Well... My swap out choke Mossy is only like an inch, maybe an inch and a half? Im thinking H&R isnt putting a HUGE amount of time or money into things, so I wouldnt THINK more then a couple inches tops. I figure 2-3 would take it off... BUT, I would call the company and ask them first to be sure.
Dont know how true it is, but I always heard growing up that the REASON not to shoot slugs through a full choke isnt because of pressure so much as it will simply shoot your choke out over time. Makes sense, I guess, as lead is softer then steel, but too many batterings would stretch it. I certainly WOULDNT take that advice TODAY, but then we did lots of things "back in the day" we wouldnt dream of today... And amazingly I still gots all my fingers and toes
... Still...Yeah, I wouldnt if I didnt have to.
Conversely, and Im probably wrong here as well (others here would know much more about this), the same people who would tell me things like that ALSO said, if I recall correctly, that the old BP rabbit ears (REAL ones passed down, not sure about the repos) would actually shoot BETTER without a choke anyway... Something about the faster burning powders needing choke more then BP did... Shrugs... Not sure how that would work with modern steel barrels, but probably the same?
Again though.. This is ALL just stuff Im figuring or stuff Ive heard second hand... I wouldnt suggest ANYONE try the things Im willing to risk.. Im too damn poor and backwoods and HAVE to make do sometimes. For example, Im not even gonna bother with the adapters, im just gonna whittle down some used high brass and tap out the primer... Or possibly cut off the crimp and reload with BP. Those who CAN afford the adapters, or better yet some real full brass and reloadable BP shells, are probably WAY better off doing so.
Either way... You'll probably get to do it sooner then I (I only get one or two high dollar things a year, and one is usually a tool of some kind), so I would definitely CALL H&R first, tell em what ya planning, and see what they have to say. Then come back here and let me know how it works out for ya, cos Im itching to try it myself!