The best ammo currently is Lapua Midas +, Lapua Center X, Eley Tenex or Eley Match. These are the ammo that the top rimfire benchrest shooters are currently using in their full custom rifles. These expensive match ammo will only realize their true potential in rimfires with high quality custom barrels and true match chambers. When I was competing in rimfire benchrest I used Eley Match in my heavy and sporter class rifles and my rifles were chambered by my gunsmith with reamers set up specifically for Eley ammo with the EPS bullet. The ammo I bought was selected by lot # and there was great variation between lots. To be acceptable for use in rimfire benchrest the rifle/ammo combo needs to be able to routinely shoot groups in the .1" (or less) range and have no "flyers" that take the groups out to anything more than .25" (the size of the IR50/50 target ten ring).
I also have a very good Cooper 57M sporter that does best with Lapua ammo like Midas and Center X. It shoots my best Eley ammo very poorly. It's also the ammo Cooper used to shoot the test target that the rifle came with. If you do not have a rifle that has a true match chamber I would not spend the money on the real expensive stuff and try either the Wolf Match Extra or SK Rifle Match. It's really good ammo but just has a few little flyers that keeps it out of top contention for big rimfire matches, but a lot of good non-competition rifles shoot it very well. I have shot some really tiny groups with it in my Cooper and bench rifles. The bulk ammo or high volume ammo lines are going to be hit or miss as to what your rifle may like and they are not capable of shooting consistently well unlike the better match ammo. You should try the Wolf and SK ammo. You may be very happy with them.
