I have a .308 in a Remington Mohawk 600. I was wondering if I could get a 243 barrel to interchange the two calibers or would it take some machining to do this.

you should contact remington to see if they still offer a barrel for your rifle , which they should still have for any repairs that may be neccessary on the older guns.

it might be cheaper for you to just trade it for a newer model seven or any other carbine that you might like

I don' t think I would ever get rid of my 308. I love the way it shoots. It will put bullets in the same hole a 100 yds. I quess I will have to buy a whole other rifle. What a shame.