You wrote:
"With only 250 made, what seriel number do I go by? The one stamped on the outside left of the reciever which is XXX397 or the one stamped on the barrel which is 121?"
Go with the receiver serial number. These guns were built using brand new un-assembled Bulgarian parts kits, as were being sold to K-VAR at the same time, so any other numbers on the gun will be reference numbers and such. The barrel assemblies were the exact same as used to manufacture the full auto AR-M1 Bulgarian AK's, so there is threading on the muzzle under that slant AKM style brake (unlike the unthreaded muzzle ends of the SLR-95/SA M-7 guns). You will only find one serial number on the SSR-99 guns, that being on the receiver and bolt head. Though some numbers may seem much higher on some than others, there were only 252 SSR-99's built, with 150 or so being done by G-Tech under contract of the Arsenal USA company. The reason for the wide range in serial numbers (mine being S004xx), is because the production serial numbers were not done consecutively. Arsenal USA even sold some of their project bound SSR-99 receivers, as stand alone receiver sales. This probably ended up being a mistake for them, because later, when Arsenal USA ran out of SSR-99 designated receivers, they had to resort to purchasing some of Intrac's SLR-100 Bulgarian receivers to finish their production run of SSR-99's. Sure enough though, only 252 SSR-99's were produced (by G-Tech & Arsenal USA's in house staff). The 150 or so built by G-Tech under Arsenal USA contract, tend to be the more desirable of the bunch. I was the first person on the forum boards at the time (circa 1999) to call G-Tech's G stamping, a "G Spot", and it kinda stuck. They say us males constantly have sex on our minds, and since most of us speak about guns as if they were females (like boats & ships), I feel the "G spot" is a more than a appropriate manner to speak about it :)
PS. The funny thing about Arsenal USA needing to purchase some of Intracs SLR-100 receivers to finish the SSR-99 production run, is that the SLR-100 receivers were originally made by the Bulgarians for a Arsenal USA project to begin with. The SLR-100 model was supposed to be an Arsenal USA import rifle from Bulgaria. It was going to be a .223/5.56 version of the SLR-95, but Clinton (with the help of the BATF) put a stop to the whole plan just before Arsenal USA was about to start bringing them in. This caused Arsenal USA's planning to go from being an expected thumbhole stocked AK importer, to a U.S. parts method AK builder. They scrapped the SLR-100 thumbhole stock gun idea, and started a new one (the SSR-99, K-101, and the used component SSR-99P variants).
Anyhows, I guess my link above explained all that, so sorry for babbling on :)