Not sure what ETEC is the acronym for?
The areas where I shoot bigger is easier to spot isn't that obvious. This is a 250 ATip a couple hundred yards short of 2 miles.
3300 yards
Most misses could be seen through the 7-35 ATACR on the gun. The camera doing the movie can go beyond 120X. It was probably 80-100x here. If the spotting is sketchier than it was in that day, I run the camera output to my phone through a wifi adapter. The DSLR gives more than magnification. Some days the mirage will be too severe for a lot of magnification. Adjusting the brightness and contrast become the camera's strong points. The point here is lighting, and the backstop composition have more to do with spots than bore diameter.
The default KO2M gun is a straight 375CT gun with a 36" 1:7.5 barrel shooting 400 Lazers at 2950-3000 fps. The rub is you'll need to be able to load ammo that gives predictable first round velocities and 10 shot extreme spreads below 10 fps. After 500 rounds, the barrel will still shoot great 100 yard groups but will introduce enough BC spread to keep it out of the finals past 2500 yards. You will not figure out how to keep the velocities that consistent with the first barrel or lot of brass. I strongly suggest you learn it with something relatively cheap.
Bore isn't a great performance metric for ELR. It's significant, but not the largest issue. That 400 Lazer has a G7 of ~0.450. So does the 250 ATip. Same BC, same velocity, same wind drift and drop. Reality starts setting in at about 7mm. At that point, the required twist rate and velocity have the rpms high enough that they start cutting into barrel life. There are a few steps before bursting that'll ruin your day. The Cutting Edge solids help with BC consistency while the barrel is fresh but are a pretty large step back on spotting. Especially for heavy targets.
A better starting metric for ELR cartridges is the Case Capacity to bore area ratio. It gives a strong indication of potential ballistic performance and barrel life. There are additional filters that need to be applied to this. In addition to the RPM limits already discussed, there is a step up in price for Cheytac case heads and another even larger one with the BMG case head. Going the other way, even the Peterson Cheytac brass won't run 338 Lapua case head pressures and the best BMG brass isn't good for Cheytac pressures. Even with the 4# dead blow hammer they seem to ship with the 50s for opening the bolt. Cheytac is the end of the line for LRM primers. BMG is a new game.
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Everything on this list in red, I've run except the Cheytac case heads. A buddy ran those. Similar practice and loading style. That more or less means 10 shot strings, pressures above SAAMI but well below the internet or what you're running the 300NMI at. The yellow background means I've worn out at least 1 barrel in that cartridge.
The 37XC will give similar ELR performance to the 7wsm, 300wm, and 338lapua.
The top level is more or less 7/300prc, 300lapua, 33xc, 375snipetac, 416barrett. Beyond that, many have found the juice isn't worth the squeeze. Barrel life isn't just a count, it's a life cycle and how predictable performance will be with 10+ shot strings. For a rough cut at balancing the juice and squeeze, every 100 fps with a given bullet effectively moves the target 75 yards closer and costs 1/3 the barrel life. Velocity comes from peak pressure, average pressure (case capacity), powder type and barrel length. Turning all 4 knobs full loud will give impressively short barrel life and a pretty useless ELR gun.
One of the real world examples that started this rabbit hole was the 2016 KO2M. Fitzpatrick won it with the 375 Lethal Magnum, Litz was second with a 338 Edge. The point most miss is Litz outscored Fitzpatrick on the longer second day distances.
