karl,
Never tried one "inside" the FPJ, but have tried one on the front over the breech plug nipple. That one didn't help a bit IMO. You might be on to something using on inside? I will have to check out the fit with a few o-rings I have next time I get one of my Disc rifles out.
I never understood WHY they put such a big chamfer on the outside of that nipple? I think half of the flame blows right out the sides between the FPJ and the nipple.
If the seal was closer to the primer, it just might help. I'll let you know if there is something that would fit, and still allow it to function.