My 416 wby had a new barrel 1 year ago (a CrMo). I foules easier than the SS I had preveiously. The barrel have given 1/2 - 1" groups and have been cleaned just like the old barrel with borefoam and a copperbrush. But the last month I have not been able to get better that 1,5-3" "groups" from it... Frustrating. It is not a bedding issue and all screws are tight. I have been thinking about a scope problem but maybe it is copperfouling which destroys accuracy? Any experiences?

Some barrels tend to copper foul badly and I never had success with simple cleaning with those guys. I suggest the following ritual.
Use JB( a mildly abrasive, NON IMBEDDING cleaner) and a tight fitting patch for 40 strokes. Then clean the barrel with copper removing solvent till the patches show no color.
1 Shoot 1 bullet.
2. Jb 20 strokes and clean with same solvents and patches.
3. shoot bullet No. 2.
4 Repeat this procedure for 20 rds, then JB clean and shoot a 5 shot group.
5. Let me know your results. This is tedious, but works!-rob

Thanks Rob. Will try to get hold of some JB paste and give it a try..

Does CrMo barrels foul easier than SS barrels on average?

It is easier to see fouling on a chrome-moly barrel than a stainless barrel. If all things are equal, fouling is about the same with either.

Ran into the same problem with a couple of barrels when using foam cleaners exclusively. I have found that foam cleaners are great copper removers but leave the carbon behind. Once the throat is carboned up your accuracy goes away and it's tough to get that baked in carbon out. Use a good carbon cleaner and fresh bronze brush on your bore, especially the throat area. (don't forget to use a good bore guide) You may be surprised how much carbon comes out. I've had good luck with Hoppes Elite, but there are others as good or better. Wipeout is now marketing "Carbout", does that tell you something? My barrels required many applications of carbon cleaner. When you think it's clean let the carbon remover sit for an hour, brushing 20 strokes every 15 minutes adding fresh cleaner then patch out after the hour, if the patches still come out black you're not finished yet. I'd try the carbon cleaner first then do the JB as Rob says.
If you are going to use foam I strongly recommend using a good carbon remover first then use the foam.

Here's an article that lists some good carbon cleaners.
http://www.6mmbr.com/borecleaning.html

I shoot Hornady Interlock 300 grainers for practice in my .375 and they copper the hell out of the bore. I use TM cleaner and a good bronze brush, but it takes a bit of scrubbing. Copper fouling will definitely open your groups up quite a bit.

a barrels tendency to copper foul has nothing whatsoever to do with whether its made from CM or SS. The manufacturing process ( button, cut, forged and how its performed) and lapping and polishing process or lack thereof is what causes the roughness in the barrels. Top flight barrels from manufacturers who know what they are doing wont foul much if at all. The lead laping and polishing is there to get rid of the ridges in both cut and button rifled barrels and the fire polishing process I described usually creates a barrel inner surface that resists fouling. Nevertheless I JB all my guns after firing 20 rds. Its worked for me for many many years and many many guns.-Rob

How many rounds have you fired since you cleaned the barrel? I've had rifles that took 20 rounds to settle down and get happy again after a good cleaning. Just a thought.

Some of my rifles shoot better after they have been shot a few times!

I always find the first couple of shots at the next range sessions, after cleaning the rifle, to be slightly off the point of aim, but after a few rounds go downrange, and that barrel gets dirty, my groups tighten signifcantly.

Also, for this reason, I like to leave my barrel dirty during hunting season.

In my experience, most factory barrels copper foul like a son of a gun, because they are usually very rough.

And, as mentioned, copper fouling kills accuracy!

Its quite true that some barrels require a few fouling shots before they produce optimum accurcacy. If you look down a bore scope at those barrels though you'd gasp at whats going on. basically its filling imperfections with jacket material. Those barrels shoot good for awhile till the fouling builds up enough to affect pressures and velocities, then accuracy falls off. Most factory barrels have lots of imperfections, so removing jacket fouling is quite popular. Try shooting copper alloy bullets in those barrels for a generally unhappy experience.-Rob