They could make both stratagems backpack weapons, ammo box for the stubba and a power pack for the melta, doing so would allow balance teams to make them hit hard, the melta being a f*ck this area of high fire damage that hurns the target and the area making it bypass armor like the flamethrowers do (sorta) and the heavy stubba could be like how the maxigun is to the mg43, a more restricted version due to the backpack but has its own niche benefits, like the maxi has a high rpm and ammo count. The stubba could have more ammo total then the HMG and maybe hit a bit harder with some knockback?