Can you help my understanding, does loading long (close to lands) result in more velocity (higher powder capacity?), better accuracy, or something else? I ask because its my understanding that Eldx and Eldm shoot very accurately at factory COAL. I haven't yet measured how much jump this COAL is in my tx3 6.5CM.
On paper, a 20" barrel looks to be better suited for 130 ELDM, 130 MK, or other high BC "lighter" bullets at 2800 fps appears optimal for a 20" barrel tikka inside of 600 yds. I see higher impact velocity, similar energy, and slightly less drop than a 143 ELDX at 2650fps, using Hornady's ballistics calculator. These are ballpark handload speeds for 20" barrel based off what folks have posted online.
I get 2578fps with factory 143Eldx out of my new 20" TX3, which is too slow for my liking. Working up some 143 gr and 130 gr handloads to determine my final bullet choice for hunting.
Loading long increases the effective powder capacity of a cartridge, which decreases the speed it'll see at any given charge weight but also increases the max speed it can make within a given pressure limit.
At the same time, backing off of the throat decreases pressure and jamming into the throat increases pressure. But, the overall speed you get from a given powder charge, seated long, is almost always going to be lower than the speed you'd get from that same charge seated shorter. In short, the theory is that depth to lands has an impact on pressure, but not as much as effective case volume.
So.....with 'slow' barrels (and 'slow' is only a shorthand way to describe them, not a detailed description) you can usually gain back some of the speed you lost *if* you find that the throat is long, as most Tikkas seem to be. You might even find that you can exceed the speed you might have expected at a standard cartridge length.
As for better accuracy, *some* makers will suggest seating close to the lands. But there are tests on several bullets in the last few years that seem to indicate that the best accuracy over the course of a barrel's life, not so much in terms of absolute MOA, but a lack of sensitivity to seating depth as the barrel wears, will occur if you seat maybe 0.050" to 0.100" off the lands. See here:
https://precisionrifleblog.com/2020/03/29/bullet-jump-load-development/
As for 130 versus 147 speeds and which is 'best' at the end of the day it's going to boil down to personal choice. I have seen that I can get 2650' or so from a 20" and 147s and I believe I could get maybe 2725' or so from a 147 if I was willing to fiddle with different powders. But I am not willing to do so. From piddling with GRT it seems as if I could drop to a 130 and get maybe another 125' of MV but over the course of 600 yards you're going to lose all of that advantage, and I rarely hunt in high winds and certainly don't think my kids are ready to take long shots in high winds, so the relative advantages of a fast/light versus slow/heavy bullet are minor, to me. Also, I like long/heavy bullets as a hedge against 'too much of a good thing' with the rapidly expanding match bullets. That is admittedly a personal bias.
YMMV.
