A brass catcher is an aptly named device that catches your brass so you don’t have to pick it up off the ground after firing. Using a brass catcher also helps to keep your brass clean which can save a step if you reload. Some brass catchers, also known as a brass trap, sit on the ground next to your firing position and you need to make sure you are aiming your brass much as you aim at the target for them to work.These work much like a soccer/futbol goal, and sometimes your brass doesn't eject in the right direction or velocity, so usually you still have to search a bit to get everything. The other type of brass catcher attaches directly to the firearm so you can hopefully ignore it while shooting. Most of these are nylon net bags that are held in place with Velcro, and need to be frequently adjusted in order to work. The Caldwell Picatinny mount brass catcher uses a different approach consisting of a nylon net stretched around a rigid wire frame. The frame attaches to the rifle using a quick disconnect Picatinny mount. This design keeps the brass catcher rigid and secure. The attachment is adjustable by removing a couple of Torx screws and can be moved the entire length of the frame. Torx tools for the attachment as well as the Picatinny mount are included. I’ve been using them for three years now and mine have always worked flawlessly. The construction is sturdy and so far they have held up without rips, frays or other issues. The generous opening doesn’t interfere with the dust cover on my AR-15’s or my AR308. The bag has a handy zipper opening to dump the brass. I usually dump after three mags (90 rounds), but it can easily hold much more. They sell for about $40, so they are not inexpensive, however IMO the construction quality justifies it. Additional picatinny mounts sell for $15, so a single brass catcher can be used for several rifles.