OWD - Your point that 'eased' closing could prevent the bolt from seating properly is exactly right. The mechanism is designed to close firmly without a well-meaning 'assist' from the gunner or loader. I think 'snap' closing is just closing the gun as you advise, allowing the mechanism to 'snap' (not slam) shut.

If there was a value to easing the lever closed with your thumb, you'd think the gunmakers would have said so...far as I know, they didn't.

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