@Blazer168
My guess, cost, mobility, and reload time for big weapons would lower the dps and/or safety to dangerous levels. Rocket launchers need time as well as a sturdy foundation to use. Both of which he does not currently have. Also it isn't something you can dodge and shoot with. Also, also, the blast could wound/kill his teammates if the missile gets deflected or thrown back, which is basically worst case. Not to mention the sound. LMGs and HMGs are bulky and, with his already seriously wounded condition, could be near impossible to keep aim. He seems to be injured to the extent where the recoil could end up hurting him more with the bigger guns too, but even his current one should then too.
If we assume weapons are not heavy to him, then ammo and reloading are the issue. You get maybe 4 rockets depending on the launcher. Then you need to reload. Same with the gun, you get 1 belt, if it breaks or jams (if it can) he is fucked. And they take a long time to reload compared to a mag in his current weapon.
And if that isn't it, then the cost to buy it could be too much.