1. does it matter what ammo you use for the break in of a barrel?
Nope
2. what is a good ammo to use in 300wm to break in a Krieger barrel?
Any reputable name brand 300wm ammo should work OK.
3. where is a good place to buy the ammo ?
At your favorite little ammo store.
4. what match grade ammo to use?
If you think you need "match grade", consider getting Federal Gold Medal Match (190 Sierra Matchking), or Black Hills, loaded with the 190 Sierra Matchking. Be advised that if it does not seem to work well for you, the problem is probably NOT the ammo.
You might contact CopperCreek and see what 300WM they are loading.
After many years (30+) of trying a variety of barrel break in methods, I finally settled on this:
1) Get a good quality bore guide, remove the bolt and put the bore guide in the action.
2) Using a good quality cleaning rod, properly sized, with a correct size jag, run a clean patch or two through the bore to remove any grit, shavings, or debris.
3) Remove cleaning rod, and remove bore guide.
4) Reinstall bolt.
5) Shoot whatever ammo I desire, at what ever target or distance I prefer.
6) Repeat step 5 until barrel replacement is needed.
7) Repeat steps 1 through 6.