colorado:
I wasn't particularly offended by your post, didn't mean to give that impression, just a little perplexed by your surprise at the ferrule thickness, you having built one and all. When chambering for the straight-wall American .45s or .50s you can retain plenty of meat around the chamber, but the much larger .577 chamber must result in thinner ferrule walls. Simple geometry.

I'm slowly working my way 'round to building another one myself too, on one of my Greeners using the pair of Shaw rifled 12-bore barrels I've had sitting in the back of the cupboard for several years. Unfortunately I've convinced myself that I need to build a new monobloc from scratch, in order to get an extra couple of pounds weight 'between the hands' while keeping the muzzles shotgun-trim, so it will be no simple task.

Unfortunately I keep blowing my budget on vintage pommie doubles in the meantime!