The gas block is under the handguard of an AR-15. It's over a hole on the top of the barrel. When you shoot, some gas goes into this block and then back to the chamber. Some AR-15s have too much gas going back, which can make the gun dirty.
How to Pick the Right Gas Block Size for Your AR-15
Referenced Strike Industries
To get a new gas block, you need to know its size. Online, you'll see a size like 0.750". This size should match your barrel's width.
Most AR-15s have a 0.750" barrel. If your barrel is thin like the old M16A1 design, you need a 0.625" gas block. Thick barrels need a 0.936" gas block.
If you're unsure about your barrel size, you can measure it. Use calipers from a store to measure the barrel near the old gas block. Then, you can buy the right gas block.
Other Things to Think About When Getting a Gas Block
Adjustable Gas Blocks: These let you change the gas amount when you use different bullets. This can make your gun parts last longer.
Low Profile Gas Blocks: If you have a special rail system for better aim, you might want this type of block.
Lightweight Gas Blocks: If you're in gun contests, think about the weight of your parts. Lighter blocks add less weight to the gun.