Whilst I am rebarreling my 243 I just wonder whether I might try for a little extra speed... Pretty dumb really as one of the main reasons I like the 243 is the huge amount of ammo available!

Rifle is an AII Sako, will have a 25" barrel and a magazine length of 2.8" It must feed from the mag well without modifications. 70-90gr bullets will be used and must seat to allow the mag to be used.

What is the real world difference between these at the same pressure limits. To give an idea I get 3,225fps from my 21.5" barrel, H4350 and 85grBTSPs.

To put it briefly, the 6mm/284 is essentially the same as the .240 Weatherby Magnum (or the 6mm/'06) because the case capacities are about identical. So naturally, these rounds can be loaded to higher velocities than the plain vanilla .243 or even the .243 AI.

However, with the .243 AI you have advantage of being able to shoot .243 Win. factory ammo in the AI chamber in a pinch. In addition, the .243 AI is a lot easier to make cases for - if you use .284 brass to make 6mm/284 cases, you have to size the necks down in two steps - once in a 25/284 trim die then in the 6mm/284 FL die. If you try to skip the intermediate step, you risk collapsed shoulders. I have not tried making 6mm/284 cases from 6.5/284 Norma or Lapua brass - one pass may be all that's necessary using these.

1894 ,
.243 Ackley is worth about 150 FPS over a std. .243 . Decide whether the velocity advantage is worth the fireforming hassles . I've got one and am not sure that I'd bother next time . Other than with the 55gn Nosler I have been unable to find accurate loads that utilise the extra velocity of the Ackley .
Ackley or 6mm/.284 will probably cost you one round of magazine capacity .

I OWNED A 6x284 RELOADED FROM 60 GRAIN TO 105 I FOUND THAT THIS IS A OVER BORE CARTRIDGE NOT WORTH THAT MUCH POWDER FOR THE EXTRA VELOCITY YOU GAIN. STICK WITH 243 OR GO IMPROVED, IF YOU WANT THE EXTRA VELOCITY.