OK, BY READING YOUR QUESTION I'D ASSUME THAT YOU'VE ALREADY PICKED THE RIFLE BRAND YOU'RE GONNA HAVE CUSTOMIZED RIGHT?? NOW YOU'RE ASKING BOUT WHICH CAL. TO GET. IF YOU'RE WONDERING ABOUT A CUSTOM, I'D GO WITH TH OLD REM M700. NOW, THE CAL. HAS A BIGGER RANGE TO CHOOSE FROM. ALSO, IT'S ACCORDING TO THE AGE OF YOUR SON TOO I THINK. THE EASIEST TO BUY AND FIND AMMO WOULD ABSOLUTELY BE THE .223, AND IT'S ALSO MY FAVORITE, BUT I HANDLOAD. I DON'T KNOW WHERE YOU LIVE, BUT IF WALLY WORLD IS THE ONLY OR CLOSEST OR CHEAPEST PLACE TO BUY YOUR AMMO, ID SURE GO WITH THE .223. BUT IS YOU HAAVE A GOOD GUN STORE NEARBY YOU'LL BE ABLE TO FIND ANY OF THE CAL'S LISTED ABOVE. I KNOW I'VE NEVER SEEN ANY 17 FIREBALL'S AT WALLY WORLD. THE .223 IS NOT VERY LOUD, NO RECOIL, AND RELATIVELY CHEAP,UNLESS YOU HANDLOAD LIKE I DO). MOST FACTORY .223 AMMO WILL BE THE CHEAPEST OF ALL THE CAL'S LISTED ABOVE, AND THE MOST VARIETY AS WELL. I DON'T KNOW WHAT YOUR SON WILL BE HUNTING WITH THIS VARMINT RIFLE MOST OF ALL, OR JUST ALL VARMINTS IN GENERAL. I'VE HAD A .22-250 B4 AND I JUST DIDN'T LIKE IT FOR A # OF REASONS, VERY NOISY AND ACTUALLY HAD ABOUT AS MUCH RECOIL AS A .243 WITH MY LOADS. THE .223 IS NOT NOISY LIKE I SAID, NO RECOIL, AND EASY AND CHEAP TO SHOOT AND VERY ACCURATE AS WELL. IT WILL WORK FINE IN MY OPINION. LIKE I SAID IT'S MY FAVORITE OF EM ALL. BUT I'M GONNA BUY A .204,NEXT GUN TO BUY) AND TRY IT TOO BECAUSE I THINK THAT I WILL LIKE IT TOO. BUT MY .223 LOADS ARE PROBABLY NOT GONNA BE FOUND ANYWHERE IN THE WORLD LIKE I LOAD EM. I STARTED SHOOTING A 35 GR. HP THRU IT WITH A PRETTY STRONG POWDER CHARGE THAT I WOULDN'T RECOMMEND FOR A BEGINNER, OR YOUNGSTER. START SMALL AND BUILD UP. TO BE HONEST I DON'T REALLY KNOW THE FPS MY LIL 35 GR HP'S ARE GOING BUT I'D SAY THE FIRE IS BEHIND EM...LOL. BUT OVERALL, I WOULD GO WITH THE .223 BECAUSE OF VARIETY, PRICE, AND EASY TO FIND...AND GENTLE TO SHOOT. AND IT'S A HELL OF A CARTRIDGE TOO, I'VE SHOT GROUND HOGS,WOODCHUCKS) AT 300+ YRDS B4 SEVERAL TIMES WITH 46 GR. HP'S THAT I USED B4 I SWITCHED. WELL THASS MY REPORT FOR WHAT IT'S WORTH, AND WITH EXPERIENCE COMES KNOWLEDGE !!! PAUL