I'm looking at an AUG at a local shop. The barrel has 1601, and caliber marking on the gas block. What is it's rate of twist?

I don't think 1601 indicates the twist.

If 7 is marked, it's 1:7 but having no marking doesn't guarantee it's 1:9.

This is because Steyr also made 1:7 twist barrels without any marking to distinguish them.

So you'll have to check with a tight patch and rotating cleaning rod to know for sure.

Okay, I was hoping that the 1601 was a part number of a specific barrel assembly, with a known twist rate. I may have the dealer order one in the configuration I want. I think I want one of the longer rails.

For FN made barrels onward, 1601 would be a date code. Sixteen being the year made, i.e. a 1301 marked barrel is a barrel made in 2013.

On the first batch of US made 1/7 barrels, there was a "7" stamped on the barrel in the area between the chamber and gasblock in line with the other markings. The second, and much later, batch of 1/7 barrels were made shortly before FN stopped making barrels for Steyr, and do not have any markings to indicate that a particular barrel is 1/7. They did come with a twist designation on hang tags looped through the muzzle though.

If you are wanting an AUG in a specific configuration or color, including 1/7 twist, a typical dealer will not be able to help you. Your choices are the factory at MSRP or Pete Athens of pjs-steyraug.com.

I'd say it's extremely likely that an unmarked (twist) barrel is 1/9. If I remember right, there were only 200 1/7 barrels made by FN that are not marked with the twist rate.

Excluding the ~1800 SDI made barrels (all marked 1/9), and assuming 15000 AUG A3s and M1s to date, then a buyer has a 1.5% chance of having an unmarked 1/7 twist FN barrel.

I'll have to check, but I'm pretty certain that the second batch FN 1/7 barrels are marked 1701.

I can confirm mine is