Hornady said they had to shorten the case with that projectile to allow it to chamber in all 45-70 chambers. I've never tried it personally because their load data is developed for the shorter Hornady cases. I have considered using the bullets but since my 45-70 isn't a lever rifle the main point of the rubber tip is wasted. I can fire conventional spire point bullets anyway so ballistic coefficient isn't a great reason to use them either.
I fire huge 500gr spire point cast bullets in my H&R handi-rifle so I could most likely chamber the FTX bullets seated in full length cases. If Hornady used shorten cases in their loaded ammo they probably ran into issues in some models of rifle. No idea what those are.