I would agree with the other posters that this sounds unnecessarily complicated.
If you want a mac than just buy one now. If you want to cut him a break on his funeral costs than just agree to do it.
I am about 99.99% sure that an informal handshake agreement doesnt have to be honored by his descendants once he dies. I am not sure if even a formal sales contract has to be honored by his descendants after he dies.
It sounds like you would need to either put the gun in trust before he dies with you as the sole beneficiary or have him write you into his will so that you inherit it.
All that said, it seems like a risky proposition to make some sort of arrangement to trade funeral costs potentially 10, 20, or 30 years down the line for such a politically fraught piece of property. There is no guarantee that future machineguns transfers are not frozen, the transfer tax raised to $10,000, the standard/high cap mags used to feed them are banned, machineguns are just outright banned or required to be "deactivated"....you know for the children.
At the end of the day who is to say you will even outlive him. Do your descendants pay for the funeral in exchange for the Mac in this arrangement?
Either way, staring at potentially 20+ years before your 60 year old/good health friend will die per the actuarial tables is a long time to bank on the idea that legal machinegun ownership will be anything like it is today and that you could even take possession of the firearm that far down the road.