Since your Hollis says "cordite. 450-400 EX and 60-400 max," meaning 60 grains cordite, 400 grain bullet, maximum; it is obviously a nitro rifle. To determine modern powder to use, equivalent to that 60 grains cordite, you need to refer to good reference; not use 60 grains of just any old powder. Whoever told you that it is a black powder rifle, is simply wrong! Hollis is an outstanding rifle---I own several, including a .450/400 nitro hammer rifle, same caliber and proof as yours, that also came out of India, having once been owned by an Indian working for the British Government at the time. Simply because a rifle is a hammer gun, does not mean it was not a nitro rifle!