As to the ammo, and to the specs, and warranties: ANY SAAMI rated ammunition that is chambered for your gun; wether it be pistol, rifle, or shotgun can be used, and WILL NOT void any warranty.
If you look to new rifle warranties, not one I have read in the last 2 years has any mention of steel cased ammo. Because- they can't really call you on it. It's rated for your gun, PERIOD.
The rifles I have read the warranty on in the last 2 years (I work part time for an FFL NOT ONE of these has any specific recommendation to not use steel case int he instruction manuals, nor in the warranty. It says "Use only high quality ammunition chambered for the firearm you are using it in. Use only SAAMI rated ammunition.".