How should I store my staple gun long term? Lever up like this:
Or lever down, like this:
I'm worried about the strength of the spring over time.
I can't say for every stapler, but this works for all the ones I've used;
While in the Open position, pull the lever handle backwards. This will disengage it from the spring mechanism, and you can then lower the handle and use the storage loop to hold it in place. When you subsequently release the loop, the handle will engage itself on the spring.
I can't confirm that this is the way is "should" be done, but...
I've had a similar staple gun in my garage for about 28 years. I almost always put the lever catch on (lever down), and I still have to squeeze pretty darn hard to fire a staple and it still sinks them all the way on every fire (unless, of course, I'm not holding the stapler firmly against the surface).
Leave the lever up.
In the lever up position the main spring is relaxed. As you press the lever down the spring is progressively tensioned until it is triggered to drive the staple. If the lever would be restrained then, then the main spring would not be under tension. But if you would restrain the lever before release is triggered, then the main spring would be stored under tension.
That rectangular metal loop may be intended for hooking onto a belt and not to restrain the actuation lever.
. . . or store it with the lever restrained by the metal loop. With the lever restrained by the loop (assuming the main spring is untensioned) then the gun cannot fire and it cannot be cocked. If the lever is completely up, the gun cannot fire a staple, but if the gun were to be forced inadvertently (say against the back of a drawer), it could possibly be cocked and fired accidentally.