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Better to engrave it first. That way if it gets lost, broken, or otherwise buggered during engraving, you haven't lost more than you already have.
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Agreed. Especially when talking about AR's.
If an engraver messed up a lower, I might ask him to reimburse me 50 bucks for a new one, and I would buy a new lower, and go somewhere else to get the engraving redone. Then, when I am happy with the engraving, I'll apply for the stamp.
(You could actually request a refund of a tax stamp, but you would then restart the process, and it would take another 9 months or more for the new application. Nobody wants to do that.
If we were talking about a PS-90, or another gun, I don't think anybody is going to get rid of a 1,000+ gun because of a mistake, so you probably have to make the best of it, and I don't think timing on the engraving is as important.