This will likely cause some accuracy problems but I do not think it would require a complete new barrel. Should be able to just have the barrel recrowned to clean up the blasted portion of barrel.
I always tape off my muzzles completely when blasting barrels. Leaves the muzzle shiny but there is NO chance that the muzzle will get effected in any way.
If this was done by a rifle builder, just get in contact with them, I am sure they will make it right, its not a big deal to correct this, even if you have a muzzle brake its not a huge deal to fix as long as the bore was not blasted way down the bore.
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