The 308 is a great round but it's limits or shortcoming make it less ideal for long range shooting out to 1,000 yards. It is a great hunting or service round that packs a punch within 800 and much better at 600 or less. But barrel length makes a difference. Especially with good bullets and good powder. Bullets have come a long way and because of their progress the 308 is more capable. How much more is still debatable and trying to use a 308 for 1,000 yard shooting is still pretty challenging.
The 308 can and has made it to 1,000 yards without going through the transonic barrier around 1340FPS in a 24" barrel, yet it's just not any off the shelf ammo that can be trusted to do the job. This is more of a handloading job that doesn't to exceed SAAMI specs in pressure to do it. The transonic effect degrades accuracy
Many people have read this
https://rifleshooter.com/2014/12/30...ato-barrel-length-versus-velocity-28-to-16-5/ and other related content only to assume a 308 becomes inadequate for 1,000 yard target yet the test doesn't tell the whole story.
The truth is barrel length, powder, and bullet choice make the big difference. Slower powder will give you more push and velocity than a faster powder and that applies throughout the gun world from pistols to rifle. The longer the barrel the more the push if there is enough powder capacity in the cartridge to get there. Though every cartridge is still limited by case capacity and barrel length becomes useless beyond a certain point.
Since 308 is one of those smaller capacity cartridges (for its bore size) that needs help to get to 1,000 with enough velocity to not lose accuracy before it gets there. It needs three things to make it more successful( really 4 to do so successfully and repeatably). A long enough barrel and a slow enough powder with the right bullet weight.
Then there is the BC of the bullet . You could say this is a fourth thing needed. Without a good BC (ballistic Coefficient) to cut the wind the bullet will bleed too much velocity before it gets there. Granted you can go through transonic flight and end up subsonic without good BC but then the accuracy tends to degrade substantially and bullet stability goes south. Not to mention fighting the wind which a fat bullet going that slow effects a 308 a lot. When you get past 500-600 yards the wind effect increases POI with increasing effect.
Then there is the issue where velocity meets accuracy. Getting the velocity that makes the 308 more capable doesn't mean you get the accuracy you need as any hand loader for accurate rifle can tell you. You need a level of precision that will average 1 MOA or better to get there reliably and if you want to get there accurately you will need both to fall into place. That doesn't always happen on the upper end of the velocity capabilities of just any slow powder and any low BC bullet of usable weight. A loader has to find one that works best with the combo and still have a small enough ES/SD to still hit within a reasonable margin at distance. A deviation of just 50 FPS and you leave the zone.
This is where the barrel length comes in and closes the deal. If you have enough length to utilize the powder charge then you can find that accurate load while maintaining the velocity best suited for long range accuracy.
If all anyone wants to do is hunt and maintain good energy out to 5-600 on bigger game then barrel length is not going to matter as much, An 18" barrel will easily fill that role. Just look at how popular AR-10 and the 20" Savage Hog hunter in 308 has become for hunting.