are there any manufacturers of big bore smoothbore airguns(aka air powered shotguns)? not CO2, I mean nitrogen or air.?

If you mean a production model, then none I'm currently aware of.

If you're just satisfying your curiosity that's your answer.

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If you're interested in buying one

There are a few guys who custom build them.

You can generally find a few at the big airgun shows in Little Rock Arkansas(in the Spring) and Roanoke Virginia(in the Fall)

Production models

There have been a few in the past

If you check Gunbroker from time to time you may run across one.

OR

On this website let it be known you're interested in buying one.

http://www.airguns.net/classifieds/place_ad.php

Look at Quackenbush.

Try this....