[dhoward]

Most AR-15s are AR-15s.
The .22LR based models aren't always.
The biggest desrepancy can come in on AR-10 (.308) component compatibility. Either a DPMS pattern or Armalite pattern. In AR-15's things are pretty much standard.
Edit: to claify Legion's post, some of the older Colts had larger hammer and trigger pins than what is common now.
Edit again: Modestly-priced - it is possible to assemble a modestly-priced, functional AR-15 using a Parts kit ($425-500) and a lower receiver ($70ish). Then build on that platform. i.e. rails systems, optics, foregrips, etc...