I fell in love with my SP1 but wanted something better.  Rather than tinker with the SP1, I opted for a Fulton Preditor, 20" 1/9 SS Krieger barrel.  I tried to use some 52 & 55 gr. reloads, but some wouldn't chamber and fearing a slam fire, stopped trying them.  Is the chamber in my setup picky about ammo?  I'd like to reload my own, but now I'm confused as to what will work and work best.  I'd really like some advise as to what kind or rounds to use in this gun.  5.56 or .223?  I've learned a lot in the past two months, but still have a long way to go. The gun is for bench shooting.

From Fulton Armory's website:

Fulton Armory uses a "5.56 Match" chambering in its rifles/uppers/barrels (in fact our barrels are marked as such), which is a slightly modified SAAMI chamber with a tad longer throat to accommodate NATO bullets. The Fulton Armory 5.56 Match chamber allows for the safe and reliable use of all SAAMI and NATO ammo, while offering the accuracy potential of the SAAMI chamberings with match commercial cartridges.