I have been reading military shooting manual from Lee Enfield 303 days and they cover the the same ground we do in the "super electronic" age. The manual notes the 303 drifts 5 MOA to the left @1000 yds and drops 350yds at a range of 2500 yds...
I don't understand "the 303 drifts 5 MOA to the left @1000 yds", that doesn't make sense on its own. (a .303 bullet would be blown 5 MOA at 1000 yards by a 3mph-4mph wind, but I don't think that's what they are talking about?)
Does density altitude include temp, press, and humidity? These all affect the "thickness" of the air.
"Density altitude" is something that pilots use. It relates the actual density of the air at an airfield (which is affected by altitude, barometric pressure, temperature and humidity) and expresses it as an equivalent altitude if the other parameters (temp, baro press and humidity) were at there standard values. Pilots are interested in this since aircraft performance (length of takeoff roll, climb rate etc) is very directly affected by air density.
Similarly with rifle ballistics, the air density is a key parameter. The drag on a bullet is directly proportional to the air density. Air density means how many kg of mass there is per cubic metre, or how many pounds per cubic foot.
It is not convenient to directly measure density, but it is very straightforward to measure temperature and pressure. It is somewhat less straightforward to measure humidity, but fortunately of the three parameters it has the least effect on air density.
Given air temperature (degrees-F or degrees-C) and air pressure (Pascals, or inches of mercury, or psi), and an assumed or estimated or measured humidity value, one can easily calculate the air density (that's why good ballistics programs ask for temperature, pressure or altitude, and humidity - it is how they figure out the air density).