primer backing out

[ken s]

minor problem with my original 40 70 SS 74 sporting rifle.
some of the time the lever has to be pushed harder to open and eject the shell. no problem. but I've noticed the primer has a 'sliding' mark on it. so it's backing out a little, and the breach has to ride over it. . I'm using MAG LR primers.
my 50 70 gvt. carbine does not do this.
Is one primer better and tighter than another?
I've just read that the old timers just used to cock the hammer and
dry fire it to push the primer back in. 1929 article.
And of course I put the hammer on safety before opening it every time. I even have a small spring in the breach to make sure the firing pin is back.
Thanks...Ken