Machinist Question - Rethreading

Can be done. The nominal major diameter of 1/2" threads is 12.7mm. The recommended drill size to tap 14mm threads is 13mm. SHould be able to drill out the 1/2"x28 threads to 13mm then tap for 14mmx1.

BTW a very WISE idea to get rid of any 1/2"x28 threads on a muzzle device. The AFT gets a bit twitchy about things that go on muzzles with that thread pitch. Regards.

On the first question I would say you can't change one kind of 1/2" thread to another. On the second question, if you are looking at external threads I'd say the answer is no. If you are trying to open internal 1/2" 28tpi threads to 14 x 1mm threads, I'd say yes, as long as it leaves enough meat for the part to be strong.

Adapters can be made to do whatever you like. I have a chinese pistol with metric threads in the shroud, the end cap has imperial threads on one side to fit the muzzle device.

My Prophet 2 has 1/2-20 threads on the end of the barrel. I don't think I've ever seen 1/2-28 on any of my air guns. Older Fx where m20-1.0 and the new are 1/2-20. I know the Vulcans are m14-1.25 and JSAR Raptor is also m14-1.25. I know there are a bunch of threads out there for the barrels, I thought the 1/2-20 was getting to be the standard for the airgun community.

Machinist here. Typical m14 tap sizes are normally x2 or x1.5. Not saying it can’t be m14 x 1 but that is definitely a bastard size tap if it is.
If you want to go from a 1/2 tap to m14 first we need to know if the thread you wish to modify is an internal or external thread. If it is an external 1/2 thread then no. M14 is bigger and you don’t have enough material. If it is an internal 1/2 thread then yes. You can just open in up to the appropriate tap diameter.
If you want to rethread 1/2 x 28 to 1/2 x 20 technically you can but it depends on if the material is aluminum or steel and how deep the overall thread is. The thread itself will be weaker obviously because you are missing material. So if you do choose to rethread to a different pitch and then go cranking on it like an angry ape you are going to strip your thread.
If it is aluminum you might be able to get away with using a forming tap which doesn’t cut but will instead crushes the thread to size so you won’t actually be removing any material. I would only advise doing that on aluminum though and again only work if you are rethreading internally not externally. I know they make forming taps. I have no idea whether they make forming dies but it’s possible. Your other option would be to go to a 1/2 x 20 STI (steel tap insert) aka helicoil. They do make helicoils in different length though so just because you found some and the ones you found are too long doesn’t mean they don’t make them in the length you need. You will also need a special tool to insert the helicoil and open up your pilot hole size. Now you can use a helicoil on a through hole but you don’t want your tap to go all the way through so that way when you put the helicoil in it stops and when you go to screw in the male 1/2 x 20 apparatus it doesn’t just push the helicoil straight through out the other side. You would also need a 1/2 x 20 STI tap.
That is about as much as I can tell you without more clearly defined information.

Listen to Pariah above he s got it down pat
I wouldnt use a helicoil on any precision job - like a barrel/muzzlebreak
any good machine shop can make a an adapter - if u were in OZ i could have helped u out
lots of adapter for sale places like midway etc. how i wish i lived in the US some days
if u had a metal lathe, u could probs make one yourself , and then u can make muzzlebreaks with air strippers etc as well, note i did not say sound altering device
cheaper to buy a 1/2 x 20 nut and try that on

Okay. Given that the thread you are attempting to modify is internal and aluminum I would identify what type of thread you are attempting to change it to. If it is the same size of thread but a different pitch, I would look for a forming tap in the desired tap size. You will still want your tap to initially start threading inside of the preexisting threads. Use some type of tapping oil. WD40 isn’t the best but will suffice. Use motor oil if you have to just don’t tap it dry. You shouldn’t have to back off a forming tap but don’t be afraid to back it off and then try to run it deeper if it starts to get too tight. Avoid snapping the tap at all costs. you snap the tap and you have pretty much screwed the pooch on that part.
Given that the tap size you need may be a bastard sized thread you may not be able to find a forming tap in that size. If that is the case you can still probably get away with using a traditional tap given how long the thread it is mounting to appears to be. You want a gun tap. Not a bottoming tap and not a spiral tap. It should have a pointy nose. You will still want the new tap threads to initially align with the preexisting threads when tapping and use oil. Once the tap starts to bite into the material, some of the preexisting threads will still bend out of the way to form the new threads. So you won’t be losing all your material. Just make sure that your new tap initially goes into the old threads. This is all based on that you don’t have to open up your pilot hole size before tapping.
For someone that does not have machining experience using a traditional gun tap will be easier than using a forming tap. However a 1/2 forming tap or anything near that approximate size is going to be pretty damn beefy and you would almost have to be trying in order to break one. The forming tap will leave you with more material and in turn stronger threads. The gun tap will make it less likely that you mess something up, like snap a tap, but you will be left with weaker threads.
This is all based on that you are going from the same size thread to the same size thread but with a different pitch. If you are going from say 1/2 to m14, just drill a pilot hole to open it up and use a gun tap to cut your threads and be done.

being that it goes on the front of a barrel any handheld tap is probably gonna lead to run out, which will probably lead to striking.
A good machinist will be worth the cost.

I would cut out the threads on the muzzle device and single point the new thread on a lathe. At least use the lathe to align a tap to cut the new thread once the old thread is bored out to size. I think trying to drill out and tap the device by hand would likely lead to axial misalignment. Probably cheaper to find a device with the proper thread if at all possible. With a lathe it would also be possible to cut the muzzle thread down in diameter and press on a sleeve (light press fit). Once installed, threading it to the desired pitch. Indicating your barrel so you are cutting true to the bore, not the outside diameter of the barrel for these operations. Similarly you could cut the muzzle thread down in diameter and cut a new thread, threading the new sleeve on with a bit of locktite, then threading the outside of the sleeve. I think reforming the barrel thread from one to another would put more pressure than I'm comfortable with on the muzzle. I wouldn't want to inadvertently choke it. I also think this procedure would be prone to misalignment as you would be fighting the existing thread.

A bit of old gunsmithing wisdom is to try to perform work to the cheaper part to replace. IE filing/grinding on the new slide stop instead of the frame or slide to fit the two together. In this case the muzzle device would be the cheaper thing to screw up.