.577Tyrannosaur or eight bore, both of them are really heavy hitters for sure, I' ve shot the .577trex, and can vouch for it' s power, I shoot .458' s, win and Lott, and .416 rigby' s, along side lighter kicking .45-70+p and .375H&H almost everyday, and the trex was more than I' d ever want to shoot more than twice. Version I shot was a cousins of mine, with two merc. reducers un-Godly heavy anyway, with a Pachymar butt stock pad, and a past pad on my shoulder and my bull riding vest (same stuff as a bullet proof vest--mines rated to stop a 9mm), and I still wouldn' t want to shoot it more than a couple rounds at once.