[r9etb]

Yeah, the Oregon cell. It is hard to tell -- but given that they'd fired a lot of rounds from other rifles, why just this one? It's just odd, unless they were doing something like seeing if it was still sited in, or something.

[Shooter 2.5]

I don't believe it's terrorists but let me help you with this one.
The Oregon group goes shooting to get some practice in but they have one .223 firearm that they know they're going to use to assassinate people. The weapon is kept well hidden because they don't want to be connected with it. They only take it out to make sure it's still sighted in. At one point, they miss cleaning up the single round.

What really could have happened was they were too close to someone else shooting their favorite toy and a single case landed into their shooting bags. It happens all the time.

[Charles Martel]

why just this one? It's just odd, unless they were doing something like seeing if it was still sited in, or something.

Perhaps they just found the lonely .223 round rolling around in the car trunk, after they'd traded-up to a larger caliber. I've got an old paper-hulled 16-gauge shotgun shell at home that turned up in a similar manner... though the 16-gauge shotgun is long gone.