My experience with HPD and C&R pistols, two now, has been pretty straight forward. Circuitous, but workable. This is unless something has changed.
My package to the thought proof bullet proof window has been:
A current permit to acquire always a long gun permit, but current
A signed copy of your C&R, They will keep it.
*A copy of the page from the ATF book with the piece high lited, and /or I guess that proof of more than 50 years old. I suggest a copy of the page that states that 50 yrs or more is legit.
A copy or original of the receipt from the supplier
The piece of course, properly packaged, hard case.
Other than that do the paper work and it it is treated like a bring in from out of state. Just do it within the required time 72hrs?
* Yes this is outdated 2007 or so. Work with your supplier on this.
You can order without a magazine and get a legal one through the aftermarket. With both of mine this was not an issue.