I had my .264 Mag (custom built rifle with the barrel shot out) re-barreled to 7MM Rem mag. I have a whole pot full of .264 brass. Would it be practicable to convert the .264 to 7MM mag? If so how would I do it?

Should work. Lube some up and run it through a 7mm mag sizing die and see what happens.

If it's old, you may end up with some cracked necks.

Get a 7 RM sizing die and a good case lube (Imperial Sizing Die Wax is one). Taking time, run the case into the die and begin expanding in increments turning the brass with each increment. Lube between increments.

Sizing up has always proven easier for my several wildcat cartridges than trying to neck down.

You will end up with a slightly shorter neck length than the original .264 WM, but should have enough to properly seat the .284 bullet.

The 7mm rem mag came out shortly after the .264 win mag, when you look the only difference is the neck size.

Get a cake pan or such and put about a half inch of water in it. Stand your .264 cases up in it about an inch and a half apart. Using a propane torch heat the necks up just red. Tip the case over in the water when the neck get red.

Now resize your cases. About every fifth firing do this again. Your cases will last a lot longer. This is called annealing and softens those fire hardened necks back to the same or near same softness they came with from the factory.

Thanx guys, I use to do a lot of handloading years ago and am now getting back to the shooting game. Relearning all over.