Dave,
A 520 gr bullet is pretty light for a 577, 560 is about the lightest I've heard of before. You may want to consider a heavier bullet with about five grains less powder to start and possibly a slight crimp on the load to get the powder burning before the bullet moves. I don't know what sort of foam you are using but I'd want to use a fairly stiff variety or as much dacron as you can get in the case compressed. I use 17 gr in my 3", a wad about the size of an orange. Above all I'd dump the Unique, duplex loads give me the willies.
Best