Thanks for the info. I will order those two books. Also will read that site.
Back a couple of weeks ago, there was a MP44 for sale on gunbroker.com, but I was traveling in a foreign country, so I did not call the seller. Also, it's a big ticket item, I thought I'd better view some samples before buy anything... but it's gone now. Will wait next one. They appear on market from time to time.
http://www.gunbroker.com/item/679368621
Roughly, how many of these are on Form 4 in the NFA system, any estimate?