It is the extremely thin jacketed "explosive" varmint bullets that do not fair well in twists of 7 - 9" at velocities much over 2800 fps. Examples are; Hornady 45 gr Hornet, 50 & 55 gr SXs, Sierra 50 gr Blitz's, Speers 52 HPs, the 50 & 50 gr TNTs and perhaps the Midway "dogtown 50 & 55 gr bullets and maybe the Nosler Varmigedden bullets.
Larry Gibson