If you don't want to buy a house/real estate, this is what I would do:
SWD 11/9mm's that Keith at GCA is advertising--about $3000 for 3 of them ($600 in NFA Taxes)--$3600
Vector UZI--$3000 (+ $200 tax)
M-16 Registered Receiver gun--I know of a dealer that has (had) stripped RR's for $3800-4200. $4200 including Tax.
HK Autosear $4500 + 200 tax = $4700
HK SP-89 $3000 and HK 91 $2000 to put the sear in.
That is the $20,000
You could then sell the guns piecemeal to recoup your investment. If you bought a $20,000 NFA gun, and had to sell it, you would be stuck with your profits to that date, vs. selling only what you had to.
As to price increases:
About a year ago, MAC's were $900 (advertised price), now they are $1100.
M-16's were $5000 for a Colt RR factory gun, now they are $6500.
The prices do fluctuate, depending on how the economy does, and other factors--when a popular movie comes out with a featured type of gun (M-16 in Vietnam movies, BARs and Thompsons in Saving Private Ryan), the price for them goes up. 10% per year is probably low, but it could be less if the economy is good at the beginning of the year, and bad at the end.
The two major Downfalls to NFA items as investments is the $200 tax that you have to pay to get the gun, and the $200 to transfer it back to a dealer when you sell it, and the possibility of having future sales blocked by law--which means your investment is not worth much. Or, they could repeal the '86 ban and allow new production for civilian sale--then your M-16 would be worth $800 and the Vector UZI worth $500. Neither scenario is likely at this point, but they are possibilities.
If you have the money to spend, NFA items are worth it in the fun they provide, as well as the potential to make money later.
The only thing I would change above, is to get a .50 caliber rifle instead of the HK 91 if you want to go that route.
AFARR