I have not personally used WWs in my fowler, Yet. However, there was a fine article about WWs in Muzzle Blasts, last year, and in addition to being larger in diameter, the balls also weigh less, than balls cast from pure lead. As long as you can make an adjustment in the Patching thickness used in your fowler, and the ball and patch combination load find, and the patches indicate a good seal, there should be no problem using WWs as lead balls in your fowler.
You may want to weigh and sort the balls, since you are going into new territory, just to see what variation you get from one casting session to the next, one batch of WWs to the next. I tend to do some test weighing every time I cast anything - balls or conicals.
The good news is that with such a large diameter ball, and heavy weight, a variation of up to 5 grains will usually not make much of a difference in Point of Impact out to 50 yards.
The only caution you need to take into consideration is the patch thickness, and lube chosen. Since these WW balls won't obturate, or expand on impact, as pure lead balls will, you don't have any " fudge factor " in the choice of your patching material, and you want to use a good lube on those patches, too. You may want to try using an OP wad over the powder and behind the PRB to seal the gases, rather than expecting these WW balls to do that job with the patch you choose.
According to the Bevel Brother's Tests in MB, they found NO real difference in accuracy using WWs compared to using pure lead balls. Now, they were doing their work in rifles, not smoothbores, but there should be no difference at the ranges where Smoothbores are usually shot.
