...call them up and explain what you've observed and tell them what you'd like.
Understood. I just have a feeling that Sig would follow the "recommended" procedure. Then again they are rocking the boat over their "break" not being a silencer.You don't need a Form5 to send it off for repair - Form 5 is only if you're PERMANENTLY transferring it.
For that matter, you could take it to any Sig Armorer, and just hand it to the guy.
The ATF suggests that you do a Form5, but "suggests" is a lot different than "required by law."
FYI, the Form 5 is typically used to transfer an NFA firearm to Federal, State, or local governments, tax-free. Same thing, for estates - i.e. when you're inheriting an NFA item, and same thing, when you've got a nonfunctional item that you're transferring as a relic or ornamental kind of thing - such as to a museum for example.
Also worth noting, while the ATF does recommend doing a Form 5, they also make it pretty clear that you don't have to - especially when you get documentation from a gunsmith that indicates what you're handing over, how long they expect to have it/when they expect to return it to you, and for what specific reason you're temporarily leaving it with them.
For details, see section 9.5.1 in this doc.
Disclosure: I am an FFL01/03, which is to say, I'm a dealer, and I have paid the SOT so I can deal in NFA items. I'm actually pursuing several law enforcement agencies for new business right now, which is why I've researched it.
They are indeed sending a new assembly with the spring. They wanted the firearm at first but as soon as I said Form 1 they put me on hold and researched the serial number and are only shipping the assembly.The firing pin breaking is not a new issue.
Is your MPX a Gen I or Gen II?
If Gen II, does the firing pin have a spring?
Depending on your answers to the above question, SIG CS may end up sending you a new Bolt, BCG and a firing pin, but should not have to ship the entire MPX.
Best to talk to SIG CS.