A Lot of Swarf
The billet of 1020 steel which I used to make the Armstrong 110pr breech loader cannon barrels weighed a bit over 10kg for each barrel.
1. 305mm long, 76mm diameter, 10+kg
2. 10+kg machined to 3.885kg
That leaves over 6kg of swarf for each barrel! How did I do it? I just removed all of the steel which was not cannon barrel (apologies to Michaelangelo).