Part 1. Max effective range is determined by your load and your environmentals (mostly elevation and temperature. I am using factory Black Hills out of an 18” barrel and am averaging about 2750 fps at the muzzle. At my home elevation of 3300 and 90 degrees, I am hitting 1817 at 500 yards according to my ballistic app. If we figure 7500 ft and 35 degrees, I’m getting 1800 fps at about 550 yards. At sea level, 1800 fps is around 425 yards. All that said, I think effective range only comes into play for the majority of people if they are using bullets that have a much higher minimum upset or fragmentation velocity. Or at least it should.
Part 2. You can’t get 5/8” threads out of a T3x light or superlight without using a threaded collar, which is basically an adaptor that is milled down to your barrel shoulder with an enlarged shoulder. Regular adaptors work just fine. 30 cal cans work just fine too. Currently, I use a 30 cal AB Raptor that has 5/8” threads and use it with adaptors on a 223 that is threaded 1/2” and a 6.5 that is threaded 9/16”. There are lots of people who tell you <5/8” thread size, small shoulders, and adaptors are all dangerous. Those people really need get out and shoot more and internet less.
