[supercat]

It's so easy to take an AR-15 and make it fully auto (it just takes a flat piece of steel with a few holes).

Depends whether you want something that will actually work without blowing up.

I have yet to hear any explanation of why it's easier to turn an AR-15 into a useful machine gun than e.g. a Ruger Mini-14.

[1stFreedom]

Can't say if it's easier or harder. I've only seen the template for an AR-15 conversion.