Having worked for TSA, I can tell you the official (technically correct) answer, which is that barrels/slides/parts are not firearms and do not need to be declared.
You'll notice in TSAs official guidelines:
http://www.tsa.gov/travelers/airtravel/assistant/editorial_1666.shtm
That they specifically differentiate between firearms and firearms parts but do not specify exactly any difference in handling.
In my mind, the implication is clear that parts are not firearms and do not need to be declared. Specific instructions are included for ammunition and magazines but not other parts.
What I can't tell you is how any given airport/officer/FSD will "interpret" the rules. Interpretation of unspecified regulations is the problem. Common sense is RARELY the first standard.
The most obvious thing in my mind would be to simply mail the thing. Inexpensive, easy and avoids problems.
Second most obvious and very likely to not be problematic, would be to simply tell the ticket agent that your bag includes a few firearms parts but NOT firearms. Let them deal it. If they think you should declare it, declare it. Have a lockable case ready per rules for a firearm.
The odds of a problem are small but would be rather inconvenient and worst case, expensive, on the off chance that you get sucked into a real nightmare.
Me, I'd mail it.