Tom,
I don't know the velocities you are working to acheive, but I certainly can't endors packing foam as soft gas checks. As stated earlier, they will positively create for you a worse problem than leading in your bore!
Considering some velocity/pressure limitations, you should be able to shoot your BB bullets without leading problems with just a little fine tuning of your combination of velocity, bullet hardness, lube quality and bullet fit. Of these, bullet fit is the key. Fit the bullet to the cylinder throat (provided it is larger than the groove diameter of your barrel), and if you have no significant constriction in the barrel where the barrel shank screws into the frame, you'll have very little leading problems.
Those folks working towards obturation so the bullet seals well in the throat and bore are having to do so because the bullet doesn't closely fit the throat to begin with. Skip the hassle and do fit the bullet to the throat and many of the problems and headaches associated with cast bullets and revolvers will disappear.
Soft gas checks? They've been tinkered with since the 20's in one form or another, and they get written up in some gun magazine by one author or another about every 15 years or so, and they are a "new idea" to that generation of handloaders, but the truth of the matter is, that they on the whole, are much more trouble than any benefit gleaned from using them!
Just my two cents worth!
God Bless,
Marshall