I bought a barrel with specs I’m quite fond of...unusual specs that aren’t really anywhere on the market. Anyways, only issue is the threads on this 5.56 barrel are 5/8x24. The barrel manufacturer claims it is not possible to re-cut the threads down to 1/2x28, but I don’t see how that could possibly be.

Can anyone help me out here?

You say it's unusual, is it steel ?
It's not some composite aluminum/steel/carbon fiber deal is it?
If it's one piece steel, sure it can be rethreaded.

It’s steel. Was more just saying I compromised on the thread pitch because there aren’t any other barrels on the market with these specs.  

And I’d like 1/2x28 threads for standard muzzle devices rather than using a 30cal. But maybe that matters way less than I think.

It’s a 16” AR. Initially couldn’t find another gunsmithing section so I posted here. Anyways, barrel is a Black River Tactical MPR. I’m not sure which muzzle device yet. I hadn’t thought of 6mm that’s a good idea too.

I’ve seen some hot rod .224 barrels threaded 5/8x24 like 22 creedmoor, 22-250, etc. you might be able to find a .22 cal brake threaded 5/8x24.

As a practical matter, if it's a .223rem and a rifle/carbine then a 1/2x28 thread is perfectly fine. At this very moment I'm having a 1/2x28 brake fitted to a 6XC rifle. I chose a .223 brake specifically so it'd have a small diameter exit hole for better efficiency so I wouldn't have to weld it up and re-drill to get tight tolerances. In this case, the exit was advertised as .26" and came out at .25" so we'll bore that out to .263".

I consider .020" total to be both the minimum and maximum desirable brake to bullet clearance (.010" all the way around). My muzzle is .820" and while there was plenty of meat in the brake's shank to open it up to 5/8x24, I decided to keep it at 1/2x28 so I can roll it over to my 223's if I ever decide to. So my 6XC barrel is getting cut for 1/2x28 instead of 5/8x24.

A typical .30cal brake exit hole is something very near .35". That allows a ton of gas to exit the front of the brake instead of the sides. which reduces efficiency a lot.

Is that taken from the mk1 shoulder sensor or from an instrumented setup? Not challenging, just curious.

In a match, 1-2% might just make that 1 point difference. Because it's only a single chamber brake, my intuition tells me that it'll be something more than what you estimate but that's all... just intuition. I had a 30cal brake on my .243AI and it definitely "worked" as far as being a brake but it seemed to work better when I had it on a .308. Admittedly the .308 kicked like a Missouri mule compared to a .243AI to begin with so the comparison may not be fair or helpful at all.

I prefer 5/8x24 threads on 5.56mm rifles and wish it were the industry standard.

It won't make any difference other than price. 5/8x24 A2 flash hiders are a little more expensive.

Muzzle brakes are more efficient and effective the higher the powder charge. The .223 being puny and using 25+/- grains of powder really can't drive a muzzle break sufficiently and doesn't recoil to begin with.

I own a .300 Winchester Magnum with a 27.5" barrel and muzzle break and the difference shooting it with and without is like night and day. I believe the only thing a .223 rifle shooter will notice with a break installed is a louder report.

I don't see why it would matter unless you had a direct thread 1/2x28 can you planned to use.

Can it be done? Absolutely.

My guess is they don’t want to deal with recutting the threads.

Simple solution would just be to run an attachment setup for 5/8x24.

I wish 5/8x24 was standard as well.

Or take the barrel to your local machinist and ask him to re-cut the threads to 1/2x28. Nearly all 5.56mm rifles that have threaded barrels come with 1/2x28 threads so it's not like it's unsafe. I run 1/2x28 on a 6mm.