Self-ignition catapult bolts?

[Tormenta]

If I checked the right chemistry book, sulphur auto ignition temperature is over 250° C. I don't believe, that the bolt can fly through the air at such speed that this will ignite it. But is possible that the friction caused by the projectile going through the loading channel can light it like a match(sulphur is also one of the main element for it). Like Harry suggested I believe, that there is a small trick behind it, or in the process or in the unmentioned element/s.