Numrich Arms made Hopkins & Allen underhammers for many years. They used a peculiar design where the trigger guard was also the mainspring for the hammer. These guns have a reputation for being accurate and affordable and serve as the basis for the later production guns by Deer Creek Products, and it was their parts & kits that were built by some of the other builders. From what I understand though, they are currently not in production. There are some receivers available from another company, and some custom builders who make them from scratch or using these receivers or existing trigger parts. So I'm not sure you will find a new gun unless it is a more expensive custom or semi-custom made underhammer.
There were some popular "heavy" chunk gun competition target rifles made with the underhammer design, and all of these actions are noted for their fast & positive ignition.
Also, some custom and semi-custom guns have rapid barrel switching capability, but I believe that the H&A design were originally pinned barrels and were not interchangable unless customized.
If you're looking at the Blue Grouse or the underhammers.com website, these guns are probably not available. However, you can find some used on some of the muzzle loading forums classifieds and major gun auction sites.
The folks who shoot them really seem to like them, but some clubs and competitions don't allow them to compete because they aren't considered a common traditional design of the trade era, and which is a debatable issue.
Ardesa makes an underhammer rifle and pistol, but it's not imported and may only be available in Europe as are many of the underhammer target pistols.
Whether these rifles have a comfortable fit or not is up to the individual. The receiver is curved which affects the fit of the stock along with the weight of the barrel. I'd recommend to pick one up and see how it fits.
These guns can sometimes send fragments of percussion caps into a bare wrist, and cause powder residue stains on a sleeve cuff, which can be dealt with pretty easily. The triggers aren't always going to be target quality because of their simplistic 2 (or 3 with mainspring) piece design. And the trigger parts aren't as protected from powder fouling and need to be removed and cleaned after each shooting session, at least with the H&A's.
Several years ago, there were many reports of breakage involving Deer Creek trigger/hammer parts that were not properly hardened which involved both new and replacement rifle parts. But it was still a rather small percentage of overall production.
Also when Deer Creek first set up the barrel making equipment that they purchased, there were reports that not all of the barrels were top quality, but there were still many more satisfied customers. And today, many people are still looking for affordable new Deer Creek underhammers to buy.
