People seem to incorrectly assume that just because it shoots black powder, it's automatically exempt from all gun laws.
If you read the law, it says absolutely nothing about type of powder.
It all depends on whether it fires loose shot or self contained cartridges (and if so, whether they are "commercially available")
Handloading some 7.62x39 with black powder will not magically make your AK-47 an antique. i wish it did, but unfortunately it will just make it very dirty.
So, no, I'm afraid you can't chop it without the tax stamp.