Minor note: The weapon should come loaded, so it's actually 31 rounds.
The 60 rounds capacity is only in effect if the heavy bolter is used without the backpack. With the backpack the clip size changes to 250 rounds (so 25 turns of full-auto). The weapon is directly linked to the backpack and doesn´t need to reload until the backpack is empty.
The only way for more ammo I can see is if other squadmembers would carry additional backpacks and swap em (which takes a while, so probably not during combat) as soon as the devastator runs dry. But that´s pretty unconventional except maybe on very long missions.
You know, I missed that line. My read was that you got 60 rounds in the belt, and then had to spend a round to reload every 60 rounds, up to 250 in the backpack. But essentially, unless you use special ammo, you never reload a heavy weapon attached to a backpack.
It does seem odd that all the other weapons have 'enough ammunition to last the mission' and the Heavy bolter and Assault Cannon is so limited (the other heavy weapons have 75 or 100 rounds of firing). I suppose you just requisition some Kraken rounds at 10 Req per clip if 250 is not enough.
I wonder if you can even get additional ammo for a Assault cannon. If so, how much requisition? +15 since it's 3x as big as the Hvy Bolter?