re: 300Blackout For Deer Hunting

The 300 AAC Blackout and 6.5 Grendel are both fine choices for a deer hunting youth gun. See my comments above re: the Grendel. In the lat 90's I bought a 12" Contender barrel in 300/221. JD Jones had just "invented" what he called the "300 Whisper." The Whisper was designed for deer hunting (supersonic bullets) or tactical uses (using subsonic heavy bullets in the AR 15 platform.) The 300 AAC Blackout is ballistically identical to the 300 Whisper, AKA 300/221. All three use the same dies. I killed 3 or 4 deer w/ my handun barrel using supersonic 125-130 gr bullets. All were 1 shot kills, including one at 190 yards. Unfortunately, the Whisper didn't see much tactical use so its use was limited to handgun hunters. Fast forward 20 years of so and everything changes: the AR is really popular, suppressors are common and AAC tweaks the Whisper to make the Blackout. Sorry for the unsolicited history lesson.

As far as supersonic bullets for the 300 BO, it's hard to beat the 125 gr Nosler BT. At BO velocities, there's not nearly as much energy or velocity as in other rounds so the 125 gr NBT expands and penetrates nicely. The 150 gr NBT/Ballistic Silvertip (same bullet) is also good. I don't have experience w/ the 125 nor 150 gr Nosler Accubond, but have read some good comments.

Yes, bullet selection is paramount. I bought a 10.5" AR pistol in 300 BO for hog hunting a few years ago. Of course, the hogs left by the time I developed a load. Nevertheless, I killed a small doe '22 w/ the BO using the 125 gr Hornady SST. Exit hole was not very big and blood trail was light. I found the deer, but won't use that bullet anymore on deer. Other bullets I'll use based on reliable reviews are the 110/120 gr Barnes TAC-TX. 110 gr Hornady V-Max, 125 gr Speer TNT, the aforementioned 150 gr NBT and the 150 gr Speer Gold Dot (there are two, get the one labeled for the Blackout). I've never used subsonic loads in the BO, but my undertanding is that the heavy, round nose work best. The longer, heavy HP bullets don't reliable tumble as well as the shorter RN bullets and the subsonic BO kills by the bullet tumbling, not expanding.

Good luck hunting and enjoy time in the field w/ your kids.