Rifleman Q&A: Roos Underhammer Muzzleloader

posted on November 28, 2021
gun muzzleloader upsidedown mechanism yellow table screens threads steel wood

Q. I have an unusual muzzleloading, small-bore, under-hammer rifle in my collection. The top of the barrel near the action is stamped “U. Roos & Sohn in Stuttgart.” The ignition system uses a percussion cap, which is placed at the bottom of a hole bored in the action. A firing pin is then placed in the hole, which is followed by a large wing nut that is threaded into the hole. The wing nut has a hole in the center for the firing pin. I would really appreciate any information you can give me on it.

A. It indeed looks like you have a pretty interesting underhammer. One of the continual problems with any underhammer design was the priming aspect—any cap that was loose would fall off the inverted cone. The wing nut retainer solution that Roos came up with seems to work, but would have been slow for the second shot. Screwed slightly on, the unit might have also served as a safety feature.

Roos & Sohn was one of the better known gunsmithing establishments in Stuttgart, Germany, from 1845 onwards. The National Firearms Museum has in its collection a .410 shotgun made up by this firm for the foreign market. Unfortunately, Allied bombing raids in 1945 destroyed all records for this company.

This “Questions & Answers” was featured in the January 2005 issue of American Rifleman. At time of publication, "Questions & Answers" was compiled by Staff, Ballistics Editor William C. Davis, Jr., and Contributing Editors: David Andrews, Hugh C. Birnbaum, Bruce N. Canfield, O. Reid Coffield, Charles Q. Cutshaw, Charles M. Fagg, Angus Laidlaw, Evan P. Marshall, Charles E. Petty, Robert B. Pomeranz, O.D., Jon R. Sundra, Jim Supica, A.W.F. Taylerson, John M. Taylor and John Treakle.

To subscribe to the magazine, visit NRA membership page here and select American Rifleman as your member magazine.


Desert Tech Trek 22 Ruger 1022 Stock F
Desert Tech Trek 22 Ruger 1022 Stock F

First Look: Desert Tech Trek-22 Bullpup Stock

Converting a standard Ruger 10/22 carbine into a bullpup rifle has never been easier, thanks to the Trek-22 bullpup stock from Desert Tech.

First Look: Samson Manufacturing Hannibal Mini-14 Rail

For the first time, owners of Ruger Mini-14 or Mini Thirty rifles can now mount optics forward of the action, thanks to the Samson Manufacturing Hannibal Rail.

The Story Behind Remington Ammunition

Through several name changes, ownership exchanges, and financial debacles over the past century, Remington Ammunition lives on today under the guidance of Vista Outdoor.

The Gewehr M2021: A Modernized Mauser 98

Despite having been developed more than a century ago, the Mauser 98 action is still one of the best bolt action designs ever conceived. Follow Jeromy Knepp as he sets up his own "modernized Mauser."

NRA Gun of the Week: Mossberg 940 Pro Snow Goose

On this week’s “Gun of the Week” video preview, American Rifleman examines a dedicated goose gun from Mossberg with increased capacity and custom finishes.

New For 2022: Savage Arms Model 64 Precision Rifle

New for 2022, Savage Arms introduces a new addition to its Model 64 semi-automatic rimfire rifle lineup with its introduction of the Model 64 Precision rifle.


Get the best of American Rifleman delivered to your inbox.