Here is how I limit the unrealistic use of Heavy Weaponry in my campaigns:
Most, or nearly all heavy weapons in fact, are so heavy, bulky or have such a feroucious recoil, that they are beyond the normal man to use, even if he can technically lift it or carry it strapped to his back. This is not something that is mentioned in the DH rulebook, but realistically, there is a reason why not even a Power-armoured Space Marine may carry and use an Assult Cannon and that Guardsmen work togehter in heavy weapon teams in the Imperial Guard (even then, the weapons are eihter mounted on a solid tripod or on a set of wheels.
The legendary "Stonetooth" Harker of the Catachan Devils is probably one of a "a handful" in the whole Imperium who is capable of using a Heavy bolter, without any sort of mount or even a fellow soldier to help him lug this massive gun around.
Listed below is the MINIMUM requirement I use for anyone who wish to be able to be able to carry and use the most common of the Heavy Weapons of use in the Imperium of man, without the use of tripods or other "fixed" placement (normal rules for bracing is not enought).
- Heavy Stubber; require an SB of 4.
- Missile Launcher; require an SB of 4.
- Heavy Flamer; require an SB of 4.
- Heavy Bolter; require the Unnatural Strength (x2), OR the Auto-Stabilized Trait and an SB of at least 5.
- Autocannon; require the Unnatural Strength (x2) and an SB of at least 10 (after all modifications).
- Multi-Melta; require an SB of 5.
- Plasma Cannon; require the Unnatural Strength (x2) Trait.
- Lascannon; require the Unnatural Strength (x2) Trait.
- Multi-laser; requre a vehicle mount (it is just made in the "turret" version).
- Assault Cannon; requre the Unnatural Strenght (x2) trait, a total SB of 12 or more (after all modifications) and the Auto-Stabilized Trait.
Nothing above (except for the Auto-Stabilized trait) influences the normal rules for bracing the weapon in any way.
This is just a suggestion for those GM's who wish to add a bit of realism to the game (in my opinion).