The AR-15 is one of the most popular rifle platforms in the country, and for good reason — it's modular, reliable, and endlessly customizable. But with thousands of parts and accessories on the market, it's easy to spend a lot of money on upgrades that look cool but don't actually make your rifle perform better. If you're trying to figure out where to put your upgrade budget first, this guide breaks down the five AR-15 upgrades that deliver the biggest real-world improvements in accuracy, …