Does Density Altitude matter at 100 yards, especially when zeroing a rifle? Will my zero shift at another Density Altitude? Or negligible?

I responded to your first post and will say the same thing, go out and shoot to see what happens. To answer your question through, I’m going to say negligible. For the sake of argument, you do have two things working against your here. If you are in thinner air you wont get the same velocity as at sea level, just a function of the combustion process. At distance since the air is thinner you have less friction in flight, so bullet will maintain velocity better. I don’t think they will offset one another so there is no replacement for good data. Also don’t get caught up in the math, go shoot and record what you get. There is more to this then just air density. Humidity, temp, pressure and probably a gaggle of other things will effect your flight path.

At 100 yards, assuming you mean to zero at 100, then go shoot in different conditions also at 100, then no, it doesn't matter much.

What does matter, at least to some degree, is recording the density altitude when you zeroed your rifle, so that can be used in your ballistic calculations for longer range shooting.

For example, you're going to take a 900 yard shot at 6,000 ft DA; it can make some difference whether you zeroed at 6,000 ft DA, or at -1,000 ft DA. (My location does often have negative DA values in winter, being very near sea level with cold weather.)

Changes resulting from different density altitude values are fairly small at 100 yards, but that error grows with distance. The farther you shoot (and the more DA changes), the more it matters.

Thank you!