How many rounds have you put through the rifle? If it is brand new it may need to smooth out the rifiling in the barrell. Check out this post
http://www.huntingnet.com/forum/tm.aspx?m=1181923&mpage=1&key=cotton%2cbal lí±‚í´¬

My brother in law has the Optima Pro and is using 90 gr. of Pyrodex RS and a 295 Powerbelt AeroTip bullet with a Winchester 209 primer and has decent groups at 100 yds with open sights. Hope this helps.