It is a $200 tax. Unfortunately, back in 1986 an amendment (the Hughes Amendment) was added at the last minute (via a voice vote that failed, but the Democrat speaker of the house refused to allow a recorded vote and declared it passed anyway) to an otherwise good bill (McClure-Volkmer act). It said that only government could own fully automatic weapons manufactured after that date.
It is clearly unconstitutional, but it is enforced anyway. The result is that there are only a limited number of machine guns in circulation (those manufactured before passage of the bill) which can be owned by citizens.
Or is that only for machine guns not rifles with the full auto?
All firearms capable of fully automatic fire are treated the same under the law. If made before the bill, you can own them (just pay the $200 tax and do the transfer). If made after the bill, only government can own them.
I remember when the price of any Class III firearm, while steep at around 3 or 4 grand was still affordable. Today an H&K MP5 can top 30K after you pay the $200 tax.