Well, I emailed my concerns to Henry this weekend, and got a reply:
:::
Thank you for your interest in Henry rifles.
Big Boys chambered for .44 Magnum can use any reputable brands of the standard or +P factory loads of .44 Magnum or .44 Special which conform to SAAMI specs, with standard profile bullets (round-nose, flat-nose, and hollow-point), and with up to a maximum projectile weight of 260-grains.
With .44 Spl, be advised that some smaller loads may not cycle well due to shorter overall dimensions.
Factory-loaded +P ammunition that conforms to SAAMI specs is fine, with up-to 260 gr. bullets. Our rifles are not designed to handle over-260 gr. bullets, as feeding/jamming, premature wear and failures could result. Also, the twist rate was designed to stabilize bullets up to 260 gr., but heavier bullets will suffer accuracy issues.
We recommend that owners experiment with various reputable, higher-quality brands of the appropriate type and caliber of SAAMI compliant cartridges for the rifle to see which works best, and then try to stay with it. These would include, but not necessarily be limited to, brands such as: Winchester, Remington, Federal, Hornady, and CCI.
Avoid using wadcutters and semi-wadcutters, and ballistic-tipped Hornady LEVERevolution ammunition. They are known to cause feeding/jamming issues in our magazine-fed rifles, due to the non-standard profile bullets.
We also do not recommend hand-loaded ammo due to greater risk of possible variations/errors and safety issues, in the case of improper reloading. If you choose to use hand-loaded ammunition anyway, make sure the resulting ammunition conforms to SAAMI specs for that type.
Regards,