True for handguns, but not true without adding that qualification. It seems for handguns to do well for .357 and .44 magnum with full loads.H 110 IS a slow burning powder, one of the slowest powers used in handgun cartridges.
Slower than A/A#9 ( slowest hangun powder in the A/A line, ) slower than Blue Dot, slower than 2400 ,it's actually the same powder as 296..athough
it's listted as a different powder in lists. ( Just as HP-38 being the same powder as 231, same powders both sold by Winchester & Hodgdon)
the slower burning powders in charts are rifle cartridge powders. Rifle cartridges , haviing much higher powder volumes, have to use slower burning powders to keep pressures within acceptable limits.
People do tend to load the 300 blackout like it is a pistol cartridge. I use 296 and H-110 for 30 US carbine and will likely use it for 300 blackout.
For magnum pistol and 30 US carbine I think pressures are close to 40K PSI.