I am attaching a drawing that I sent to the folks that made my 35 caliber barrel. It is in PDF format. you can open print it give a copy to your gunsmith. On this drawing I wanted the barrel to match the original contours to lessen stock work so it should be pretty close out to 17.5 inches, the length of my cut off barrel. The last measurement is a guess as that part was gone. Barrel threads are 1 inch, 10 threads per inch with square threads.
Just a word to the wise, recontouring a barrel is problematic, they tend to chatter and bend while machining. It is done regularly but with some difficulty.
This info should help you check you existing barrel to se if there is enough material to work with. You might check with CMP to get a comparison price for one of there new barrels just in case the conversion price is too high. The CMP barrel will have stuff that will cost you do to your 03-A3 barrel (front sight and rear sight screw holes and extractor cut).

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