Ares Armor has released their 80% lower that are SR25/DPMS patterned.  They are on sale right now on their site for $99.  Unlike a finished receiver, an 80% lower is not required to be shipped to an FFL because to the ATF, it is not a firearm until the final 20% of the mill work is completed by the user.  Once completed, the user can purchase a parts kit, and complete the lower for a fully functional large frame AR lower.

The challenge to the 80% lowers can be finding a mill to complete the lower on though.  Ares Armor sells a jig with templates for each of the processes that the end user needs to complete in order for the lower to function.  I know some have used a simple drill press and had good success, while others have not only failed, but irreparably damaged the lower.  There are also companies that will hold meetings where they will help you set up their mill, and as long as you push the green button the lower is still yours, for personal use, and not subject to being serialized.  So if you are looking for a challenge for you next AR build, you might just start by making your own lower from an 80%.

For more information, http://aresarmor.com/store/Item/TACMHDM8

Like all of our products we hold them to the tightest tolerances and stand by them for life. The operations left to be completed are as follows/ fire control group, trigger pin, hammer pin, trigger slot and safety selector. This is a not an FFL item. This is not a complete receiver and still requires machining to be done. We have a copy of our ATF determination letter and our determination update stating that it is not a firearm.

Technical drawing to show the cuts needed to make the lower 100% functional.