If you replace any parts whether it's a stock sights or anything any alterations what so ever you need a manufacturers license. From research that I've done about the only thing you can legally do is clean one for somebody without a manufacturers license.
And if you take a gun in to do any type of work it must be entered into your permanent record book and you have to run the background check and your customer needs to fill out the purchasers form what that is called can't remember the number or name of it but you know what I'm talking about.
Sorry, but no. Anyone can work on anyone else's gun without a license of any kind, as long as they don't do it for profit. If you DO work for profit, you need a basic type-01 FFL or dealer's license. If you are operating as a pawnbroker, you need an 02. If you are collecting curios and relics, it's an 03. You don't need an 07 (manufacturers license) unless you are actually building guns for resale.
As a gunsmith, I can even sell a customer all of the parts to build an AR-15 and then assemble all of the parts without a manufacturer's license, BUT, I can't buy all of the parts, assemble them, and THEN sell the completed weapon.
If just doing gunsmithing, you only have to log the customer's info in if you are keeping the gun overnight. I can go to the local gun range and work on guns there all day long, or I can fix a gun and return it during business hours or before I leave at night, without logging anything into my A&D book unless I keep it overnight, then it has to be logged into the A&D book and then logged out when it is picked up. If it is picked up by anyone other than the person who dropped it off, a 4473 (NICS form) has to be filled out by the person picking it up. In my mind, this is a very touchy area because the form asks if the person applying is the person actually "buying" the firearm (I don't remember the actual wording, all of my forms are at the shop), but it is there to prevent the whole "strawman" situation. I haven't talked to my ATF agent about it, but I have several customers who work out of town and a couple of them have dropped off guns and wanted their wives/girlfriends/brothers/etc. to pick up the gun. In the interest of covering my butt, I just tell them that they need to come get it when they get home.
FWIW, I have even installed a 10" barrel on an Auto Ordnance '27 Thompson replica , which makes it a SBR. I did not need a manufacturer's license to install the barrel, I just needed my 01 FFL and a copy of the customer's tax stamp in hand to perform the work. In the ATF's eyes, all I did was swap a barrel. FWIW, if you ever have to remove a Thompson barrel, you dang well better have the right equipment or you will regret it...:whiteflag:
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