You don’t have to go crazy on a scope for that purpose. Most anything would work fine, but the Vortex Copperhead line is decent without putting down too much cash. If that’s too much, Walmart used to (haven’t looked in a while) 3-9x32 Tascos or Simmons scopes for like $40.
Or, watch...
This is exactly right. Impact velocity and bullet construction are what matter to terminal performance; the name of the cartridge and how fast it went at the muzzle are both totally immaterial.
It’s all about compromise between velocity and weight/handling characteristics for your intended use. Good quality barrels can be accurate no matter the length.
For long range hunting, impact velocity is important for terminal performance once you make a hit, so that’s another factor that...
I assume this was just a typo, but just to avoid causing the OP any confusion, the above quote is quite inaccurate. The 6mmCM and 243 are basically a toss-up velocity-wise with the same bullet weights. In fact the 243 has slightly higher case capacity.
6mmCM case, ammo, and rifle designs are...
Since you have to go with lead-free, you might want to take a close look at Hammer bullets for whichever chambering you go with. They reportedly have better terminal performance at distance (where you generally need a softer, more readily expandable bullet) than some of the other lead-free...
For your son, a well-designed stock without too much drop and in a LOP that fits him, with a decent recoil pad, along with practice, will make far more difference in felt recoil problems than the step down from 6.5 to 243. Of course you can always brake it, but in my experience with kids, noise...