Hopkins Allen originally was a fairly well known manufacturer from the 1800s, but the company changed hands many times since then. Originally in the mid 1800s they DID produce original "Underhammer" rifles and pistols that became their claim to fame, but they made VERY few of them relatively speaking, and most of them are spoken for with period collectors, I doubt yours is an "original."
The "Underhammer" Pistols were also known as "boot" pistols at the time, as they could be carried concealed in a boottop, and would not snag when drawn quickly.
MOST (not all) of the "Hopkins Allen" underhammers and flintlock/percussion "Kentucky" rifles that you see today were made in Italy and imported for Numrich Arms after the Name was acquired by Numrich, in the 60s and 70s, (yes,the big firearms parts distributer,) that again has changed hands and is now known as "Gun Parts Corporation."
They were big players in the "rebirth" of BP shooting which started in the 60s, even before CVA and the other big names started. You used to see ads for them in the back of magazines like Guns and Ammo and the American Rifleman all through the 70s and 80s for a $100 or so.
Many of the "Underhammers" WERE sold as kits, but many were sold as complete guns.
My buddy has a "complete" underhammer rifle in .45 that I borrowed for a few muzzleloading seasons until I bought my first Muzzleloader.
He (actually his wife for a Xmas gift right after they were married) paid $250 new for his. They have been married for almost 30 years, so you can tell how long ago it was, in the late 70s/early 80s. It is a reliable and accurate muzzleloader.
In good shape it MAY be worth that much now. However, the first time I borrowed it my buddy told me his wife had "overpaid" for it, he was sure you could buy a new one for less. The last HA Underhammer repro I saw at a show was for sale for $175.
However, the "early" "modern" BP replicas are experiencing some early collectors interest, like anything else, so you MAY find someone willing to pay more. It is kind of neat if you think about it, the whole idea of "reproductions" of antiques in themselves becoming "collectible,".
Coincidentally, my first (and still only) flintlock is a Hopkins Allen "Minuteman Brush" .44 cal carbine made by Numrich that I bought new-in-box for $100 in about 1992, from Dixie Gun Works at the NMLRA shoot in Friendship. It is a "rare" version of the "Minuteman" flintlock repro "Kentucky Rifle" that HA/Numrich made in the 60s and 70s.
I have been offered up to $300 for it.
It is NOT as good quality of a repro as is available now, but it works well.
I have had to replace the lock. (I was lucky when at DGW one time in there showroom in UC, TN, right after they bought the last available locks from the original manufacturer in Italy, I should have bought TWO of them!) The good news is the new lock is of better quality than the old one.
The thing that you have to keep in mind is even though it is probably marked ".45 cal," the bore is actually .437". Speer and other manufacturers offer roundballs in .435 and .437 that will work, with either .015 or .010 patches. I can shoot .440 balls, which are more readily available just abaout anywhere, with .010 patches, for no more than two shots before I need a hammer to ram it home from fouling, if I don't clean it first. When shooting target, I use the smaller balls. It can be tough to find .437 balls, usually you have to order them. Most all of the "repor" HA guns, both "conventional" and "underhammer" rifles and pistols made for Numrich had the same bore size.
I hunt with a clean bore with a .440 loaded, carry a speedloader with another .440 ready to load, then carry loose .437 Speer balls in case I have to reload a third time or more.
The good thing is I have shot at and killed two deer, both one shot kills, my last last year a 150" dressed (over 200 live weight) doe from about 40 yds. My first was a button at about 80 yds.
I shoot 65 gr FFFg behind the .440s, 60 gr FFFg behind the .437s for target, and get better accuracy than I can hold.
SOME (not all, since they quit making them 30 years ago or more) parts are available from both Dixie Gun Works www.dixiegunworks.com
Go ahead and shoot it, you won't hurt the value if you clean it properly, and they are fun to shoot.
I remember having to "pinch" the cap a little to be sure it wouldn't fall off the upside down nipple, but other than that it was relaible.