Yes, the owner could give them permission to come onto the property. I would think that the owner of the rifle could still decline to allow them to review it though. I don't forfeit my 4th amendment rights just because I'm on your land. You can ask me to leave but you can't compel me to let you or someone else rummage through my belongings.
It's the same theory with the big box stores that do bag checks. You are completely within your rights to decline the bag check and walk right out of there. I don't even break stride when they ask me if they can search through my bag. Just a curt "No, thank you." The store can ask you never to come back but they can't compel you to let them search through your property.
Now if you had a firearm out in the open and there was some obvious issue with it, the LEO could seize it under the
plain view doctrine. The 4th amendment doesn't protect you if the incriminating object is in plain view.
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“The inherent vice of capitalism is the unequal sharing of blessings; the inherent virtue of socialism is the equal sharing of miseries.” -Sir Winston Churchill