I own maybe half a dozen slogan slings. You do NOT need the magnum sling. It's very heavy duty and not as handy as the regular size. I carried an 11lb magnum rifle all over hells half acre in Africa with just the regular sling. I tried to buy the magnum and Slogans owner talked me out of it. It was good advice. When I finally saw a magnum sling in person I realized it's way overkill.
Seriously, I would change your order.
Their slings start a little stiff and soften a great deal as the rubber starts to stretch and wear in a bit. The grippiness never changes and the backpack functionality is great.