and is there places that'll let you rent time on one?

I mean is it as simple as say getting a block of aluminum and like downloading a file from the internet like ar15reciever.cnc or smallblockchevy4bolt4inchbore.cnc and feeding it to the machine?

Man thats not just a post thats a hole new web site!

Are you talking like a cnc mill?

If so you can probably download a program with the coordniates for it but there are a few problems.

1. Finding a program for a reciever. Most companies probably dont want to just give you the prints for there stuff. They cant make money unless you buy ther stuff. If you have a reciever or know someone who owns one that you could borrow, you could get a micrometer,caliper, and a couple other measuring tools and make your own print. Than you just enter all the cordnates on to the program using the x y and z axe.

2. Im not sure but Idoubt you will find a cnc to rent. These are not small pieces of equipment. If you have material and design though you may be able to find a machine shop and hire them to build your piece. The machines are quite expensive, even for moderatly cheap ones you can pay thousands for even a regular vertical milling machine or lathe. It would be a lot more for a computerized cnc machine.
[sniper]

it is not "that" simple.

i use a program called mastercam to convert surfaced 3d models into toolpaths for our cnc machining centers.

dspite, better post processors, the process is still very labor intensive. fixturing and machine tooling must still be carefully considered.

the design time to create a complete ar-15 lower from a 2-d engineering drawing would require about 100 hours of engineering time.

we do contract machining. glad to help you out any way i can.

I didnt say it wouldnt take a long time and work, just the general idea. Have you heard of something like  a CAT program that you can draw a 3d image and get the coordnates for the design? And good old hand tooling like filling to get to your tolerance is still going to come in to play.
[sniper]