The set screw behind the trigger guard limitations over-journey in the trigger and allows make fast trigger manipulation that a lot easier. At the center in the 940 JM Professional is really a strong aluminum receiver with a grooved prime which is been drilled and tapped for any scope mount. https://mossberg940jmprosemi-auto66501.angelinsblog.com/28685846/mossberg-590a1-bolt-disassembly-for-dummies