Thank NSW Dev-gru for this system.
With the 220gr Sierra leaving the muzzle at 2985 fps... sectional density is secondary
That sounds like they chose the right bullet.
The sectional density and bullet shape determine Ballistic Coefficient. Here is a paper on 30 cal bullets:
http://www.appliedballisticsllc.com/Articles/ABDOC110_WhatWrong30Cal.pdf
They conclude that the Sierra Matchking 220gr and 240gr are the best of the 30 cal bullets for long range so my guess is that they are using the Sierra 220gr Matchking. The heavy for caliber 6mm bullets have a better ballistic coefficient so they are often used in long range matches. I think a 6mm would have been a better choice, but the 300 Win Mag is better than 7.62X51.
The 220 gr matchking is a boattail hollow point. The hollowpoint is not intended to expand so it probably meets Hague Convention regulations. These bullets are cup and core construction meaning that the copper cup is open at one end and closed on the other end. The open end can be either an open base or a hollow point after the lead is inserted and the bullet is formed. The open lead base type is deformed under heat and pressure of firing so match type bullets are formed the other way around with a copper base and the open end becoming a hollow point. It is more accurate so given the caliber this would seem to be a good bullet choice.
Also a consideration is rifling twist rate. Longer bullets need a faster twist rate to stabilize. But longer bullets and faster twist rates cause more rapid copper fouling of the barrel. The Sierra 240 grain bullets are known to foul a barrel faster.
I reload for long range shooting and have been doing this for about 25 years.