Anyone think of a reason not to have M60joe change the 9/16x24 threads on my 13 inch barrel to 1/2x28?      I can't really think it would hurt resale value.    Not interested in selling anytime soon anyway.

I have an M4-2000 that I run on my ARs  and have been thinking about trying it out on the AC.    Not interested in thread adapters.   AAC does make a 7.62 blackout flash hider mount in 9/16x24,  but I've read that the Ruger barrels may not be threaded concentrically and  would prefer the muzzle brake mount, to protect the can better on FA.

Any thoughts?

I can't.  Standard threads give you more options for muzzle attachments and suppressors.

I personally wouldnt rethread a AC556 and/or run it suppressed.

The AC was never designed to be run with a suppressor and you will be putting more gas back into the bolt/carrier and it will slam home harder into the back of the receiver.  The AC has no buffer in the back so it metal to metal contact as the bolt and slide come back.  If you put a suppressor on a AC and you will be battering the bolt back into the receiver harder than it was designed for and potentially develop a crack in the receiver.

You will also be modifying a factory machinegun from its stock form.  If I was buying a second AC I would want one in its factory original configuration with the original ruger AC flash hider vs. an aftermarket A2.

If somebody had cut and rethreaded the barrel, I would be concerned on the wear/tear on the receiver since the previous owner ran it suppressed and I couldnt return it back to the way an AC should look with the distinctive Ruger flash hider.  All else being equal I would offer less money for a rethreaded AC than a factory correct version, especially if it was a factory 13" model as those barrels are difficult to come by.

That said, its your gun and unless you plan to sell it you can certainly do what makes the most sense for your use case.  (there may be other buyers out there that would pay a premium for a rethreaded AC as well)

Got a e-mail back from M60joe today.  He said he would "pass" on threading the barrel.   My plan was to put one of Accuracy Inc.  adjustable gas blocks on the AC.
Will have to think about it awhile.

Thanks for the input.

I had one shortened and threaded by AWC many many years ago.  Still shoots great suppressed but the real fun is running the 22kit suppressed.  Tends to heat up faster suppressed than not.

Send it to Accurate Ordnance.  They do some amazing work.

How about a standard mini 14 barrel cut down to your 13" and threaded to your liking?

I'd be leary about modifying it to a point where you can't bring it back to original condition.  Who knows what these will be worth in 20 years!?