Looking at replacing my old tasco 4x32 scope on my ar15. I bought a NcStar Colt copy and sent it back to Optics World for a refund. But I have a buddy that will sell me his Nicon PROSTAFF 3-9X40 MATTE BDC for 100$. Is this a good scope to put on my gun that will imporve the results of my bad groupings of 8 inches at 100 yards? I may have another problem. When I bought my upper and built my gun on a PWA lower I bought it from a friend who had a [legal] machine gun in Florida many years ago. He was putting on a different twist for his gun and sold me his upper to build my gun. I have always wondered if the guns barrel was heated up too much and could this be why it seems like the gun will not shoot as accuratly as my Colt? Or is it the 29$ walmart tasco scope? The thing is I have had several of these cheap scopes on guns like my Remington 700 and they shot at 100 yards fine.