You can not legally build a fully automatic machine gun in the United States. Section 922 of the National Firearms Act, prohibits machineguns (any weapon capable of firing more than one cartridge per trigger squeeze) manufactured on or after May 19, 1986. Furthermore, modification of any semiautomatic weapon to render it fully automatic or even capable of firing 'bursts' is strictly forbidden and subject to penalties of up to a 100,000 dollar fine, 10 years in prison, or both.

In other words, don't do it.....it's not worth the risk.

You can still build one with a registered pre-86 sear, but the sears are so expensive it really doesn't make any difference and like you say... if the sear breaks or is defective you are screwed out of a whole lot of money, whereas if you bought a machine gun with a registered receiver you could replace the sear easily. If I were you and had the money I would just buy a registered receiver and buy up some parts kits so you always have replacement parts down the road. The receiver isn't going to break if you take care of it. Sears and other parts will fail due to wear eventually, especially if you plan on getting a Mac. Those have such a high rate of fire they tend to eat up sears.

PLEASE, Limit your questions to Hunting on the HUNTING Ask Page.
You will receive more accurate and detailed information if you will place your questions in the proper forum.