no you do not need a FFL to alter a firearm... you can work on your own firearms without a FFL.
You, as an individual american, can go build a gun from barstock and the ATF wouldn't care.
abarnsley, there are times when ATF declares that "parts alone" is intention to complete. There have been numerous cases along those lines with the black rifles. You can own an auto sear, or M16 disconnector or bolt carrier- all legally. BUT if you own them AND an AR15, you are in hot water. Not true of all firearms, but the AR has been a big target. Probably due to the millions of DIAS sold back in the day and the many ways to missassemble a semi and end up with a MG.
Yes I can file the papers and pay the tax and wait and legally shorten to 14" or whatever but I just want a rifle, not a NFA-registered rifle. Been there, and the paperwork was killer. You are required to file a form14 I think every time you transport the rifle. I believe now it has been relaxed to file when you transport across state lines. This rifle would see at least 2 trips across state lines every week. Lord forbid I forgot the paperwork or needed to make en extra stop... not worth the hastle for me.
A shortened rifle is not just a pistol.
A rifle with the barrel trimmed <16" is a short barrel rifle - NFA registration, transfer/creation tax
Rifle with butt trimmed total length <26" is 'any other weapon'. Again NFA & tax.
Rifle cut down barrel & short stock = not sure if it stays 'any other weapon' or becomes 'destructive device' at that point.
Works for pistols too, like a 18" barrel on a 1911 is NFA+tax. But on a pistol you can add a barrel and stock at the same time and make a rifle - legally. BUT you can't shorten a rifle into a pistol.
Contenders are pistols, I believe, and most of the barrels are 10-15.5". Pretty sure there is a note in their literature to always use the buttstocks with rifle barrels 16"+ and never with the shorter barrels.
If you buy a new, bare receiver, you must declare on the first transfer and logbooks if it is a pistol or rifle receiver. If it's a rifle receiver, you can't later "make" it into a pistol, regardles of what parts you put on it.
Just like once a firearm is a machine gun, you can't "unmake" it. I went around and around with ATF for a while on this one- I was issued a M14 with the selector brazed in semi-auto. Their stand is that is was a MG regardless of any subsequent alterations. Manufacturer paperwork upgraded it to 'MG' and there it sat, impossible to downgrade.
I know this isn't a super mainstream question, but ATF has a zillion subcategories for what happens when. Based on their prior "parts = intent" convictions, I wasn't in a hurry to dive in blindly. I think I've got my answer now, based on prior examples.
Of course as a machinist, even "permanent attachment" is an issue. What's permanent ? I've got CNC mills, tig welders & bandsaws. Nothing is really all that permanent. I could weld a brake on today, and pop it in the lathe and machine out the weld tomorrow...
Oh IANAL but hope to not be prosecuted by one either.