After shooting 250 rounds of Tula, my new WASR 10/63 has quite a lot of carbon build-up on the muzzle crown. How would I remove this? I have tried using Hoppes and Break-Free but nothing seems to work that well.

I use Gunslick foaming bore cleaner.Let it sit for @20 minutes and carbon melts off.Great to use on the bore,gas tube as well

http://www.midwayusa.com/product/474487/gunslick-pro-foaming-bore-cleaning-solvent-12-oz-aerosol?cm_vc=2110BrandPopProd

Do you know if that stuff will neutralize the corrosive salts from surplus ammo?

Usually the build up on the muzzle can be easily scraped off with a piece of brass sheet or the smashed mouth of a brass cartridge case.
It just flakes right off.

Another great method is to buy a jar of Slip 2000 Carbon Cutter.  This dissolves and removes carbon with a 15 minute soak.  It also is water based so it flushes off corrosive residue.

A last method of removing the carbon is to buy a "lead-away" cloth.  This is a chemically treated cloth that wipes leading and carbon off.  It was developed to wipe carbon and lead off stainless revolver cylinders but also works for the muzzle and gas piston.
DON'T use on a blued gun, it also wipes bluing off.

Get Mpro 7 http://www.mpro7.com .

No, it will not.  You have to use something like very hot soapy water.

You don't "neutralize" salts. You remove them. The salts are water soluable. Water or any water based product will remove the salts if you use enough to flush them away.

I do that a smashed 223 case works great.

Corrosive.
Dont be scared to remove all the small parts and hose it off with hot water in the sink. Small parts separately. Then i take it to the shop and blow the water off with compressed air..
Oil liberally afterwards.

I do that with my 74 with plastic furniture, but I'm a little hesitant to go soaking my Yugo with wood furniture under hot water. Always looking for a better way, you know!

You'll find quite a few guys that shoot corrosive ammo use Windex to clean with. Of course the ammonia in it cleans but the water in Windex it what actually removes the salt residue.

I take the hadguards off, hold the buttstock up and spray away from it.