I have a problem, i stand to inherent a 410 gauge H&R handigun, with all parts intact, now the problem is, thats it's a unregistered AOW, and i need to make it legal, i'ver chosen to do this, by removing the "defining feature" which makes it a AOW, aka teh smoothbore, does anyone know of a reliable gunsmith who can cut rifling groove (doesn't need to be deep, only visible) into a 410 gauge barrel?

It's already an AOW.  Nobody in their right mind will touch it.

Shitcan the walking felony ASAP.

Here's an interesting thread on exactly your situation.

http://www.gunvaluesboard.com/have-acquired-an-h-r-410-smooth-bore-handy-gun-that-would-grade-excellent...-30425650.html

Stop, first things first make sure it hasn't been removed from the registry as a C&R.

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Quick research shows it is listed as a C&R, but not removed from the NFA, your handy gun is "contraband" according to the ATF. Sorry.

Remove the barrel from the frame and give it to someone to hold for you. That way the frame and barrel are not in one persons possession. Then quick google search for barrel smith turns up this: http://saternmachining.com
They make their own barrels, might contact them and see if they can cut a few grooves in your project, if not maybe they can give you other suggestions. Another thought is weaponsguild.com , a guy on there made a jig to cut a couple grooves in a 9" or less barrel. Maybe he would loan it out.

ETA: not the one I was thinking, but maybe this guy could help you out: http://www.weaponsguild.com/forum/index.php?topic=42103.0

I'd be willing to try the reamer method, just need to send me the barrel and whatever a .420 helical reamer will cost.

There is a smith in Oregon that specializes in this very thing. Hopefully the OP didn't follow the ignorant advice above and destroy this...

FWIW, within the last few years in Indiana a fellow had such thing confiscated. Arresting officer wanted this heirloom to go back into the hands of the owner but legally couldn't allow it so he reached out to the firearms community on an Indiana gun forum. A guy did all the legwork and found this smith in Oregon that specializes in rifling handy gun barrels just for the purpose of removing them from the purview of the NFA. I'll see what I can find...

ETA: Discussion in INgunowners.com: H&R Handy Gun Rifling
The gunsmith's website: http://www.oregunsmithingllc.com/About-Us---Services.html

This smith comes highly recommended by a guy named Eric Larson who is considered an expert on the Handy Guns. His response to that issue was: