1 ounce of gelatin powder for every 8 ounces of water. It takes a phenomenal amount of gelatin to make a decent size block. 1 gallon of water is simply not enough and 2 gallons is really going to leave you with a kinda small block still IMHO. I usually make mine in 4 or 5 gallon batches. I make a super concentrated pot (about 2 gallons) on the stove and then add hot (like near boiling) water to my mixing bucket and then pour in the super-concentrate and mix with a whisk in a drill for a few minutes. Then empty the fridge and let it sit in there for a day or two. I find it easier to leave it in a sacrificial bucket while I'm shooting at it and then I cut the bucket off afterward. Keeps it from bouncing all over when hit, jumping off the table and getting dirt covered. If I need to have more depth than a 5-gal bucket provides, I can always cut the end off of one and epoxy the tops together. It always breaks while taking rounds but it's good enough to hold the gel. Usually I'm only concerned with about the first 16 inches of penetration looking for terminal effect, cavity size and depth and bullet fragmentation. YMMV. You could do better though, clearballistics.com gel is actually clear and easy to use/re-use.
