Note: This article still needs polish; it started off as "idea-hamstering" for the disclaimers at the bottom.
Potential Legal Issues When Building Receivers
ATF guidelines state that a "less than 80% complete" receiver blank (or flat) receiver is not a firearm, and may be freely transferred like any piece of metal until such time as it is completed. Anyone who, without a manufacturer's FFL, completes a receiver for someone else or for resale/transfer, is committing a crime. It is absolutely not prohibited to make a receiver for yourself, or if you have a manufacturer's FFL.
Lend a tool, not a hand! Enthusiastic but ill-informed gun owners frequently fantasize about forming an assembly line of folks at a build party. While such a line could well churn out firearms like a well-oiled machine, it would churn out felony charges even faster!
In order to avoid the appearance of wrongdoing, it is helpful to ensure that the critical step of completing the receiver - the point where it crosses the magical "80% line" - is always performed solely by the receiver's owner. ATF is vague as to where that line is crossed, so it is best to ensure that the owner perform all, or nearly all, of the work. For instance, it's probably not a big deal if someone helps trim down the top rails on an AK receiver. It is a completely different matter if the owner does nothing with his/her receiver once the flat is bent! Again, the owner should perform all or nearly all of the work on his/her own receiver.
Have a Compliance Supervisor!
It is very useful, for all concerned, for one attendee to be tasked with monitoring the legal compliance of the rest. The qualifications of an ideal supervisor are, in diminishing order of importance:
Legal competence: The supervisor should be familiar with Federal and State firearms law, including 922(r) import issues. He/she should be particularly aware of where constructive possession concepts do and do not apply.
Willingness to sign statements: The entire point of a legal supervisor is to provide the attendees with a witness to certify that their build was compliant with the law at all points up until the attendee's departure.
Position of civil authority: It is possible that an attendee may, at a later date, be challenged by law enforcement over the legality of his/her weapon. Possession of a signed statement from a law enforcement or legal authority acting as supervisor should defuse this situation, particularly if the LEO involved happens to know the supervisor. Stapling one of the supervisor's business cards to the statement should reduce the risk of the LEO suspecting a forged signature.
Availability during receiver construction: Ideally, the compliance supervisor will have nothing to build at the event. He/she will be willing and able to state, in court if necessary, that he/she had an eye on the attendees' actions when critical build events were occurring. See the 'Access Control' section for an alternative to this.
Ability to monitor critical workstations: At an AK party, if all of the flat bending jigs were segregated in one corner of the event area, with no other needed machinery nearby, it would be very easy for the supervisor to confidently say that he/she supervised all of the flats being bent. The most Federally critical stage of any build party, is when the non-firearm blanks become firearms. This suggestion is made so it can't be said that the supervisor couldn't have possibly kept an eye out for improper transfers or folks doing each others' work. Ideally, photographs should be made of the location of the jigs relative to the rest of the party.
Access control: An alternative to the supervisor not getting any personal work done, would be to segregate the flat bending jigs (or other critical "move it past 80%" hardware) into a room which other attendees do not have access to. For instance, if the host is the supervisor, it would be simple to put all of the flat bending jigs into a bedroom when all of the attendees' flats have been bent. If no one has access to the bending jigs, it is safe for the supervisor to go about his/her business until it's time for exit inspections on departing attendees' firearms.
Willingness to stay late: Two of the three sample disclaimers below are for attendees, and are intended to be signed by the supervisor as they leave the event. As such, the only way the supervisor can actually make a final inspection of each attendee's firearms, is if the supervisor stays until the end of the party.
Each party should have a few disclaimers to protect both host and attendees. The sample disclaimers below are written with Californians in mind, but they could be readily adapted for any state. My own disclaimer is that I am not a lawyer, and these represent my best effort at a reasonable set of disclaimers. I welcome any constructive criticism or suggested changes which actual lawyers may be willing to provide.