We've used a lot of fusing methods over the years, but now tend to use pyrotimer where we want accuracy between shells or the Jubilee strings where we are less concerned about accuracy but more interested in quickly fusing and a quick set up. For those that haven't seen them the Jubilee strings are a length of quick match with a number (usually 5 or 10) leaders coming off of it with delay fuse of differing lengths leading to a "bucket" end into which a cut shell leader is placed and then...
Shell Fusing
Shell Fusing