I used to make custom arrows and tapered and barrel tapered them on request. Normally I found that if I took a parallel shaft of a particular spine and did a single taper on the nock end I might loose 1# in spine. When the same shaft was barrel tapered it lost almost 5# and in some cases 10# from the original spine of the parallel shaft. Once the shafts are tapered they should still be in the same spine range that you normally shoot. In other words, if you shoot a normal spine of 55# a barrel tapered shaft should still be spined 55# but it probably started out as a 60-65 parallel shaft.