Instructions in Will as a solution
Although instructions in your will are useful, assuming that the will is found, read, and understood, it will not address the same types of issues a business entity like a trust, corporation, or LLC can. Individual ownership creates additional problems. Some of these involve the concepts of constructive possession, invalid possession, or unintended transfers. That can occur when:
1) someone knows the combination to your gun safe (your wife or a friend)
2) the Class 3 item is in a vehicle with other riders; or
3) if you allow someone else to shoot the weapon.
These all create serious problems under the NFA and can not be dealt with by an individual owner. On the other hand a Corporation, LLC, or Trust can resolve these issues if it is setup properly.
What happens in your will if you leave your Machine Gun to your son who is now five? Lets say he is 18, in college. Your will does not give your wife, or someone else the ability to say, wait a minute, maybe this is not a great idea right now, maybe we should look at it in a few years when he is ready or more mature.
What is your son is in college in California, New York, or somewhere where the items are prohibited. Your will cannot tell them not to transfer the items because of a geographical ineligibility.
In addition, with a trust, you can list your spouse, friends, relatives or children as co-owners, managers of the property and avoid the transfer taxes.
There are around 30 significant differences like the ones mentioned above in a properly drafted NFA trust as compared to individual ownership or using a generic trust.
If you are single, and have no wife, children, or relatives, you may not be concerned with these issues, but most people interested in these assets (class 3 weapons) have a substantial net worth and would not consider the risk a prudent move.
I am not saying that a trust is right for everyone and I have met many people that it is not necessary for, I just think that people should be able to make an educated decision and are sometimes blindsided by the acquisition issues and do not consider the long term effects of ownership.