I have trouble believing the throat is small than groove diameter. Not impossible I guess but unlikely I'd think.
Just my opinion.
Regardless, I'd be trying a boolit of at least 0.001" over groove diameter and if it will chamber use that. At worst it will be resized by small "throat" but will be the best fit you can get. If it won't chamber and the throat is that small I suspect you will get poor accuracy and bad leading from gas cutting.
Again, just my opinion.
Try the largest boolit that will chamber then let us know how it goes.
Merry Christmas and Happy New Year!
Longbow