I have a few ideas of what could be going on.
Option #1: Two or three guys are all firing as fast as they can.
Your response should be : No biggie, as long as they are being safe, at least in how it relates to you. (hey, if he only risks his own well being, that's his business)
Option #2: The guy is 'bump firing' (hold the gun somewhat loosely, put finger on trigger, fire gun not by pulling back on trigger, but by pulling gun body forward. Recoil brings gun body backward, continual forward pull causes the trigger to activate again...resulting in VERY RAPID semiauto fire)
Your response should be : No biggie, as long as he is being safe, at least in how it relates to you.
Option #3: The guy legally owns full auto toys.
Your response should be : No biggie, as long as he is being safe, at least in how it relates to you (Followed by 'hey can I try that?)
Option #4: Guy is a law abiding citizen who follows the believe that it is Illegal to Tax the Execution of a Right (as the Supreme Court stated regarding a Tax on the Right to Vote) and does NOT possess the $200 tax stamp he should for his machinegun (home built, home conversion)
Now here it gets tricky. Right now you have no idea which of these 4 conditions is going on. Lets just say that somehow you magically know it is Option #4. Now things get interesting.
Morally, should a man turn in a lawbreaker with the law being broken is immoral? Example, you are a Christian in Germany, you see your next door neighbor walk around all the time looking like a generic German. One day you see he has a Menorah lit. German law states that Jews need to wear the Star of David on their clothing. The man is technically breaking an unjust law. Do you turn the man in?
I hope your answer is 'no, if the law is unjust, I will NOT turn in someone who is breaking that law'
Then you must consider the morality of our laws regarding full auto firearms. Is it right that we are taxed to express a personal freedom, especially when the SCOTUS has stated regarding OTHER freedoms that they cannot be taxed?
What about the fact that the 'machine-gun registry' is closed, so only machine-guns made prior to 1986 can be registered, causing an artificially limited supply, which drives up the price, meaning only the RICH can afford to own them legally.
(In parallel, what if your city stated: 'only offical ballots will be counted in this year's election. We will only print up 100 ballots for the entire city, these 100 ballots will be up for auction, this is fair because every person has equal access to bidding and winning of one of the 100 ballots' The real effect of this falsely limited supply is that only the RICH would be able to vote)
and finally, let's say you come to the conclusion that yes it is your duty to report on those who break immoral laws, OR that you believe our current system of only allowing machine-guns to the RICH is moral, and therefore you feel it is your duty to turn him in
The question then becomes: How do you know it is option #4 not options #1-3? Can you legally invade his privacy to 'investigate' that option #4 is the one being used?
As it stands right now, as MOST LIKELY his behavior is legal, you need to not worry about it. IF somehow you find out that his behavior is technically illegal, THEN you need to decide if you are going to report someone for breaking an immoral law.
Finally, ask yourself this. Let's say he just is capable of very fast semiauto fire, but you report him for 'suspicion of having an unregistered machinegun'
The ATF comes in, shoots his dog, battering rams down his door, torches open his safe (which destroys some of his guns) seizes all his legal firearms, punch holes in his wall looking for hidden machineguns, cut holes in the furniture cushions, upend all the cabinets and bookshelves, etc etc
All the while the guy is sitting handcuffed on his front porch for HOURS while the ATF searches...and eventually find nothing, and leave the guy to deal with the mess.
(Note: this IS standard ATF practice for 'reports' of machineguns...and as long as they didn't fabricate the report, they have no legal duty to pay to repair the damage they did)
What is your legal obligation to the man for jumping to conclusions? (probably none) But what are your moral obligations to the neighbor when you jump to conclusions and he pays for it by having $10,000 of damage inflicted upon his property as well as being humiliated.