I used the Speer plastic bullets back in the 60's and 70's. They worked OK at indoor target distances, across the basement.
The revolver rounds were all plastic, the .45acp's used the bullet alone, and a standard case. Its been awhile, but Im thinking the flash hole was reamed out too, like you do with the X-Ring's.
Ive used the X-Rings as well, in .44, and had about the same experience with them as Mark did.
Prior to the Speer plastic bullets showing up, we used to use canning wax "bullets" in the same capacity. A primed case pressed into a layer of wax on a baking sheet, was all that was needed. They worked OK too, but tended to be a bit messy.
Keep in mind, any of the primer fired rounds, are not quiet, and indoors, are quite loud.
One thing I will advise against with the plastic bullets (rubber or wax too, for that matter), is using them in a force on force capacity. DONT! They HURT (and leave nasty bruises)! Trust me.
We were using BB guns back then in that capacity, and the idea that using your real gun was appealing, as it still is today. The first "hit" cured us of that.
Airsoft is a much better alternative, and also a great indoor training aide shooting wise. The gas and electric guns Ive had and use are capable of of good accuracy at basement distances, and outdoors at longer ranges than you might think.
I used to routinely put 5-6 out of 10 into the 1" to 1-1/2" holes in my burn barrel across the yard at 25-30 yards using my red dot equipped airsoft MP5. I could easily hit 18" trees in the wood line at 50 yards with it as well.
The handguns work well at the longer distances too.
Personally, with the airsoft guns available to us now, Id go that route over the plastic bullets for all around non range practice.