It's easy. It's even easier to screw it up.
First, use a narrow permanent marker and with a scale, extend the flute onto the cylinder for visual reference. Time that mark to one of the jaws on your chuck. Pay attention and do your best to accurately index the position. Note the number of flutes on your barrel. (6,7,8, etc)
Next, thread your tennon as you would. Leave the breech face features alone for the moment.
Now screw your action onto the barrel, get it tight enough by hand so that you can gauge where you'll end up once torqued to final value. Next, back off the receiver until it clocks with the indexing jaw on your chuck. If you are using a chuck that has a number of jaws divisible by the number of flutes (evenly) then back the receiver off to next closest jaw. Measure the gap between the lug and the shoulder.
A little math:
Lets say you have six flutes. On a 16 pitch thread that pencils out to .0104" of linear (z axis) travel between flutes as you "climb" the threads. (1 / 16 ='s .0625. .0625 / 6 ='s .0104")
So, you back it off till it times and find that your gap is .006". .006" subtracted from .0104" is .0044".
.0044" is how much you want to peel from the shoulder to get the flute timed.
NOW understand if you do this, you will screw it up everytime. Threads crush. Parts compress under high load more than they do when your hand tightening. There is a fudge factor here. It varies from receiver to receiver and barrel to barrel.
I generally leave a few .001's as a safety margin. It is far easier to solve an underclocked flute than it is one that's gone past. Sanding a .001" off a recoil lug is pretty easy with a good piece of emery paper and a flat surface. (surface plate being the optimum tool, but a slab of granite tile will work too)
Headspace concerns. Again, easier to squeak a thou' or two afterwards than to pull it past and have to take ten or more to get everything back in sinc'.
The best answer:
Stop ordering fluted barrels. Buy them as a regular blank, fit and chamber, index the 12 o' clock position, then send them out for fluting. (what we do is use a small spring loaded center punch through the front base hole. It puts a tiny nick in the thread crest that we can use to index in the rotary axis on the cnc later)
This is how we do it here at LRI. We never buy fluted barrels for customers. The fluting operation is one of the last things we do (in house) after all the other work is completed. It solves the entire problem.
Good luck and hoped this helped. This is the exact process I came up with prior to deciding to bring all the fluting services in house. The biggest voodoo is sorting out the crush your going to have. There's no real magic formula for it. least not that I've found.
Fun!
C.
