I don't think the "paintball gun vs. rifle-with-a-scope" issue is going to help find the kids, although it certainly may be relevant to the investigation into Tammy's death. I, for one, think the masked man may have been Alex Cox, though I don't know why it would have been him - or anyone else for that matter. The whole case is just bizarre.
My opinion re: the gun though, is that if Tammy called it a paint gun, it was paint gun. There's no reason to think she wouldn't know the difference.
Paintball guns do have hoppers, which I think could easily be referred to by someone not familiar with them as a "scoop". To dispute her paintball gun identification and rather claim it was a rifle, one has to assume that either Tammy wouldn't know the difference, or that she couldn't see the gun clearly (but for some reason opted to go for "paintball gun" instead of "rifle"), or that the police and/or media is guilty of a typo ("scoop"). I guess it comes down to whether you think it more likely that Tammy called a rifle scope a scoop than she called a paint gun hopper a scoop. I don't think rifle scopes look like scoops, but I think paintball gun hoppers do.
Rifles don't have hoppers or scoops. Hoppers on paint ball guns are larger (and sit much higher) than all but the largest rifle scopes, and they have a completely different shape. They can be shaped like a hopper or funnel, but they can also look almost like a small bicycle seat or small football sitting well up above the chamber (do a search for paintball gun hopper). I also think that Rexburg police would not have so easily dismissed the incident as a "prank" if they believed Tammy had been confronted by a man with an AR15 or shotgun rather than a paintball gun.
If it were dark enough for Tammy to confuse the two, I think it more likely that she would have identified it was a rifle than a paintball gun. To me, the fact that she identified it as a paintball gun means she actually got a good enough look at it to notice the odd looking hopper.
Just because police used a metal detector when later searching certain small areas of the yard doesn't mean they believe the paintball gun was actually a rifle. A rifle casing would not eject from a rifle unless it was first fired, and an unspent round would have to be manually removed/ejected from the chamber using the bolt release and charging handle (and this before another round could be chambered either manually or from a magazine). A rifle casing or unspent round (at least from an AR15) would be much larger than a casing from a small handgun, and would certainly have been noticed on the ground by the officer in investigating the earlier incident (unless Chad picked it up...)
If looking for a small caliber handgun casing (which is possible, especially if a bullet was found during the second autopsy), they would probably be assuming that the gun had been fired at that location. I think it more likely though that the police with the metal detector were searching for a cell phone, a SIM card, a small laptop, a knife, keys, jewelry, or even a syringe - something that wasn't there when they investigated the paintball gun incident back in October
moo.