The story
By surface analysis, hand + loading. The verb (handload) is possibly a back-formation from the noun.
Etymology from Wiktionary, CC BY-SA 4.0.
handloading
Today it means: The loading of cartridges for firearms when it is done by hand and (especially) at home (in semi-homemade manner): assembling them from the individual components (e.g., casing, primer, gunpowder or other charge, and shot or bullet). Full definition →