Bore guides (perhaps more correctly termed rod guides, since that’s what they guide) are inserted in the chamber end.
They minimize flexing by the rod. They also help guide the rod into the bore. This makes inserting the rod into the bore faster and easier, plus if you’re using – say - a patch on a jag, it keeps the patch from falling off. It also prevents crud in the barrel from being pulled into the locking lugs.
While not much of an issue with AR’s since you can separate the upper and lower, a bore guide also prevents fluids from getting into the receiver area.
Solvent ports help you even further in avoiding getting solvents or oils all over the place. They are less important in an AR than a bolt gun, since the latter may have a stock finish or an action bedding that can be damaged by these fluids. Also, you want avoid getting fluids in the area of a bolt gun’s action or trigger mechanism.
There are some generic guides for bolt guns (I use a Stoney Point for bolt guns). However, for an AR you really need one designed specifically for an AR.
I prefer a plastic rod guide to a metal one since the metal ones seem to get stuck in the chamber (for me, at least) and are hard to remove. Admittedly, the Stoney Point has a metal barrel, but it has a plastic nose that goes into the chamber.
I very much like the Sinclair’s AR bore guide and add-on solvent port myself. Both units are made of Delrin. However, they stick out the rear of the receiver quite a bit and require a longer cleaning rod. [url]www.sinclairintl.com/[/url]