Thanks for all the replies, this is really interesting stuff! It does seem likely that most are shoot shafts from the sound of it.
WillS, when you are making arrows by splitting, what are you using for wood? Cut your self or store bought? Split dry or wet? Can you provide some more detail on how you are creating the shafts in a quick efficient way?
From what I've seen, I would suggest the opposite. Some were certainly shoots, but the majority that I'm familiar with were split from timber.
For mine, I aim to be as historically correct as possible so the timber that I'm using will be alder, linden and mainly aspen - not "poplar" as this is now the common name for tulipwood or "yellow poplar" but true aspen or
Populus tremula. I get most of my aspen from contacts in Scandinavia, and occasionally from within the UK. When it's available I also use black poplar or
P. nigra but as it's so endangered and rare I don't get the chance to use it often. That said, it's almost identical to
P. tremula so makes very little difference except to the real purists!
If I'm using readily available English timber such as the alder and linden I will harvest it myself, taking straight lengths of trunk or bough wood, and splitting numerous times once seasoned until the blanks are the correct size. If the timber is aspen or other less accessible woods I'll get them shipped to me in crates as square blanks.
The corners of the blanks are then planed off using the block plane and "chuting board" until a many-sided cylinder is formed. At this stage the tapers are established according to individual shaft weight and rough spine. If I'm making exact copies of specific arrows I'll select shaft blanks that seem most likely to finish at the weight of the originals and then copy the profile of the arrow I'm replicating. Once the shafts are almost circular they are sanded either with modern sandpaper or two small blocks of sandstone with a groove filed through the middle.
If you're struggling to split accurately, it may be worth doing it in a similar way to splitting bowstaves from a log that you think might twist or warp - set a primary split line using a sharp axe that follows the line you want, but doesn't go deeper than about 1/2" into the log, on both sides. Then start at one end holding the axe vertically between the two shallow split lines and begin opening it up. The split will follow your primary guidelines instead of running off to the side.