After market Side-Shot FX Impact magazine.

Douger, you literally can load this mag using two hands, so things speed up a lot. But yeah, loading 10 more pellets seems like a very unreasonable and huge amount of work.

As an experiment, while I was writing this message, with no special training, and preparation I tried to load FX mag and Side Shot mag with JSB King Heavies MKII, JSB King Heavy.
JSB King Heavy MKII - Side Shot, 39 seconds. FX Mag, 82 seconds.
JSB King Heavy - Side Shot, 36 seconds. FX Mag, 119 seconds, I had troubles with putting pellets in.

Outcome? I can load more pellets much faster in the after market mag.