I am sure someone will post that Rifleshooter article but that isn't really a good source of info on barrel lengths as it says barrels slow down after 24". They don't.

If you want a 18" then go for it. You just lose some velocity but you can still be able to get enough to get the higher BC bullets fast enough to outperform the .308s with same length barrel and maybe a little longer. My semi has a 22" barrel and I have only shot the 140 AMAX factory loads through it. Velocity was 2778fps average with one lot, which was fast even in my bolt guns, and the other lot was 2617fps, which is a more realistic number for 140 loads. So you can figure you will be in the low to mid 2500s with an 18" barrel.

Yes get an adjustable gas block as you will want to be able to adjust it.

Guess you found that article. Don't take that as fact. Something was going on in that test. After discussing it with some others it seems like the barrel being new and still speeding up skewed the results. Also that is a very slow load for a 142.

Yeah the 150 is reported to need a faster than 8 twist from people who tried them. Some with longer barrels and higher velocity get them working in an 8. As you said though to get them in an AR mag would be tough anyways. In an AR the 147 ELDs would work at mag length as that is what factory ammo comes in.

I wouldn't be looking for those chop articles. There are so many variables and so few done.