What’s the single most important upgrade you can make to your AR-15 in order to keep if functional when wave after wave of brain-eating zombies are headed your way? Simple. Upgrade the extractor spring.
Anybody who has spent any amount of time around AR 15s will tell you that by far the most common malfunction is failure to extract, followed only by failure to feed (get an upper with M4 feed ramps and good magazines to fix this one). The reason for the failure to extract is usually a weak extractor spring. Open up your rifle, remove the firing pin and pull the bolt out. Push the pin out of your extractor and take a look at the spring. If it has a blue insert, and the spring only has four rings, you’re going to need to upgrade that bad boy.
How do you do it? Easy peasy. Just order this extractor spring upgrade kit from Bravo Company and replace the spring on your extractor with the new, more robust spring. Here is a post with pictures if you are scared. Doing this one simple task can add tons of reliability to your weapon.