It is unorthodox, but I have seen it done and by some of our club top junior shooters. These kids shoot at high levels in Service Rifle.
I had been skeptical but before they had to go off to college, they were using this technique across the course.
The typical 600 yard ammo is slow fire and single feed, so it doesn't have to be mag length. Generally the 77 SMK is the practical limit, but like I said, these guys were thinking out of the box and sunk those 80's in deep and just ran them. Now I don't hear any more about it. They are off at school and competing for slots at international levels and Olympic seats.
A typical RL-15 charge for 600 under an 80 SMK wouldn't fit if the bullet is sunk that deep. Those loads they used were in so deep the case mouth was unsupported.
The 77 SMK and the new Tipped ones are good to go at mag length for the 200 and 300 yard lines, and some are forced to use them at 600 just becasue they don't have hand loaded 80's. An 80 will shoot inside a 77 at the 600 when there is much wind to speak of.