The H&R rifles are built on the 1884 design (low arch block, wider receiver, etc.) and tend to probably be the strongest Trapdoor out there when you consider the age or originals and questionable Italian steel (though so far my Pedersoli is just fine). What is a MAJOR problem with these rifles is the locking cam and can be dangerous if not corrected!
The problem is that the originals are a one-piece affair with the thumb piece on the shaft and locking cam with the bridle between the cam and thumbpiece that screws into the side of the block, holding it all together.
The H&R and Pedersoli rifles on the other hand don't use the bridle and the cam is seperate from the shaft and thumbpiece and is instead held on by a set screw. Now the difference between the H&R and Pedersoli repros is that the shaft where this set screw sits on the Pedersoli rifles is square where the H&R is round.
Now on the Pedersoli rifles, as long as the set screw is tight, it won't move on that shaft at all since the hole in the cam is square to fit over the square section of the shaft. On the H&R however with the shaft being round, the set screw doesn't have that good of a purchase on the shaft and if it loosens just a little bit, can lead to big problems. Problems like either not being able to open the breech at all (push on the thumbpiece and it and the shaft just spin under the cam) or worse, with the tang of the thumbpiece no longer held securely under the hammer, the block can fly open under full pressure and spit the empty case back at the shooter!
Yes, it does happen, I've seen it a few times both when it locked up the rifle and when it blew empty cases out.
But now for the good news.
It's an easy fix. There's three methods you can use to cure this problem.
First is to remove the cam and slide the shaft out, file a flat spot on the shaft where the set screw sits and put it back together. The flat spot gives the screw a better area on the shaft to set against and won't allow the shaft to spin under the cam.
The second method is to remove the shaft and cam again then drill a small hole where the set screw sits so again, you have a better surface.
The third method is not one I recommend by itself but is an option if you want to keep the H&R absolutely original and that is to simply loc-tite the set scew into the cam. Yeah, keeps it from backing out but doesn't make for a better area.
Personally, I like method one and as either of the first two methods, use a bit of loc-tite on the screw too. Now you will have a flat surface so the cam can't spin on the shaft and the loc-tite will prevent the scew from backing out during use as well. See, it's an easy fix and you get the strongest trapdoor out there to boot.