AR 15 gas block helps to move the bolt carrier group for auto-cycling of next chamber. There are different types of AR 15 gas block are available such as Seekins Precision .750 Adjustable Low-Profile Gas Block, Faxon Firearms Ultra Low-Profile Gas Block, JP Enterprises .750 Adjustable Gas Block and many more. Few AR 15 gas block are adjustable and few are non-adjustable. You can a gas block with 0.625 inner diameter. Gas blocks can be purchased on online gun store or gun retailer shop.