Well for starters I wanna be sure you're not talking about the tension you get from cocking the hammer back after it's been fired. Dry-fire it and pump it to see if that's where the problem might be. Since the chamber opens up from recoil that much, it seems unlikely that a rough chamber would explain your symptoms. A rough chamber is common in 870s and can create the need to slam the forend back to break the shell loose. That can be more or less solved by honing the chamber. However the bolt doesn't usually open up much when that happens; and you say it moves fine when it's already cocked which makes me think your problem is elsewhere.