Soviet AGS-17, AG-30, and etc. In a reality have 29 grenade.
"AGS-17 is fed from special belt drums that hold 29 linked rounds. "
http://world.guns.ru/grenade/gl20-e.htm
American Mk.19-
" Ammunition is usually supplied in belt boxes with capacity of 32 or 48 rounds. Loaded 32-round box weights about 19 kg (42 lbs)"
http://world.guns.ru/grenade/gl19-e.htm
Now 20 grenade in SPMBT is very little on one battle (4-5 turn fire). In my opinion, it is necessary to make a duplicate set ammo (2x29=58, 2x32=64).
And still, crew Auto GL - 2 soldier, instead of 3 as in SPMBT.