Back in the early eighties, I managed a 250-19x and a 249-21x ; both were fired with a 6PPC. A thirty would have gained me some x's but not enough to give me 25 of them. It would not even have turned that 249 into a 250. When I miss, I miss big! I am truly awed by the shooting of 250's with hunter rifles and impressed with the same thing with varmint class rifles. It's not easy. With a hunter class rifle, I've never even come close. I have not even tried for close to twenty years but I doubt I could come any closer today. GD.
A few years ago I shot a 250-20X with my group gun, a 10-1/2 lb. .22 PPC .100" short. I took the targets home and carefully plugged the non-X's with a 30 cal. plug. It would have been an easy 25 X. That was when I was younger and could shoot straight.
Anyway, that was when I had Dwight Scott chamber another gun to a 30 BR. It kicked a lot more but that is a really good cartridge.
We ran those matches a long time... since 1991. To my knowledge the 250-20X was the highest score we ever had with a .22. Over the years we had a couple of 250-25 X's shot, both with 30's. There never was a 25 X shot with a 6 mm.