What's a "universal" gun? Simple: it's a model so ubiquitous that you can build one with few (if any) parts from the original maker, a gun so widely accessorized that you can get just about any practicial widgets for it you might want.
For center-fire pistols, the 1911 and Glock 17 fill the bill; they are so widely supported that you can, with very little effort, build a polymer-frame 1911 and a metal-frame Glock (and go shooting in Opposite World?). Parts and accessories for each fill pages in Brownells and Midway USA catalogs, not to mention the back page ads in any gun magazine.
A generation or more back, the Baby Browning and hundreds of "Eibar" copies (in everything from .25 through .380) kind of filled the role, though usually only as complete firearms.
In center-fire rifles, the AR-15 stands head and shoulders above the rest, followed by the slightly less multi-sourced semi-auto AK-47. Tam argues persuasively for the Winchester 94 as well, and she's got a point, especially in the "ubiquity" aspect.
Shotguns, it's a dead heat between the Remington 870 and Mossberg 500. Even the folks who shoot high-end shotguns usually own one or other of these as well.
In .22s, Ruger dominates, with the 10/22 rifle and Mark I/II/II pistols (especially if one includes the the 22/45 as a subset of the "Mark n" line) I can't think of any other adult-sized .22 rifle or pistol that comes close.
And the point of this list? There are a lot of guns out there, with a wide range of virtues and failings. No firearm is perfect for every shooter or application but some are a lot easier to keep running, a lot easier to find experienced users to get advice from, a lot easier to find and buy. You should own one or more "universal" guns if you're serious about the Second Amendment.
W9BSP: A HAM'S HAM, A TEACHER'S TEACHER
3 weeks ago