To add a bit more, here are some of the things I've picked up over the years. This might give you an insight on working with firearms....
1. Drop in parts....aren't.
2. The best money maker for the fledgling gunsmith is knowing the proper way to disassemble and clean a gun.
3. The next best money maker is the "box o'gun". (which happens when an industrious customer can not get the darned thing back together.)
4. ALWAYS cut the cheapest part first.
5. Measure twice/three times/four times; cut once. And cut SLOWLY. You can always take more off, but you can't put it back.
6. Buy once, cry once. You don't have to purchase a tool van and a machine shop to start off. Buy the tools you need as you need them--but buy QUALITY. If you don't, you'll find yourself frequently replacing tools, and you'll spend more on replacement tools than buying the best in the beginning.
7. The probability of having a part spring out/fall out/roll away for parts unknown increases exponentially if you have thick carpet. Clear plastic bags are your friend.
8. NEVER be afraid to admit that you don't know something--but always be willing to learn about it.
9. Do not be afraid to call an experienced gunsmith to ask for guidance. Brownell's, for instance, has a tech staff that will spend time with you to guide you in the right direction.
10. Finally--ALWAYS to the best job you are capable of. NEVER do the "quickie" repair job or patch work. ALWAYS do it RIGHT, instead of doing it cheap or easy.
Best of luck to you, and I hope that you truly enjoy working with guns.
(By the way...while working on the M16/AR family is straightforward, it takes attention. There are eeevil little parts, lurking in the AR series rifle that want more than anything to make your life difficult. Don't ask how I know....