I dont mean actual weight, it's volume. Freshwater weighs 8.3ppg, I'd be willing to bet by volume it holds 350 lbs of water. Although protein and corn are more dense for the same volume it displaces less. I misrepresented my statement but thought I got the just across.
Since my pellets are smaller and more concentric than corn thus I can put more protein "per gallon" than corn due to "free space" between kernels.
I understand what you are saying but go get a bag of protein and a bag corn and see which is bulkier or bigger in the bags. Pellets for the most part are in bigger bag than corn for me. Then get a bucket and weigh it full of each. Both will have free or air space inbetween. Just been my experience weighing out feed for mixes in the past. Yours might be different. Call a feeder manufacturer and ask them what the weight capacity is on their feeders with corn and protein and see what tell you. My protein bins are 14 tons capacity, but I can not get but 12 tons max of protein in them since the measured weight is in corn capacity not protein. I know this from when I order feed. Always can order more corn than protein.