This is kind of a GURPS post.
High Tech 3rd Edition for GURPS 3e had rules for attacking an area under the beaten zone. They were cumbersome and bothersome to use. Players avoided it like the plague, even though this is how a machine gun is normally employed!
Machine guns are area weapons, not point weapons.
Another issue with the beaten zone rules was there weren't any guidelines at all for determining the size or shape of the area attacked, other than it being an ellipse.
I dug and dug for a good rule of thumb and never really figured one out.
Why, because it's complicated!
The first variable that shows up is in the ammunition. There's a tolerance range for bullet weight and velocity. If you put up a target and only had this variable, the hits would string vertically. Impacts on the ground string in a line along the axis of the barrel.
The next variable is dispersion. This is gun simply not hitting the same spot between shots. On a target this creates a pattern that falls within a circle. On the ground it's a nearly round oval.
Combined, the impact pattern on the ground is an ellipse with the long axis aligned with the direction the barrel is pointed.
The farther from the gun, the bigger the ellipse.
The problem for making a rule of thumb is finding out how much the ammunition tolerances affect bullet impacts and how much dispersion is built into the gun; then calculating the shape of the ellipse.
It's doable, but generally not worth the effort for a game, especially when the rules for using it are so damn irritating! One roll per target in the zone, per shot fired... So if there's 4 targets with a RoF 12 gun, you're rolling about 48 times with an effective skill of 9. The good news is, if you hit, then you roll one time less per hit on the remaining targets. It's taken an hour to make all the required rolls.
4e has the suppressing fire rules which replace the beaten zone rules and are MUCH easier to use.