It can only work if you can buy low and sell high. So how exactly can you buy ammo for lower than Remington can manufacture it, and sell it for higher than Remington can sell it?
It’s not enough to note that the price of a commodity has gone up to make a profit on it. You have to either have a source for that commodity lower than the current market value (for example, you bought it back when it was lower in price), or you have to expect that the price will continue to rise, such that if you bought it now at the current price it will be even more expensive later, and you can sell at a profit.
Notice the problem in both these scenarios. Do you have the commodity already? Probably not. Will the commodity continue to rise in price? Yes, no, maybe? Maybe it will continue to rise, in which case selling now is the wrong call, and you should buy now. But if you buy now because the price is rising, when exactly do you sell? When the price starts to drop?
This is what causes volatility in commodity speculation. But it’s fueled by people losing money. They buy when the price rises because hey the price is going up! And then the price drops, and they’re stuck with selling at the lower price. Buy high, sell low is a no-good way to invest.
If there really is a permanently higher price for ammo, the people to capitalize on it are the ammo manufacturers. They only have so much excess capacity in their manufacturing chain, and so when there are temporary price spikes they can only increase production by so much. To do more they have to add capital goods and hire more workers. So will they do that? Only if they think the increased demand is going to continue. Will it? Yes, no, maybe.
But the real answer, if there really is a supply-demand mismatch, is that manufacturers will try to increase supply to match demand so they can make more money. If you think you can buy ammo for lower than Remington can make it, and sell it for higher than they can sell it, then go right ahead with your plan. If that doesn’t sound very likely, then stockpiling ammo probably isn’t a good investment.
It might be a good idea for other reasons–if you expect TEOTWAWKI to happen then a couple crates of ammo in your gun safe might be a really good idea. But that’s not an investment, that’s disaster prepping. You aren’t going to sell that ammo after The End of the World, are you? And if the price is high now, remember that you’re buying high. But that’s because you want the ammo today so you can have it tomorrow. Not because you’re going to buy it today and sell it tomorrow.