Plans for anything like that are first amendment protected and ARE NOT illegal.
DO NOT try to construct the device! That is VERY MUCH illegal.
There are still several types of auto drop in devices on the market you can buy LEGALLY with the $200 transfer if you want one.
Since the device it's self is registered, you can move it between rifles without an extra tax stamp,
And the rifle is LEGAL for use and common resale when the device is removed.
Personally, after 16 years of active military and a lot of 'Experience', I don't have much use for a fully automatic rifle,
I'd rather have a hard hitting/accurate semi auto than spend the money on some ammo wasting device.
In my OPINION,
Your money would be MUCH better spent on a semi-auto AR-10 in .308 Cal. with a good barrel than used on some ammo wasting device that will probably make the rifle jam so much it's virtually unusable.
An accurate .308 semi-auto is a beautiful thing and a joy forever!
It will take down every kind of game you can think of, is accurate, has VERY fast follow up shots and hits REALLY hard.
Like I said, that's just my opinion...