Pull a bullet from one of those rounds, weigh the powder, and see what loads for similar types of powders exist in data. Keep in mind that Mfrs use powders that us mortals can't get, but it would be worth a try, post a nice pic of the powder so someone might be able to ID it for you to pick one that may be a match. Once you have an idea what the powder is, use start data with the suspected powder to work up a load. One member did that trying to duplicate the Win Whitebox 45gr JHP ammo that shoots so good in many rifles and isn't available any more. The other alternative which may work just as well is to just pick a Hornady recommended powder for that bullet and work the load up, which is what 99% of handloaders would do more than likely, there's no guarantee that either method will work, but ya gotta start somewhere.
Tim