It's the Tiger engine , bungie built it years ago to do Halo and they tweak it for every game. I read it, I think Reddit, that it's the same engine, with a few minor tweaks to allow easier access to certain things. Like moving a spinmetal node in D1 took 24 hrs because they had to load the entire map overnight then access jumbled spaghetti code. That will no longer stop them. However actual graphics, mechanics, etc, are still the Tiger engine from Halo. It was confirmed I think after the super glitch was still active in the Beta, and bungie got called out on using the same engine, and it was checked (idk how) to be the same stuff. The base gun model code is the same too, so same guns different appearance