The proper process is to put the gun in a milling machine and mill off the metal so as to convert it to a semi-lug barrel.
Then the sides of the cut are machined to conform to the roundness of the barrel.
Once (and never again) I was called on to convert a S&W Model 617 to a semi-lug barrel using nothing but hand tools.
I used a high tension hack saw frame and high speed blades, a good flat file (it wasn't when it was finished) and wet or dry sand cloth.
I sawed slots in the barrel lug TOWARD the bore to act as depth gages and to prevent cutting too deep into the barrel.
I sawed each section off before moving to the next to prevent the cut from drifting off.
I filed the barrel to round the cut and match it to the contour of the round barrel, and removed file and saw marks and gave it the correct finish with the sand cloth.
I finished it by using a Scotchbrite pad to give it the correct grained finish to match the rest of the gun.
DON'T TRY THIS AT HOME.
It's a VERY risky job because it's very easy to get the cuts off and ruin a barrel. Plus, sawing stainless steel is HARD work.
I was not at home so I didn't have access to my tooling or the man who did my raw machine work.
I only did this under strong protests, but it was for a relative and he wouldn't take NO as an answer.
A good gunsmith with a milling machine can do this. A machine shop "might" be able to do it, but since most machine shops don't have an FFL for gunsmithing it's not legal.
In any case, this is a hard job to do, and an easy job to botch. Botched and you send the gun in to Ruger for a new barrel.
