Re: long range caliber question
If LR means out to 1000yds, then the choices are many. If you intend to go to say 1mile, the choices limit and the costs go up...way up.
From the 3 cartridges you suggested, the 7RM gets my nod. Simply because the RM case will launch very high BC bullets at 3000fps and not remove your molars in the process.
The 308 is fun but ballistically very challenged. The WM superbly accurate but limited in case capacity for 200 to 240gr bullets. Simply will not reach 3000fps at sane pressures.
For target shooting as the main use, the 6.5 is tops due mainly to reduced recoil, followed closely by the 7mm, then the 6mm. This ranking based on moderate case capacity (say up to 75gr of powder), portable barrel length, non hernia inducing rifle weights.
If more bullets came out in say 25cal, the tables would change.
For target shooting, recoil and barrel heat will not increase your accuracy. The smaller the case the better. That is why the 6.5-284 is so popular. However, there is already smaller cases being campaigned.
The 6mm is really coming on strong due to better bullets. Cheap to shoot and easy on the shoulder and barrel. The 6BR and better choices based on the Savage case are really making inroads into F class and LR BR.
If you feel that the challenge of the 308 appeals, why not shoot the 223 and 80gr bullets. Ballistically identical and you want to talk cheap shooting, low recoil. This may just be the best entry level cartridge combo out there now.
Best part you can pick up a Savage heavy barrel with Accutrigger and get shooting for well under $1000. Now that is a great place to start and competitive in many new NRA classes.
If you want to use the rifle for hunting, then the bigger cals and heavier bullets start making more sense. Here I would lean towards the 7mm and 30cal. The 338 will come but you need to start somewhere.
There really is no right answer as so much depends on your intended uses. Do a search as this topic has been discussed many times and will be discussed many more times.
Bullets are really driving the direction of this sport. Every year we get new and better choices. This changes what works..
Welcome and enjoy the reading..