Cleaning out EMT

My thought is that the EMT will have a rougher inner surface than you want. It is welded seam pipe. It is heavier than you will prefer. If you remove the interior coating it will corrode more than you will desire. If you want to get started on the cheap I would recommend schedule 40 1/2" 1120 series irrigation pipe from Home Depot or Lowes. buy a 10' length for $3 and graduate to a commercial blowgun when you know you are hooked. Either way you will need to make your own darts and will be limited to either low quality darts or to high detail work to create better than commercially available darts.

1/2" irrigation pipe shoots miniature marshmallows decent to about 20' but accuracy is not available in such a setup.

You might want to price out high quality blowguns. In my opinion those are available for under $40 with enough darts to get you started.

All the best in your quest.

The pipe I was looking at was type M which has a wall thickness of 0.028". Type L has a wall thickness or 0.040" and the OD is constant so the ID is going to be 0.024 smaller. Then there is type K which has a wall thickness of 0.049". That would bring the ID down to 0.042 less than the ID of type M pipe.

1/2" copper pipe has an OD of 5/8"

5/8" copper pipe has an OD of 3/4"

5' of type K copper pipe will weigh just under 33.5 ounces and have an ID of 0.652" or still .027" larger than .625.

Copper straight pipe is not so easy to bend. But, copper is still a soft metal. Even hard drawn pipe will bend some under mild abuse.

5/8" type K copper pipe may be harder to find as even 5/8" is not often used. It may be something that requires ordering and awaiting as well as paying a premium for.

3/4" type M will be available off the shelf but it has a 0.763 ID which provides 50% larger cross section than a .625 BG. Ability to power a heavy dart increases by 50% while ability to reach high velocity could decrease perhaps as much as 33% under worst case circumstances. Not a lot of complaints about velocity drops going from 0.50" to 0.625" BGs though. the relationships would be similar.

It takes about 11 ounces water volume to push a dart out of a 60" 0.625 tube under 1.5 psi pressure. It follows that it would take larger healthy lungs to do the same job with a larger diameter tube. To be frank I don't see how any of can blow 1.5psi but to get 4fpe we must be about there.

https://www.engineeringtoolbox.com/copper-tubes-dimensions-d_357.html