I have several model 51's and a model 651. After firing 6 rounds, when I try to eject the spent shells, it is very hard to get them to eject. I can't believe it could happen to all four guns. What might be the problem? Please advise and Thanks.

When I had this issue with my m48s, I took a bore brush and some flitz, a drill at low speed, polished each cylinder. First with the bore brush, then with the .22 mop. Then I waxed the cylinders with Renwax. Several hundred 22WMR later, all eject easily.
This may work for you, it did for me and I've done the same to my others, just because.
Your problem may be ammo, but well polished cylinders may overcome whatever the ammo may be doing(dirty or expanding cases).
BTW, welcome here.

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Halfway and one more step

I would try different ammo before altering the guns too much. Of course, .22 WMR is currently the toughest ammo to find.