Re: Drilling port holes ONLY in the grooves. How?
This is pretty easy on the surface, but requires a little faith in math and your skills to execute.
First: Twist rate
Second Land grooves
Say its a 9 twist and has 4 grooves
360/4='s a groove every 90*.
If your twist rate is 1 rotation per 9" of travel then your rotating 40* per inch. We'll call 12 o clock "zero degrees".
If it were me doing this:
I'd make a lead lap and shove it down the bore till a portion was sticking out the crown. Then I'd use an centering microscope to find the root of the groove.
Since this kind of job would almost certainly be done with a 4th axis on a CNC although a dividing head could easily get the job done too (if time is of no importance to you)
You center your A axis so your spindle is directly overhead the lead slug. You datum off the muzzle. Now focus in on the groove feature. Rotate one direction till you catch the edge. Zero out in A. Rotate the opposite till you cath the other edge. Note the degrees of rotation, divide by two, and you now have the center position.
From here you just start rotating the proper "pitch" based on your X location change. Say it was 2.5" from the muzzle. To keep it timed with the groove you'd rotate 100 degrees.
Drill your first batch of holes, then return to zero. Incrementally rotate 90* for the next groove and repeat for the remaining 2 grooves.
For a six groove just divide 360/6 to get your index, so and so forth.
Note that the more grooves you have the small the drill you'd have to use to stay out of the lands.
To do this "really right" you should angle your drill by around 2.5 degrees to mitigate the "cheese grater effect" the interruption will have in the bore. This complicates things a little as your setup just got a whole lot more kinky.
Can still be done though.
My guess is that anyone doing this in any sort of quantity has mandrels made up that will fixture inside the muzzle a bit. This would greatly simplify things in a production status.
Hope this helped.
C.
PS:
In all my years of building NM AR's and whatnot I never paid attention to hole indexing. TILL a conversation with a very talented barrel maker/AR builder took place.
In all his years of building them he never paid attention either. Then when guns started coming back for new barrels he got curious and started looking. When he got "lucky" and nailed the groove he noticed these were the guns that shot exceptionally well.
So he/I do all of them that way now. Keep in mind the 2.5* relief on the hole is another critical aspect.
If I'm a bullet traveling down the bore and a small sliver of my jacket is peeled off it changes my rotational center of gravity. It has to.
If I'm a 80 grain bullet in a 8 twist barrel traveling at 2800fps I'm rotating at 252,000RPM. That's ALOT of RPM's.
Just using the common sense were born with try to envision the centrifugal force at work here. To grossly/poorly offer an anology, go tape some fishing weights on a football and try to throw a nice pass. It jumpropes right?
Same thing here.
Gnite.
