It seems to me that if a 26" barrel is inaccurate, a 22" barrel won't be much more accurate if the barrel length is the true cause of the inaccuracy. We're only talking about a difference of 4".
Is 22" long enough for the 300 mag? I don't know. You may want to use a longer barrel for that magnum caliber. I'll let someone who knows more about the mag calibers advise you about that.
Wish I could help more....
