Ar-15 20 in. BB model1sales question

[Fieldman_.308]

Hey guys! Well I've had this rifle for about a year now. Up until recently I've only shot ultramax 50gr vmax. Being that in my area this ammo was getting hard to find I decided to start handloading ammo for it. The ultramax ammo always maintained better than .5 moa in this rifle. Now to give you further info, this rifle has a 1 in 8 twist. I've had hell with a lot of different combinations of reloads. I ended up purchasing the rsi shooting lab to help determine a better start. This program said with the fast rate of twist my gun would be able to stabilize larger bullets better. I've tried a lot of bullets!! Now I went back to the 50gr but in a nosler bt and today I was able to get a good varmint load. The group was around a .25 moa. Good enough for the large prairie diggers where I come from. Now after all that, my question. Why is it that my rifle is suppose to like larger bullets, but seems to find its home with these 50gr nosler bts? Just for info, the worst groups I got were @ 1.5moa, and I consistantly got .75moa to 1.5moa. Nothing terrible, but I like the best for longer range dog killin. I was wondering if it could be powder choices to fit my barrel length(20 inch). Any ideas and information would be appreciated thanks!