This is a built in safety feature to ensure the user does not mount a firearm with a round in the firing postion. You must lock the bolt carrier group (BCG) back into place using the bolt catch lever before it will fully seat on the mount. This will prevent neglegent discharge. Some aftermarket brands of internal components for various firearms may still permit the user to fully mount the firearm without locking the BCG back. We strongly discourage mounting without fully unloading the firearm. Use at your own risk.