It certainly sounds like a bedding problem.

Acquired a used Weatherby ultra lightweight. Very eratic groups. First shot is always low and the left, the next three are usually stacked on top of each other. Climbing higher each shot. Any suggestions? Might rebarrel to a wildcat cartridge something in a 25 caliber any suggestions? Thinking a lilja barrel?

check the action bolts. If they are loose then when the gun heats up you will get this problem. Also check to see the scope bases are tight and not broken.

If it has one of the synthetic stocks with the aluminum bedding block, you may need to glass bed the action. I've seen a couple of these that shot eratic groups. They were very accurate after glass bedding. The aluminum block doesn't fit the action very well.

I'm having a similar problem with my 257 Weatherby. I was about to have some work done on it, then was reminded by the folks here that Weatherby guarantees 1.5" groups at 100yds with factory ammo. I forked out a few bucks for the factory ammo, shot three 3-shot groups, then took it and the targets in to a Weatherby service center near me. They have it right now and are working on it. I don't know what they will end up doing, probably crown and bedding work, but the work will be free and the gun will have to shoot at least 1.5" groups for them to give it back. Depending on how tight the groups are, I will decide whether to have any additional work done on it. You may want to try that option before you do anything else, unless you are dead set on the new barrel. My guess is once you start messing around with it, the warranty will be voided.

If the group of 3 is climbing or in a line, then it's definitely a stock problem. It's probably touching somewhere and as it heats up it pushs on that spot harder and harder. That's a rough explanation anyway. Bed it, and if it still happens, take some material out of the stock. Or, if it was me, I'd take some material out of the stock to make sure that the barrel didn't touch anywhere, then if that didn't work, I'd bed it.

I'd disregard the first shot since it's coming from a cold barrel. The next 3 sound like a possible
bedding problem. Does this rifle have a synthetic stock by any chance? Lately I've been hearing
much about some flimsy synthetic stocks that are out there. If that's the problem you may have
to strengthened it or switch to a laminate. Just my opinion. Best wishes.

Cal - Montreal

No offense Cal but what technique does one use to disregard the first shot when hunting?