Owen Grady wields a Marlin Model 1895SBL lever-action rifle in Jurassic World, which is outfitted with a scope and, as Chris Pratt alluded to in his tweet, uses .45-70 rounds.
It’s unclear if Pratt just chose this gun all on his own or if it was one of a few selections set before him, but either way, considering Owen’s profession as Jurassic World’s velociraptor handler, it makes sense he’d use that firearm in case one of his raptors starts causing trouble and he can’t talk them down.