Re: does bullet speed change with altitude
I'd agree with Rijndael.
I shoot through variable air densities that occasionally dip below sea level (certain days at Quantico) to air densities that approach 6,000'.
With your .260 (I shoot a .260 and a 6.5-284), it would not surprise me to see you experience 2-3 moa variation in impact at even 1,000 yards between extremes like this. Air density is substantially affected by altitude, temperature and barometric pressure and a certain combination of these factors could cause unexpected misses.
I too monitor Density Altitude, which I use in conjunction with ballistic tables run at various altitudes. I simply check the environmental factors and turn to the correct chart.
The beatings will continue until morale improves.