Double-gunning (Boyer 60s?)-- never done it using this method, just single-gunning with a solid bucking bar, although not on boilers, just passenger car siding and framing (and a fair amount of it). There are also air operated holder-ons and a hammer holder-on, which is one step below double gunning. The locomotive builders used giant, long jawed press-like riveting apparatus rather than guns with buckers.

One of more intriguing riveting videos I've seen (I think it might have been one of Michael Palin's RR videos) showed some Indian State Ry workers riveting a smoke box with hand sledges. One person held the head forming tool, which had a wooden handle, while two others alternated smacking the large end of the tool with what were about 4 lb sledges (the bucker was out of sight inside the smoke box). I see a lot of this sort of blacksmithing tooling for sale on ebay, and the sellers are uniformly clueless as to what it is.

It's nice that the ASME etc. are bringing riveting standards back, but I also notice that the video isn't demonstrating riveting of a pressure vessel, like an air tank or the boiler itself, something I am leery of trying. Nor does it really show too many details of what has to be done and the tricks on how to do it, as in how long to cut a rivet, how much to allow for the formed head when cutting it to length, how to heat it and to what temperature, how large to drill/ream the holes, how to form the driven head, the proper use of fit-up bolts, etc. Undoubtedly this is because this company wants you to pay them to rivet, and I definitely would hire them to rivet on a boiler (mud ring, etc.). The non-pressure vessel and car work (psgr and frt) I'd rather do myself.

I also notice some of the other rivets were too long (malformed heads), including what look like some driven by the builder. The consistently best riveting I've ever seen was done by the St. Louis Car Co (the "Quality Shops") -- Amtrak still runs some stainless steel St. Louis-built baggage cars. The worst were American car co transit products.