2500 yards sounds a little far even for volley fire. That's nearly a 1 1/2 miles. I've never read of an instance of where volley fire was actually used, that I can remember. But it was shown in the movie Zulu Dawn. You may remember them setting out the range markers.
Volley fire and wasting ammuntion aside, the period of the Krag was also a time when longer range aimed fire was an expected skill. It is another question as to how many soldiers were up to expectations but the Boers, using issued Mausers, mostly, taught the British a thing or two about shooting during that war, chiefly the value of marksmanship and rapid fire. American soldiers during that period had no such experiences in Cuba or the Phillipines but nevertheless adopted a clip-loading Mauser.
Everything I've read about basic loads suggest that a typical basic load for a submachine gun armed man was something over 200 rounds, or about the same as the basic load for an M16 armed man (seven 30-round magazines). Oddly enough, photos suggest that Soviet and Finnish soldiers armed with submachine guns did not carry more than one additional drum magazine. I don't even recall seeing a photo of a Soviet soldier armed with a submachine gun with a stick-type magazine who had anything for additional magazine, though undoubtedly they used something. Even now, Russian soldiers do not seem to carry the number of magazines carried by US or British soldiers.
The British took to heart the idea of rapid fire during the period leading up to WWI and the basic ammunition load using 1908 webbing was 150 rounds in the pouches, which was even greater than the 80 rounds in pouches for the M14 when I was in the army.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.