weatherbynoob : I do agree with Doc a lot depends on where, terrain, and vegetation you are hunting. Where we live in AZ a little bragging here is very well known as the trophy elk capital, extremely hard to get drawn even for us unless it's cow. Our terrain at the house is very thick juniper, travel 3 miles down the road and you are in open meadows 2 & 3 miles across then going into heavy timber all public land. There are a couple 370's and a few lone 400 + as we speak pushed down from the snow into the flats that we see almost daily near dark. They head out into the meadows at night then bed in the juniper thickets during the day. It's almost impossible to get on them, probably why they get so big. Both my son and I didn't do well this year in my case 1 very nice bull I couldn't get to him fast enough where my buddy spotted him, and lost him in the fog, and my son archery hunting as the crow flys 4 miles from the house passed on a small 5 by, and had 350 + bulls on him every day for 2 weeks before and after work that hung up out of range with no way to pattern them just ambling along, very weird year. Everyone thought it was too warm in my son's case, and the peak rut started late. Got a little side tracked there, sorry.
We watch the treelines at the meadows right at daylight and dusk where they generally wander down thru some honey spot canyons into small creeks & grassey areas. This year I counted over 150 cows and honestly lost track during my hunt, and about 3 week earlier at least a dozen herd bulls that just vanished guess they knew it was hunting season. My friend and his son both took 370 & 380 late rifle about a hundred miles to the west along with another friends family in the same area 6 for 6 cows/bulls archery.
In our area you really do need a long range rifle in the 270 class on up althought the 308, and smaller bullet diameters do work pretty good if you hit them right with the right bullet. When we were kids on the reservation all any of us kids had were 32 specials and 30-30's with just plain jane factory winchester or remington/peters and to be honest the elk didn't know the difference. Many old cowboy & indians to this day still use beat up old carbines because that's what they grew up with, granted some of them have much nicer rifles, but a lot of us still carry old beater/truck rifles in my case a 6.5x55 mauser just in case that is fully capable of taking the largest elk just so long as I'm not trying to shoot country miles. The one thing with the Partition and have used them for years is if you are very close with a broadside shot usually they blow through, sometimes and have had this happen more than once the nose section comes off reducing penetration and finding the remainder of the bullet when butchering. I have used some Sierra Game Kings and Speer BTSP in a 270 win at under 100 that just grenaded I didn't lose them, but it was a mess, they were fine for deer. I do have a good friend from calif that hired a guided hunt private ranch in New Mexico last year that took a 370 7x7 with 30-06 Innerbonds not a long shot, and he said worked great. I have pretty much switched to Barnes TSX for elk, but I still have a pretty good supply of partitions, coreloks, & others that I wouldn't hesitate using. At least for me a lot of it depends on which rifle I using.
I have seen X-ray pictures of bullet fragmentation with lead bullets and would say that's probably true in some cases with less than good bullets. In CA I know there is an issue with the Condor range as we do here in Northern AZ, but as of yet we here are not resticted to copper only. Waterfowl is a different subject. We could probably argue till the cows come home on the subject, but I'm still not sold on the issue, and think there is a little more bias on their part concerning findings especially having first hand knowledge of how some of these groups work. I do my own butchering and use good bullets and never had any issue, but I can see if a person wants to be on the safe side it's really up to them. The only reason I use the Barnes and it's usually on elk is I want complete penetration. Ron