Being patient and shopping around pays off. It took me from Oct of last year to August of this year to find and put the parts together.
Here's some links...
BCA Barrel
https://www.bearcreekarsenal.com/bca...s-block-system
Anderson upper $39.99
https://aimsurplus.com/anderson-manu...pper-receiver/
BCA 10" Mlok Handguard $37.99
https://www.bearcreekarsenal.com/bca...-rail-anodized
Gas Tube and Block $13.99
https://www.omegamanufacturinginc.co...bo_p_3062.html
450 Bolt Carrier Group
https://www.bearcreekarsenal.com/bca...-carrier-group
450 Bolt Head
https://www.surplusammo.com/products...ushmaster.html
https://sandersarmoryusa.com/product...hos-extractor/
Now that I add up all the parts, minus the scope, it would have been cheaper to buy a complete upper in 450BM from BCA, than part one together. $229 at BCA vs. about $260 made.
https://www.bearcreekarsenal.com/bca...wist-w-15-mlok
The 450 BM is not fun to shoot. It has pretty heavy recoil. Most ARs don't have recoil pads like the Ruger American bolt gun (which wasn't bad to shoot). I shot my AR 450 just enough to sight it in and will used it for deer this year.
Next project is an upper in 350 Legend. The good thing about the 350L is that it uses a regular 5.56 bolt/carrier. Plus the recoil is supposed to be a lot lighter. Figure it will be about the same cost, maybe a bit less than the 450 in that I already have an upper, bolt/carrier/handle, hand guard and scope rings/mount. All I need is a barrel and gas block/tube and a scope.