Don't get me wrong guys, I like the 6mm and 6.5mm bores myself and I think these departments provide amazing bullets for target work and some hunting too but I also
think we have to be pragmatic and careful with what the sports magazines, internet,and marketing guys are trying to sell us.
The problem with the Internet and youtube calibers and fashions is that we loose a bit sight of what is important here.
The most important component often overlooked and rarely discussed is getting professional training and half of the work is done outside the range studying the ballistics,
loads, calculations, and then later interpreting the results. Then having access to a good location or installations where one can practice a lot like Llama mentioned.
Having training and perhaps some one on one coaching from a professional is probably the most important investment the average shooter will ever made.
You can apply ballistics calculations and principals to whatever you have to work with and regardless of the choice, consistency is more important than power, reach or
whatever the marketing and sales folks and enthusiasts are trying to sell us.
The 223 loaded with 70-90gr VLD bullets is a great way to train long range on a very reasonable budget. The black-hills 77gr TMK has a transonic range past 1000 yards
in most locations and conditions so this is more than plenty to learn in pretty challenging distances. Even shot from a compact 16" AR will stay supersonic to 1000 yards
with 10.7 mils that is amazing for such small round.
But, like I said, the 7mm-08 for the same or even less cost than the latest 6.5mm fashion will leave any of those 6.5mm behind in terms of performance and reach.
For fans of hornady even this mid budget 7mm bullet is launched by the 7mm/08 at 2,800fps from a 24" barrel...
https://www.hornady.com/bullets/rifle/7mm-284-162-gr-eld-match#!/
For those who own a 308, don't worry because this is an amazingly versatile round that can extend the average 800yards effective range to 1300 yards w/o much trouble if one doesn't
mind paying a bit more for high end bullets.
In fact a load that can be shot from AI magazines and many 308 gas guns at a max COAL of 2.8030 can achieve similar trajectories than the 6.5 but shooting from a
20-21" compact rifle vs a longer 24" 6.5mm barrels. Not only the trajectories will be nice but it will put more energy and momentum on the targets.
Of course one needs to spend more than the average Sierra Match King.
Beware of the salesman and study the science of ballistics. In the end all starts and ends with a good bullet choice for the work required, the rest is just part of whatever
the delivery recipe.