I do 99% of my knapping from spalls, but chip a slab occasionally. I can see slabbing rare or costly materials. If you can't get spalls bigger than 2" long, then you're not using a big enough spalling implement. The bopper is for percussion flaking the spall down, you need a big, soft gritty hammerstone or a big chunk of solid copper to spall with. I like to use a big, heavy hammerstone that's sometimes as big as the piece I'm spalling.