Mark: If you have a copy of John Meredith's compilation "Omdurman Diaries 1898," the original sources there make it clear that Gatacre's command were using Dum Dum style bullets. Here is a quoted letter from Gatacre:
"The present-shaped bullet .303 Lee-Metford rifle has little stopping power. Well, we have this class of ammunition, so I am altering the shape of the bullet to that of the Dum-Dum bullet, which has a rounded point. I do this by filing the point off. Before I left Cairo I provided four hundred files and small gauges to test the length of the altered bullet, and daily we have 2,800 men engaged on this work. I borrowed fifty railway rails and mounted them flat side uppermost, to form anvils on which to file. We have a portion of men unpacking, and another packing, so that the same men are always at the same work. The men are getting very sharp at it; it would make a capital picture. The men are working very well; we have no drink, and therefore no crime or sickness. I am getting on well with altering the ammunition. We have 3,000,000 rounds to alter, but are making good progress, altering 80,000 rounds per day." (p.26)
Other of the diaries describe this project:
Lieut. Samuel F. Cox (Lincolnshire Regiment):
"... [G]ot damned long lecture in the afternoon on the subject of filing down cartridges -- discovered that, when altered, magazines cannot be used. However, G.O.C. says he doesn't care, in fact says its rather a good thing. Don't know that I agree with him." Feb. 1st, 1898
Pvt George Teigh (Co F. 1st Bn. Lincolnshire Regiment: "I was on wood cutting for bakery in the morning. After dinner I was filing bullets till tea. That is what the rails were put round the square for to hold the bullet firm against till we filed off the top of the bullet. Feb. 2nd, 1898.
Lieut Cox: "Up at 7. Ammunition fatigue the devil, all available men and officers 6 hours daily for 30 days to file off the tops. Makeshift anvils made out of inverted railway lines resting on some pins with mud as mortar." Feb. 2nd
Lieut. Ronald F. Meiklejohn (Royal Warwickshire Regiment): "The General says our bullets have not got sufficient "stopping power" for savages, so we are to file off the tops to make them expand. Gauges have been issued and every bullet has to be gauged by an officer. When Gatacre was standing near, Colonel Longbourne audibly remarked of these gauges, "Damned things are all a different length." An evident fact, but the General was clearly annoyed and said we must judge for ourselves." Feb. 2nd.
Pvt. Teigh: We paraded at 5:30 and went on route march. We are to have one every Tuesday and Saturday. We formed up and marched a couple of miles over the desert and then the alert sounded and we formed into line and fixed bayonets and then we tot he order to fire 2 rounds of ball ammunition. After we had done that the General said we cold fire all the old ammunition, so we are going to use it up every route march. I think it will be good practice for us. He said we get the new Dum-Dum Bullets up now. May 10th.
And so it goes throughout the diary entries. I take it from these entries and others like them that a clear distinction was being made between the standard issue .303 L-M bullet -- referred to as "ball" and the filed .303 L-M cartridges referred to as Dum-Dum. Maybe not "specification" Dum-Dum, but D-D in every real design and effect. Whether Gatacre's filed bullets traveled all the way up the Nile to Omdurman the diaries don't relate, but is seems reasonable that if 3 million were filed, it would be hard to have expended them before then.
Hope this goes toward you question. Annapolis