The 6 UM is definitely a sweet spot.Like most things in an engineer's brain, firearms fall into a category of things to be optimized. Just like backpacking gear, vehicles, etc.
For a hunting rifle, the attributes to be optimized are wound channels, external ballistics, shootability, and weight. For most of my hunts that rifle is a 6UM at about 7.5 lbs scoped (current one is 11.5, so another will be built).
The following is based on western hunting and plains game, in generally quite open terrain. This use case prioritizes external ballistics at long range without giving up the ability to achieve sufficient penetration at any reasonable shot angle.
Common magnum cases don't truly optimize 7mm+ bullets. They have the capacity to optimize up to ~6.5mm bullets. As mentioned above, it takes a really big case to optimize a big bullet. Lapua Improved, XC, HCM, etc. They have a place on hunts where difficult winds and extreme ranges are the rule. At that level, the extra recoil comes with an actual advantage over the 6UM.
Mid-capacity 7mm-338 magnums just don't offer anything in terms of terminal or external ballistics vs the 6UM for these types of hunting. The recoil is detrimental, and there is no advantage to go with it.
An extreme capacity magnum that comes in a little under 16 lbs is on my wish list, but that's about my only practical use for a big magnum.
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I haven’t shot one, but my 25 SST shooting 133s isn’t a whole lot more gun. Said it many times, after I saw what my 7mm was doing at long range, I knew I could drop down in barrel length and keep very similar ballistics.
Not gonna switch to a 6 UM as it is too similar, but will build a 22 creed. I already have a 22 BR I built for inside 500-600 depending on DA.