What is it about (some) Americans that somehow the answer to everything is bullets?
Believe it or not (and most people reading this in the US probably won't believe it), it's a firmly rooted part of our culture.
During the 1600 and 1700's, firearms were necessary for the well being of the family, especially the further you got away from the cities. Domestic farming wasn't what it is today, and firearms were needed to defend the home against invaders (in some rare cases Native Americans, but also Spanish and French settlers who disagreed about boundary lines) and put food on the table. Firearms were expensive, and difficult to acquire, but lasted a long period of time (typically handed down generation to generation). Powder wasn't produced domestically, so when it became available it needed to be stockpiled.
The mentality of the US changed from "stockpiling" to "hoarding" when Great Britain attempted to quash questionably rebellious activities by seizing blackpowder stores, leaving many americans defenseless and vulnerable to food issues. They ended up resorting to the "blackpowder black market" if you believe it or not, but the point is the same. When many americans see a political problem arising, their solution is to stock pile ammo in the fear it will be needed later.
Rational or not.
If you want more information, I'd suggest reading "Paul Revere's Ride" by David Fischer. A really good read anyway.
Back to the regularly scheduled programming.
Is there some sort of legal limit on the amount of ammo a person is allowed to have in their home?
Nope. As mentioned, if you buy too many guns in too short of a period of time, the ATF will get involved (in part to make sure you aren't a dealer, but in part to check to see if you are planning some type of invasion). The ATF also gets involved for fully auto weapons, suppressors, and a few other related items. But otherwise, it's your right as an american to own as many, or as few, rounds of ammo as you like.
Generally someone with 10,000+ rounds falls into one of the 4 camps below:
1. Watches The Walking Dead like it's a documentary from the future, and is preparing for there to be no more ammo available forever.
2. Thinks guns/ammo will be made illegal (or effectively so) soon, so wants to get in while the gettin's good.
3. Shoots a LOT at competitions or recreationally, and buys in bulk and keeps enough around to weather a temporary shortage. A lot of people shoot well over 1000 rounds per month.
4. Hoarders and/or people who think of it as an "investment" since prices usually go up.
Personally, I understand the mentality to hoard ammo. I do believe we are heading in a dangerous direction in the US. I don't know what will happen, and I highly doubt it will require even one round of ammo, let alone 10,000, but I can still see the thought process. I probably have 2-3,000 rounds myself, but mainly because I fall into category #3 (kinda). I don't shoot as much as I would like (cost wise), but I shoot often enough to buy in bulk and let it last me a while.
But to the original OP's problem, I think it'd be productive to ask him
why he's hoarding ammo. My guess is he believes "shit's coming." If so, why is he only buying ammo? Ask him how much ammo he thinks he needs. Ask him if he thinks he only needs ammo, or does he need freeze dryed food, gold coins, medical supplies, bottled water, gasoline, ect. Most "preppers" believe true survival depends on all of those items, not just ammo. Getting him to talk about what his motivation is may be able to get him to see bigger picture as to whether buying more ammo is truely going to put him in a better position related to what he's looking to accomplish. Not that he should stop spending everything he owns on ammo and instead buy gold coins, but getting him to think about it may be a first step.
Alternatively, you could support his ability to buy ammo, but only when he can get it at $X price per round. For example, if it's normally sold at $0.40 per round, but you say he can only buy it if he finds it at $0.35 per round, it may reduce his spending, and show that you support his desire to buy more by making sure the funds you have go further. The solution here is that ammo keeps its value. If you bought it for lower than market rates, you should be able to turn around and sell it later for the cost you bought it for, if not more. Which may provide an opportunity for your MIL to sell about as much as your FIL buys. Again, not perfect, but a short term solution.
Long term, therapy is probably the only true "out."
