It is fairly easy to pack a blanket or a small sleeping bag in a haversack, but in cold weather, one would sometimes like to have a warmer bag. A mummy type sleeping bag usually does not roll up easily, and it is somewhat hard to pack it. Does anybody know a good way how to pack such a sleeping bag in M10 or M28?
I guess it all depends on the pack size of your mummy sleeping bag. Typically army issues haversacks have dimensions of anything ranging between 12x12" to 13"x18" (30x30cm and 33x45cm). If the compression bag that comes with the sleeping bag does not fit when you place it in the haversack, you could use a different compression sack where you could change the dimensions of the pack size to fit the bag. But you will be lucky as most of the cheaper mummy bags typically compress to the size of the bag you want to place it in or larger.
My recommendation would be to buy a different maybe a more compact and light sleeping bag which will fit better. But depending on your budget, say something like Snugpac Softie Technic 2/3 would be good. I'm sure with a little bit of searching you could find a cheaper one.
Bear in mind technology improves all the time!