Build and AR15 in 6.8 SPC. Very little recoil, up to a 120grn pill dang near 3000fps. And is a hog buster!!! It takes a regular upper and lower, but requires a dedicated bolt, barrel, and mags. Around a $1.00 a round for Hornady SST.
The only thing the cost would be a little higher in the 6.8 you could build one for less than $550 for sure. The bolt would be the most expensive at around $200 and then the barrel at $130. The rest of the gun is standard AR15 parts. Parts now are easily found and cheap now.
It's nothing to these ARs, very easy to work on, tons of how to videos.
Ya'll building it together will be priceless and that AR can be handed down for a long long time.
WARNING tho!! It's hard to have just one AR. I got fo now. Absolutely a blast to plink with.