If your H&R will fire without pulling the trigger by pulling the hammer partway back and releasing it, then the transfer bar mechanism is malfunctioning and should be returned to H&R for repair. The transfer bar should drop out of the way as the hammer drops if the trigger is not fully depressed, preventing the hammer from striking the firing pin.

Is there a reason the supplied lock won't be adequate. If the action is held open, there is no chance the gun could be fired. I, personally, don't use gun locks. I have a bad tendency to lose small keys, so I'd leave the keys in the locks, defeating the purpose. Then there's the issue of the value of a disabled gun should you ever need one it in a hurry.

But I have seen hammer locks in gun stores for T/C Encores and Contenders. Don't know if they would work for your H&R, but they do exist.

Mike