Second, take the volume of the entire shell around earth from 400km to 450km up-- that's about 3 * 10^19 cubic meters. 1.3 million objects scattered randomly over this entire volume yields a density of 4 * 10^-14 / m^3, which sounds like a small amount, until you consider that a moderate sized object orbiting at that altitude might sweep 20000 m^3/second. You'd expect this indiscriminate attack on LEO from a single launch to hit 2.5% of objects with this 2.5 m^2 cross-section in the year that the BBs stayed in orbit. You'd expect to hit the ISS about once every 2 hours and to accrue hundreds or thousands of hits.
To the extent that you focused your attack on certain planes and on a narrower range of altitudes, you could expect it to be more effective. And, of course, to the extent that there's any further fragments created / Kessler-type effects, you can expect more effect.
One, Kessler syndrome should really be called the Kessler hypothesis since we’ve never actually proven it’s a problem in any real form. To the degree it’s been rigorously modelled, it would create a lot of property destruction in a limited number of orbital planes, not block access to space. The problem is breaking stuff. Not blocking. Two, where it’s been rigorously modelled and shown to be a problem (again, only for a limited set of planes) it derives from something big breaking apart. Not lots of small stuff. And again, we’re talking about a bunch of property destruction. Not annihilation of everything in the plane let alone orbit.