I believe the "small disk of some metal" to be pieces of the primer.  I have to clean them out of my firing pin channel frequently when shooting.  They will jam the firing pin and the gun will not fire.
I have examined the fired brass and there are little "pin holes" where for some reason the brass has flowed out of the primer and into the firing pin "hole".

Thought I would mention a "new to me" ammo that I tried today.  It is made by a company called Atomic Ammunition.  It is a +P 124 gr. Hollow Point.  I found it at Cheaper Than Dirt.  Fired 100 rounds today.  No problems except occasionally the last round in magazine would not load.
I was shooting at 20 yards, at my swinging steel disc.  This ammo was hitting considerably harder than anything else I have ever used.  I normally carry Hornady Critical Duty 135 gr. +P+.  The Atomic would almost make the disc swing back over the rod it is suspended from.  cheaperthandirt.com lists the energy at 465 ft/lbs. and velocity at 1300 fps.  I did not chronograph any rounds today so I do not know how fast it is really going out of my R51 but, I am absolutely impressed with the energy with which it hits the steel.  I would love to see some reviews from others on this round.  My only real concern at this point would be possible over penetration.  Will definitely be doing some more work with this ammo to see if I want it to be my new "carry ammo".
Has anyone experimented with Atomic Ammunition?  What do you use for your "carry ammo"?

I emailed him yesterday and have not heard anything either.

Greetings fellow forum members!
I am a newbie here.  I have  had a love affair with all guns most all of my life.  I started going huntin' with my granddad before I could walk through the woods!  He used to carry me on his shoulders as he would squirrel hunt along an East Texas creek bottom.  Later he raised "bird dogs" and we chased after Bobwhite's.  Always a small game hunter until college, at which time I got into a like long pursuit if whitetail deer, mule deer and now feral hogs.  I started reloading with a "Lee Loader" handloader when I was 9 yr. of age.  I have now been loading for shotgun, rifle and pistol for over 50 years.  I have a covered bench and rifle/pistol range in my backyard.  I love to bow hunt, both compound and crossbow, and hunt with handguns.  I killed with my first deer with a .41 Mag. Blackhawk back in 1976.
As I have aged and the times have changed I got into conceal carry back around the mid '90's when Texas implemented their CCW license.  I have maintained it to this day,  I am a Life member of the NRA since the 80's.  I enjoy shooting handguns with my wife and in the last couple of years have gotten into pouring my own lead bullets for hunting.  I recently bought my R51 9mm and have really grown to love this gun. 
I look forward to hearing what fellow R51 owners have to say about their guns and learning from y'all!