Yeah- I don't know if the shot load would act as a single unit, ie: they'd be a mass weight behind the barrier and move it as a single object. Thus, I susept they'd act as a whole, being held behind the card, but I've not heard of any problems nor had any myself.

A few years back when testing my Brothers Manton 15 bore double ML shotgun, the load in the second tube never shifted from the recoil of the right barrel. His shot load, while fairly heavy for a 15 bore, was still only 1 1/8oz. thus not overly heavy. The 2 1/2 dram powder charge (68gr.) gave 72% at 25 yards, good for a cylinder bore. I use the same charge in my single shot 20 bore flinter, but with 1 ounce of shot only, for 85% to 87% paterns in it's slightly choked bore. Deadly on birds, clays or feathered.