I got some: I put 20 rounds through a 16" 1:8 barrel and measured muzzle velocity averaging 980fps with stdev of 17fps. At 50 yards that's good enough to keep every round inside a 2" diameter, and given how much volume is in the case I consider it pretty decent variation in velocity. The charge is 9.2gr of a flake powder that takes up 1.0cc. (The necks are also crimped so hard that the bullet jacket is dented by the cirmp.)
I also tested 10 rounds through a 26" 1:10 barrel. At that length they're actually slowing down to about 700fps at the muzzle!
So for a bolt gun with a shorter barrel it's not bad.
Excellent follow up, much appreciated.