Barrel ID?

[Toestr]

The middle barrel. Looks to be a 16" one piece with AC threads and has SP style spiral porting but I've never seen a SP barrel with milling like that at the rear.

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[Ecapnation]

Might be a modern one piece gog barrel.

[pghp8ntballer]

It could be the one piece all American barrel they made right before smart parts went under

[Toestr]

You might be right. I found a picture of a 14" one piece AA barrel and it was similar but the milling at the rear wasn't the same. They might have changed the milling for the 16".

[Psycho91]

Pretty neat, reminds me of the kapp barrel backs