Though screw-in choke tubes were invented decades earlier, it wasn't until Winchester started equipping its production model shotguns with interchangeable chokes in the 1970s that they gained wide acceptance. Soon most American gun manufacturers were equipping their guns with interchangeable choke tubes. Browning introduced Invector Tubes in 1983 and introduced Invector Plus tubes for its shotguns a short time later.
Browning makes Invector and the various grades of Invector Plus choke tubes to fit Browning shotguns. A skilled gunsmith could machine the barrel of nearly any brand of gun to allow using Invector Plus choke tubes, but only Browning shotguns come from the factory ready to use with these chokes.
Browning manufactures several grades of Invector Plus choke tubes. A selection of Invector Plus comes standard with 12-gauge and 20-gauge Browning shotguns. An upgrade from the Invector Plus is the Invector Plus Extended Tube models, and the top-of-the-line grade is the Midas Grade Tubes.
Browning manufactures shotguns in 12-, 20-, 28- and .410 gauges. The original Invector Plus tubes are made only for 12- and 20-gauge Browning shotguns. The Invector Plus Extended Tube chokes are made only for Browning's 12-gauge shotguns. The Invector Plus Midas Grade Tubes are available for all current gauges of Browning shotguns, as of 2011.
Flush or Extended
The original Invector Plus choke tubes screwed into the muzzle of the gun and ended flush with the end of the gun barrel. These chokes are only a few inches long, so the taper of the barrel constriction (or choke) is relatively short. Many shooters believe a tapering the choke over a longer distance gives them a better shot pattern. Both the Invector Plus Extended Tube and Midas Grade tubes protrude an inch or more longer than the gun barrel, allowing for a less abrupt taper in the choke.