Any firearm is only good as long as its working; broken, they make less than ideal clubs. Most military style rifles are fairly easy to maintain and repair, even in field conditions, but if parts break, you need to have replacements on hand to get 'em fixed.
The AR Essential Parts Kit from Choate Machine & Tools packages many of the must-have spare parts for your AR-15/M4 in a compact package, designed to fit any compartment that will hold two CR123A batteries. Very many grips, vertical foregrips and stocks have built in compartments for CR123A batteries, which allows you to carry potentially mission-critical spare parts with your carbine. Choate has a stock with two spacious battery compartments, which would allow you to carry this kit as well as spares for a weapons light.
While the EPK does not contain a full field repair kit, it has a good selection considering the compact size.
The parts list includes: Extractor, Extractor Pin, Two Extractor Spring Assemblies, Selector/Safety Detent Spring, Selector/Safety Detent, Takedown/Pivot Pin Detent, Takedown/Pivot Pin Detent Spring, Takedown Pin, Pivot Pin, Hammer/Trigger Pin and Three Bolt Gas Rings.
I would like to have seen a firing pin retaining pin and disconnector spring, which could have been added without increasing the package size. Other important spares, like a firing pin, cam pin, hammer and trigger springs, and so on would blow the package size. The plastic tube could also be a little bit more robust, but since it will ride within a grip or stock, it should be fine.
Anyways, a cool product that I wanted to pass along.
Get it from Choate >