I suspect that the concentration of machineguns is actually much more skewed (concentrated) than most people realize.
First let's look in general at how skewed U.S. gun ownership. About 30% of people own a gun in the US or about 99 million people. But 50% of all guns are owned by just 3% of the U.S. population (about 10 million people). Quite concentrated. But the transferable machinegun world is another matter entirely.
There are basically 4 types of fully transferable machinegun owners:
1) Wealthy persons - usually own numerous and multiple rare machineguns (often dozens - if not hundreds or more)
2) Average income/asset individuals - usually own 1 or 2 machineguns at the most, they just can't afford to buy anymore
3) Heirs - rarely keep the machinegun which is usually quickly sold for the money at auction or to a dealer.
4) The "old" guys - "older" individuals who have acquired dozens of machineguns for decades well before the runup in prices.
Hence, the wealthy and the "old" guys really have the bulk of the transferable machineguns out there.
Everyone I know who owns a machinegun doesn't own just one, they own dozens or more and they are either an "old guy" or wealthy but are usually both.
The incredible run up in transferable machinegun prices has made this concentration of machineguns held by just the few even more concentrated because only the wealthy can afford to buy them now. As prices continue to rise, the effect will increase. Transferable machineguns are a rich man's game.
My guess is that a couple thousand wealthy/"older" individuals own the bulk (70 to 90+%) of all the transferable machineguns in the registry.
And several thousand other private individuals make up the remaining ownership.