I too have one of the little Mini Mark X rifles in .223 Remington. I have not checked the twist in it but the previous owner stated that it was 1-12" which may or may not be correct. I have shot almost everything from the 45gr. bullet to the 55gr. bullet, nothing heavier. But this little rifle will shoot 1/2" off a rest with several bullet weights and powders. It seems to shoot about anything 55gr. and under in less than 3/4" at 100 yards with several powders; IMR4198, H335, W748, H322. With a Winchester 55 gr. SP bullets and a heavy charge of H335(which I won't tell anyone because it is slightly above the listed book charge) it has produced more than one group of under 1/2" off a rest and I have one group which I saved the target that is 3/8" center to center. That is the load that I have settled on and haven't shot it from the bench much since I worked it up--no need to--it works. I reworked the stock from the club that came on it into a minature British Express rifle, no glass bedding, just free floated the barrel. It has one of the old Burris minature 4-12 target scopes on it. Its one of my all time favorites. My experience anyway, James