While not necessarily knowing the specific technical details, I had read some info about its construction and have seen some line drawings of the M576 round.
So I was aware of the "container" (for want of a more appropriate word) for the pellets and the sabot but there's never been any specific detail about the size of that "container".
I had assumed this is what you were talking about but I wanted to be certain. Like you I have checked the details of the M576 in the past but always found conflicting information, some sources state it holds 20 pellets of 00 buck while others say 21 or even 27.
I've also seen images in Jane's Infantry Weapons 1986-87 (page 445) that show two different configurations for the early round, the XM576E1 and the XM576E2. If anything, they serve to confuse the issue because the E1 had the typical "shotgun hull" style container and the air scoops on the sabot while the E2 has a solid sabot with a wide though shallow central cavity apparently filled with 27 pellets.
However I have never seen one of these "in the wild" so all my understanding has come from whatever references I could get (I've managed to get my hands on most 40mm Low Velocity rounds the Aussie Army has used but we never issued the M576 as far as I'm aware).
Now I don't know if you're aware of the following site but it's the one I have usually referenced because the info and images apparently come straight from US Army training manuals.
The only straight-forward info I have on the pellets is from this site and it states that the round holds 20 metal pellets of 24 "g" each
If that's 24 grains then it's half the weight of an average 00 buck as far as I know (roughly 54 grains for 00 shot). If it's grams then it just starts getting silly (24 gram = 370 grain)
So yeah, overall, the whole damned thing leaves me confused!
http://inetres.com/gp/military/infan...ammo.html#M576
P.S. The Jane's Infantry Weapons yearbook is the twelfth edition and ISBN is
0 7106 0829 2 if you want to chase it up