Here is the difference with a picture
Not sure how you would make a rear block in the magazine and have the follower still work. You could also change the bolt stop in the 243 so the long mags would work, but then you would essentially have a long action .243. Some have done so to seat longer bullets out longer than the short magazine will allow. As I recall, the Tikka rifles are throated quite long in .243 anyhow.