Assuming the frame and barrel are clean, the latch and shelf are clean and free of oil, most Pop opens are usually caused by not closing the action with enough force, especially when new, if you have an owner's manual, it mentions that. If it still pops open after addressing those issues, it needs to go back to H&R for repair. Please read the Centerfire Handi basics 101 sticky, most of those tips apply to shotguns as well as rifles.
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I'm pretty sure slug gun loads are still limited to the same as shotgun pressure limits set by SAAMI, which in comparison to centerfire rifles is very low, so I wouldn't be concerned about that issue.
http://www.frfrogspad.com/miscellk.htmTim