:8 if you wanted to be able to do subsonic ammo well.

Think of these long and not-so-common bullets as like using a Sierra 77-80 in a .223 rifle. It seems like the best analogy to a 1:7 5.56mm rifle is a 1:9 6.8mm rifle.

If you want to take advantage of the increased terminal performance of some bullets with faster-twist, then err on the side of faster twist. I am going to test that out. I have a 1:4 twist 308 barrel on order.