I have used Epson Salts orally several times, taking the purgative dose of about 2 level tablespoons dissolved in a large glass of orange juice on an empty stomach. It kicks in quickly, always less than 2 hours for me. No cramping, but building pressure and sheer volume cause a high volume flow that is impossible to resist, and at these higher doses, I keep running until I am running almost clear. The most pleasant part for me is when the gurgling starts, and waves of peristalsis begin to set in. I know that the inevitable will happen soon. Any diaper I have ever tried, including the latest from Rearz that exceed a 10,000 ml capacity, will be overwhelmed before it's over. But the taste is terrible, and it is rather harsh. It can also cause electrolyte imbalance, magnesium or sodium overload, and other unpleasant things, although that isn't a problem for most.
I agree with many who have posted here that Sorbitol is a great alternative to any of the saline laxatives. At higher doses, 100 ml - 150 ml, it produces a very high volume, along with some cramping. Consistency of the output can begin as soft, but inevitably becomes more and more runny until it is nearly liquid. But the best part, especially when taken with someone who shares the experience, is the gas that the undigested sugar produces. It ads to the urgency, and after a while, it is impossible to predict what will emerge when you let something out to relieve the building pressure. Never trust a fart. After a while, the expulsions are rather frothy, giving great sound effects. Sorbitol is now my favorite laxative.