Ultradyne USA Quantum, Gas Block UD12001 The Ultradyne USA UD .750 Gas Block allows users to custom tune their rifle's performance to a specific rifle and ammo combination for smooth and reliable function. Perfect for competitive shooters" this gas block allows for a smoother" cleaner" and cooler cycle that improves performance by leveling the feel of the shot and reducing recoil. This gas block is constructed from durable steel with a corrosion-resistant nitride finish.