Re: .243 Ackley Improved
You do not say what your intentions are for the rifle.
My goal was to be able to shoot the heaviest bullets I could, up to the 105gr VLD. If that is your plan, be sure it is a 1:8 twist. Also, be aware that depending on how abused it was, the barrel will be gone anywhere from 800-1500 rounds max. The lower number if used for competition or prairie dog shooting.
I would not buy a used 243 Ackley without having the throat examined with a borescope to be sure there is some life left in it. A Remington is relatively expensive to re-barrel. You would be looking at close to $600 and possibly more and fairly lengthy wait times unless you have an excellent gunsmith close to you who is experienced on working with match rifles.
There are members on this forum who had a custom 243 Ackley barrel made for their rifle with a 9 twist and a throat that was slightly too large in diameter, and the result was that the rifle could not shoot any boat tail bullet accurately. Some of the Remington contours are not fit for setting back the chamber and cutting a new one. If you go with the 3.5" straight section followed by a straight taper to the muzzle then one has more options in future. Remember the throat is first to go with the muzzle second.
So decide what you want to do with it. 65gr Vmax at 4000fps ? Or 105gr VLD at 3000fps ? For the heavier bullets it also helps to have enough barrel length (26" vs 22 or 24").
For fireforming, load your new brass with a "normal" 243Win charge. It can even be a "hot" 243 Win charge. You need enough pressure to properly form the case, so weak charges are out. For me it is 41gr of Hodgdon H100V powder under a 95gr Berger VLD and it shoots to 1/2 MOA. It is my hunting load until I get together a big enough batch of fireformed brass to start load development with 105gr VLD's.