The markings on the rim are un-readable. Several co-workers are big time hunters so I'm pretty sure I can get a bullet from one of them.
Commercial 223 Remington brass typically has a headstamp of "223 REM" and the initials of the manufacturer. Military 5.56 NATO brass typically has the initials of the manufacturer/arsenal and the year made. It often also has a cross inside a circle (the NATO emblem). Some examples are:
"223 REM / R P" - 223 brass made by Remington (Remington/Peters)
"223 REM / F C" - 223 brass made by Federal (Federal Commercial)
"L C / 6 3" - 5.56 NATO brass made by Lake City (for the US Armed Forces) in 1963
"IMI (or TZ) / 8 2" 5.56 NATO brass made by Israeli Mility Industries in 1982
223 REM is not a popular hunting caliber. It's too small for deer, and not flat-shooting/fast enough for varmints. Your hunter friends might not shoot it. It is a popular fun caliber though, so good luck. In addition to drilling the bullet, you will probably also want to trim the brass. The neck on a regular case is shorter than neck on a blank.
On a whim, I drilled out some Sierra Matchking .224 62g HPBT bullets I had laying around, so it certainly can be done. I also checked to make sure I still had a trimmer and sizing die for 223 REM (I used to reload that caliber et. al.). I haven't ever made a pen from a 5.56/223 case, but I don't see any problem with it.
Regards,
Eric