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Since the Scar upper is the numbered part wouldn't I have to send the upper to Orion for engraving..?
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No. It's a common misconception that the serialized part must be engraved. Per the ATF, you must mark the weapon in a conspicuous place to include "receiver, barrel, or frame" (paraphrasing, you can easily find the supporting documentation online). On an AR, for example, one could engrave the upper receiver and be acting legally.
Most people engrave the lower so that they can run multiple NFA uppers on it. Otherwise, each upper would have to be engraved.
I will be engraving my SCAR polymer lower. That way, should I ever need to return it to Title I status, it's a $125 part that has my info rather than an unobtanium "upper" receiver.
If one never plans to get rid of the SCAR--I certainly don't plan to--it's a nonissue. But I prefer to preserve the option value associated with engraving a low-dollar item.
Back to the OP, Marvin is the go-to guy for barrels. I'm patiently editing for factory CQC barrels since they should require no additional tuning and, if you look closely, cut-downs are a tad longer than factory 10" guns. I also echo previous posters who suggest sending the barrel and part to be engraved to separate shops.