I am seriously considering 1:8 twist. I have some of the older Lehigh 62gr CC , and Hammer 68gr HHT. I have some Lehigh prototype 32gr and 38gr CC's.
Finally, I have the older Hornady 75gr AMAX.
I see that Midway has the Hornady 62gr ELD-VT on sale.
I'll save the Hammer 80gr HHT, Berger 85.5gr, and Sierra 90gr for my 22-250AI.
I wonder how light can I go with the traditional lead core/copper jacket with a 1:8 twist?
Depends on the bullet.
Even though 300k rpm is considered the start of the danger zone by many including the bullet manufacturers, I've never had a bullet fail at 300k. 340-350k and above though, absolutely, but it's very bullet and barrel dependent.
In my 223 Ackley I had the 50 nosler btlf start to experience about a 40% failure rate at 362k rpm, but if I dropped the speed down 150fps which dropped the rpm to 340k they held together.
My 6 dasher with nosler 55 btlfs ran at 3800 / 355k rpm great until around 2500 rounds on the barrel, now it's popping about 10% in flight... But that barrel is getting very rough in the throat and starting to damage the jackets. 87s-105s still shoot great out of it though and haven't lost velocity yet.
The other issue is a bullet may hold together at crazy rpm with a new, clean, smooth barrel, but they may start shedding jackets as the barrel gets dirty or as the barrel gets older and the throat gets rough.
Run the numbers for the bullet and twist you want to use and see. Personally, I don't worry too much until things start to get above 320k rpm, and at 340k rpm and above you are really starting to ask for trouble in my experience, but there's also a good chance it *might* work.
Monos seem to hold together regardless of rpm as there is no jacket to shed. I tried some really light monos in my 6 dasher, 22br, and 223 Ackley and they all worked even at 400k+ rpm, but I wasn't happy with accuracy and the cost was too high for me for high volume ground squirrel blasting.
Back to the 8 twist, you'll need to run the numbers, while it will run the mid weight bullets ok, 8 twist is somewhat borderline for the 75 eldm, and the 8 twist may not stabilize the 85s-90s especially at low altitudes in the cold.
With the 8 twist, 3800fps is 340k rpm, and that's well into the "pushing your luck" zone with a jacketed bullet but my experience is most bullets will hold together at that rpm. 3800fps is "book safe" loads for 40gr bullets in a 22br.
The safest choice is always the slowest twist that provides adequate stability for your bullet / velocity / environmental combo, but if you want to run multiple bullets in the same barrel you have to find a happy compromise, and you need to decide if that compromise is a slower twist to run the light bullets but a borderline stability with heavies, or a faster twist to stabilize the heavies and a risk of over spinning the light bullets and having them come apart.