Quoted:
If you have an upper that is less than 16" in the same location as another AR-15 that IS NOT registered as an SBR, this [i]IS[/i] considered intent to build an illegal SBR by the BATF.
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Where I have italicized "IS", I would substitute "may be". To my knowledge, there hasn't been a ruling, but it isn't much of a stretch to imagine being charged, either.
A barrel by itself should be fine, as it would require tools and 10-20 minutes of work in order to install. "Intent" would be far harder to prove than, say, a complete upper that can be attached to a lower in under 10 seconds.
Of course, it is obscene to be charged with "intent" to do ANYTHING, but why should the government care about individual rights? The sheeple certainly don't...
-Troy