On all 4 of my fullcap mags (Mecgar mags, from the factory) the baseplate didn't fit in the grooves on the HKS speedloader. It did fit perfectly on the 3 Clinton mags though (also Mecger, also factory.)
So I dremmeled off the top rail, so that the base of the mag just sits against the second rail. Don't look pretty, but works great.
My 2 cents.