OK, first off the 07 FFL is not $1,000 per year. I'm an 07/02. The 07 FFL is $150 for 3 years. The 01 FFL is $200 for 3 years initial and $90 for 3 years on renewal.
My company is a firearm, NFA firearm, and ammunition manufacturer. It's formed as an LLC. My desire was to incorporate as an S corporation but in MI there can't be a sole owner corporation, has to be at least 2 people. LLC can have one owner so I went that route instead. Cost me $50 to file the paperwork and $25 to renew each year.
I started as a bullet caster and ammo maker with a sole proprietorship and DBA. When things looked like they were going to work out, I got the new FFL and incorporated into an LLC.
Benefits of an LLC is you get the protection of personal assets that a corporation does but by default also enjoys pass through taxation like an S-corp does. However an LLC may elect to be taxed as a corporation, so the LLC files a separate tax return and the members (owners of the LLC). Reason for the second method is to save on the self employment tax. Helpful if you are making several hundred thousand dollars a year in profit. You can have a low salary and pay out dividends or owner draw/officer draw throughout the year much like a bonus. They are still subject to the federal and state income tax but they are not subject to the self employment tax because they are not wages or salary. You still pay tax on the money but you save the 15.3% on self employment tax. Like I said, if you are only making $20-30,000 gross profit a year in this venture it doesn't make sense to complicate the taxes just to save a few hundred dollars.
With the standard tax filing of an LLC, you just file a Schedule C and Schedule SE along with your 1040. The business stuff is listed on C and your self employment tax (ie the Social Security, etc) is calculated on the SE. It's filed along with your personal income tax, to streamline the process, and is due when your personal income tax is due (April 15).
***Disclaimer: The above is not legal nor tax advice, which can only be given by an attorney and/or tax professional.***
With that out of the way, the trigger guard you are making, be it for a bolt action or what have you, it's really a low risk part as far as liability goes. The trigger guard does not have a significant role in the safety nor operation of the weapon. A portion of the liability is shared by the gunsmith who will install it, the company that will sell it (Brownells) the company that will wholesale it (your buddy), and the end user if they install it. In the event of a liability suit, the suit will bring everyone in that is associated with the part and put them on the chopping block trying to figure out what happened and whom is responsible for the failure, negligence, etc. If it's a design or defect, it's on you. If it's an installation failure, it's on the gunsmith and could be on you if you fail to provide proper instruction with the product. If the end user failed to properly install it, part of the liability would be on them, but then the next in line would be you. Most gun parts, even a slip on grip from Hogue for your pistol, will have some language that it should be installed by a competent gunsmith. This doesn't save your bacon in all cases, but if the end user does not install properly, you have some defense to use.
In any event, contact guninsurance.com. Joe insures a lot of the big name manufacturers in the industry and will insure you. Prices vary but for the coverages I have, it's very reasonable.