What should my primary and secondary mods be for HK47?

I just want to emphasize that speeding up HK is not about making him "fast" -- he will never fit anyone's definition of speedy. It's about making him fast enough so that his tm fills to 100% on recalibrate. If you are not 80 yet and cannot omega recalibrate, then you might need a lot of speed to get HK to fill. The speed of your JE matters so you'll need to do a little math to see how fast your HK needs to be. U want HK, 86, and 88 to all go before a speedy Biggs or STH. Whether to single target attack or aoe will depend on the context, but you at least want that choice.

Unlike 88, potency is not really an issue for HK. Once you put the scanner on him at G9, he has a 39% base potency. Almost no one mods for tenacity, especially people running rebel teams. So that is more than enough potency to get your debuffs to stick. If they are getting resisted, it's more likely that you got tripped up by the minimum 15% chance to resist rather than anything else.

Imo, ideal mods are crit damage and crit chance set, with a crit damage triangle and speed arrow. Basically, the same way you would mod 88 and 86. I've done the math and so have others, and your expected damage is better off with a crit damage boost once you cross the 40% crit chance threshold -- a given with HK's leader ability.

You want as many speed secondaries as possible. Crit chance secondaries are also desirable. Potency on the cross is fine as insurance but not really necessary. I would not use an accuracy arrow unless you are facing a dodge team (which are pretty rare beasts in high end arena).