Congratulations to you and your lady! When it came time for me to pick a cartridge for my son's first deer rifle, I chose the .257 Roberts. I ordered a Ruger 77mkII Ultralight and loaded 100 grain Hornady SP's.
If the good Lord lets me live to 100, I'll never forget the look on my son's face and the excitement we shared when he bumped off his first deer at the ripe old age of 8.
Since then, I have also been blessed to be with him when he collected several other whitetails with that rifle, and I have never regretted my choice of the 257 Roberts.
