What is Reddit's opinion of Mighty 109 Greys Pea Gravel, 40 LBS?

From 3.5 billion Reddit comments

Yes, you can make a 300 lb sandbag. But, I don't know if your duffel bags are big enough to do that. You need to measure them first and do a few quick calculations to determine if you can. For example, Quikrete Play Sand (most tutorials recommend play sand as filler material) come in 50 lb bags that are 0.5 cu ft in volume (also, don't go on Amazon to buy your filler material, it is very overpriced due to the weight of shipping; just go to your local hardware store). That means you need 6 bags for 300 lb, or 3 cu ft of sand. Most recommend you only fill the bag up three quarters of the way, to allow for the material to move around. Therefore, you need a 4 cu ft bag for 300 lb of sand.

Lets say you measure you duffel bags, which are cylindrical, and they are 42in. long and 16in. in diameter. That means the volume of the bag is

V = L*3.1415*D^2/4 = (42/12)*3.1415*(16/12)^2/4 = 4.9 cu ft

Which means they would be (3/4.9)*100% = 61% full. A little on the low side, but I think it would work. Therefore, you'd be able to use these bags for your sandbag. If your bags are smaller that 4 cu ft in volume, then you would have to use a different material. Pea gravel is sometimes recommended as filler material. However, a quick search seems to suggest that pea gravel for sale has about the same density as play sand (50 lb/0.5 cu ft = 100 lb/cu ft). So you need to be careful and make sure that it is denser. This gravel looks to have a density of 120 lb/cu ft, but again, don't buy from Amazon. It is way to expensive. This is just an example of the variability of pea gravel.

In summary, measure the bags, find their volume, and compare that with the 4 cu ft of sand you'd need for 300 lb.