I guess I am completely misunderstanding which parts you already have. I thought you already had the bcg from your Rattler, the charging handle from your Rattler, and the longer handguard.

Unless I missing something, that leaves the barrel, the longer recoil assembly, and the upper assembly to give you a second upper. Am I wrong?

If not, I was wrong on the price being 450$, it is 499$. So for 500$ more and tax and shipping, and with the 800$ for your barrel and assembly you will still be under 1400$ for your second upper.

I am sending you an email to the address in your profile with some information that might be of interest. The subject line is rattler parts, and it is from the address in my profile.