With my experience of backfiring, I don't know why, but I think this is not the average 'backfiring'. This must be something else? It's a sharp peculiar cracking noise, tappets and what not. Like something is loose and is just flying around somewhere, hitting the sides and making that noise at certain revs.
And this is coming from a UCE owner. No experience at all with iron barrels lol. :|
That kind of backfiring can be caused by an air leak where the exhaust pipe enters the cylinder head.
I don't own a single-cylinder Royal Enfield, but I do have a BSA 441, and if I don't take great care to make sure the exhaust pipe is well sealed when I put the pipe on after working on the engine, it will do this.
The pipe on my BSA just pushes into the head, similar to the way Royal Enfield does it, and in order to guarantee that it's sealed, I use the red hi-temperature RTV sealant on the section of pipe that goes into the head, and I also add some to the face of the clamp-on finned collar. When I tighten the collar, I pull out on the pipe, so that once the clamp is tightened, the pipe will want to spring back, and hold the clamp against the cylinder head more securely.
Then I wait a few days for the RTV sealant to cure before I start the engine for the first time.
If done right, this method will assure that there are no air leaks to cause backfiring.