I did see the massive damage rules in the DMG, but I didn't think they fit a modern campaign. This is my draft of a massive damage threshold more like d20 Modern, using the exhaustion track.

Modified Exhaustion Levels

Level   Effect
1   Disadvantage on ability checks
2   Speed halved
3   Disadvantage on attack rolls and saving throws
4   Hit point maximum halved
5   Speed reduced to 0.
6   Hit points reduced to 0 and dying*

When you reach level 6, you are reduced to 0 hit points and dying. While no effect can increase your hit points, you can still be stabilized and any effect that regains your hit points will stabilize you instead. Stabilizing only lasts for 1 day, to which you have to make the saves again. A Wisdom (Medicine) check (DC 15) adds another day to this time, but this check can only be done once per day.

Massive Damage Threshold and Wound Levels

Guns are deadly, no matter how many hit points you have. Most living creatures have a Massive Damage Threshold, or MDT for short. For most characters, their MDT equals their Constitution score.

If you take more damage from one attack, spell, or damaging effect (only count an ongoing effect’s damage per round) than your MDT, you must make a Constitution Saving Throw. If you make a roll equal to or higher than the damage dealt, you suffer no effect. If you roll less than the damage dealt, you take a wound level. If you roll less than half of the damage dealt, in addition to the wound level, you drop to 0 hit points and start dying.

A wound level counts as an exhaustion level. If you have two wound levels and a level of exhaustion, you suffer the effects of three levels of exhaustion. Wound levels can only be removed with surgery or one week of light activity per level.

A creature may declare their attack nonlethal (most melee attacks can be declared nonlethal; with ranged weapons, it may require special ammunition for the weapon or taking disadvantage to the attack roll). A nonlethal attack triggers Constitution saves as above like lethal attacks. If you roll more than half of the damage taken, but less than the total damage taken, you are stunned until the end of your next turn. If you roll less than half of the damage taken, you are reduced to 0 hit points, unconscious but stabilized.