My only observation is; why are you using 4064 powder? It's too slow for .223, I think you're seeing signs of way TOO low pressures. Especially when you consider the very slow velocities. Might just be lack of pressure NOT creating a good seal around the outside of the primer cup.
IMR4064 should be ok with a 69gr SMK, I didn't have any problems using 4064 with 68gr Hornady's. Now I'm using Power Pro 2000-MR which has an even slower burn rate, but with 75 and 80 gr bullets.
But I think a faster powder would be smart with such a short barrel.