The key aspect to getting the longest barrel life possible is keeping the bore cool. I have been building monster, fire breathing wildcats for customers for two decades now and there are a few things i have learned.
1. the throat needs to be designed properly. No larger then 1/2 thou over nominal bullet diameter. The tighter the better. This limits hot gas blow. Hot gas blow by greatly accelerates heat cracking in the bore just ahead of the throat.
2. Its best to seat the bullet within 10 thou of the lands. This allows the bullet to SEAL the bore quickly, not allowing the hot gas blow by.
3. Neck diameter in the chamber matched up with proper neck diameter of the loaded ammo can also greatly effect gas blowby. Loose necks allow significant blow by, accelerating heat cracking.
4. Perhaps most critical, as correctly mentioned, powder burning in the bore causes the most heat build up. The hotter the bore surface, the less able the barrel steel can stand up to heat cracking and erosion. As such, and this is something i preach to my customers, never shoot more then three shot strings. Looking at a three shot string and what it does to the barrel, shot number 1 does very little if any damage to the bore. Shot two also does little damage. Shot three will do as much damage as shots 1 and 2 combined. if you shoot a fourth round, that shot will do as much damage as all three previous shots and if a 5th shot is taken, that 5th shot will do twice as much damage as the first three.
now this is referring my wildcats not a 6.5 creedmore but it does hold true to some degree.
with large hunting cartridges i cringe when i hear people say "if your not shooting 5 shot groups it means nothing"......
i tell my customers, get your rifle zeroed and get off paper. Then work on your drop chart by doing Practical Field Shooting. That being shooting at random measured ranges, setting up each shot like you would in a hunting situation, taking a single shot, recording the impact location and then letting the barrel cool and doing it again at a different range. Targets can simply be small rocks, nothing fancy is needed, just need to be able to clearly see impacts.
in 10 shots you will get more practical practice then 100 shots fired on paper and your bore will never get hot.
with rounds like my 7mm Allen Mag, you can get easily a decade of fine long range accuracy by using the rifle properly, or you can change your barrel every year if used incorrectly.
in my opinion, neck length means nothing to accuracy life..... but again, i am not referring to small rounds.....
Appreciate your input 
