Thread time and twist

K

kopcicle

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Jun 19, 2023
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Location
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Having seen my share of shot out mouse guns during a penance in the armory many decades ago I noticed that the gas port was drilled wherever it landed. In the groove, on a land, half and half, it didn't seem to follow any pattern.

Fast forward to recent builds and tune-ups and I find that a fair portion of the quality barrels I have seen are drilled in the groove.

So here is the question. How is the barrel extension timed to the barrel such that the gas port finds the groove?
I have several ways I can "time" threads but each has its own problems and inaccuracies.
I'm kind of reduced to the build a dummy and measure off that process.

The whole point of this was I wanted to experiment with a few uncommon bores and cartridges but that is now moot.

Our Governor, Attorney General and Legislature feel that the public safety is more important than following their oath of office, honoring the constitution of the state of Washington, the constitution of the Divided States of America, and the word of law as given by the Supreme Court of the Divided States of America.

I still want to know how it's done ;)
 
So
16 TPI is
.0625 per thread
0.03125 for half
0.0017361" per 10 degrees of rotation
Then 1" 16 un 3a thread spec
then

I have a variety of curious blanks here and I want to thread, drill, chamber them myself.
Sure enough the 22, 25, and 277 bores could be considered long range.
The .358 bore, not so much. More like reach out and have it fall out of the sky.

No matter, I still need a way to do this when I'm starting with the gas port and finishing with the extension thread.
 
Chamber the barrel, mark barrel where the pin lines up, remove barrel extension.

Make a collar to bottom out on the tenon shoulder. Preferably, the end would be solid with just a hole drilled for your rigid shaft bore scope shaft to pass through.

Calculate where your port needs to be relative to tenon shoulder.

Make a stop collar for the borescope shaft.

Taking into consideration the length of the tenon collar, lock the borescope collar to the shaft so when you slide it into the tenon collar hole, the shaft collar will bottom out to your calculated distance from the tenon shoulder to the gas port centered to the LED view.

Mark the borescope shaft where the LED is orientated.

Now, rotate the chuck till you see the bottom, actually the top of the groove.

Mark the barrel where angular location is from previous step.

Screw the barrel extension on, make small cuts on the tenon shoulder, if need be, to align the pin to the barrel mark.

Final finish headspace, however you do it.

Drill gas port timed to pin.
 
Last edited:
I figured it would/could be as follows..................
1) chamber
2) screw on extension and headspace
3) mark the circumference of the barrel where gas port will be
4) (as mentioned before) borescope where you want the port to be drilled (lands or grooves)
5) drill hole for index pin in extension accordingly

Sounds easy, but I might not have a clue?

Note about port size..................1st drill undersize hole, then step up to desired size as not to create burs in the bore.

Again, I might not have a clue?
 
ARlife4me said:
I figured it would/could be as follows..................
1) chamber
2) screw on extension and headspace
3) mark the circumference of the barrel where gas port will be
4) (as mentioned before) borescope where you want the port to be drilled (lands or grooves)
5) drill hole for index pin in extension accordingly

Sounds easy, but I might not have a clue?

Note about port size..................1st drill undersize hole, then step up to desired size as not to create burs in the bore.

Again, I might not have a clue?
Click to expand...
Drill... ream to size. I also have a deburring tool that goes through 0.096 hole. Once tip gets past the hole into the bore, two spring loaded blades pop out. Pull the the quill up with spindle running, it deburrs the inside hole. Or just fire rounds through the rifle, small burrs if any, cleans up

Step 5.. just drilling a new hole on the hardened extension, might work with carbide bit. Try it, let us know if you have any surprises on anything else.
 
Last edited:
Somewhat of a subject matter?
Any idea if it's better to have port on grooves or lands? Seems like having it in the groove would have less fouling in the port?
 
We never really made any extra effort to do. I know a couple of custom finished barrel gunsmiths will charge you extra if you want the port timed.

Anecdotal, I have 2 barrels in the shop I spun up for a friend. By accident the ports landed in the groove. Both were melonited and they lasted over 10,500 precision lives and probably would have lasted longer had I convinced coerced my buddy in swapping out. In the one with 10741, shooter posted 198-8 at 6 before I pulled the barrel.

Another barrel returned from the JR program, had less 3000 rounds when it was pulled out due to severe drop in Xs at 600, and shots not on call. I have not inspected this barrel.

During the M14 days, this subject was debated extensively. Hole in groove provided better precision, only to be debunked, then argued again.

I do not have any preference for my own weapons.
 
Last edited:
ARlife4me said:
Only fouling was mentioned, not accuracy on my post.
Click to expand...
Sorry, I did not address your question. That is one that never crossed mind as I do not borescope my own barrels. I shoot them till they quit, mostly meaning lower X count at 6 or off calls appearing more frequently. I also test my long line ammo at 200 on the reduced 600 at 2. If the system is incapable of shooting 50% + Xs from the bench, time to think about the next barrel.
 
bamban said:
Sorry, I did not address your question. That is one that never crossed mind as I do not borescope my own barrels. I shoot them till they quit, mostly meaning lower X count at 6 or off calls appearing more frequently. I also test my long line ammo at 200 on the reduced 600 at 2. If the system is incapable of shooting 50% + Xs from the bench, time to think about the next barrel.
Click to expand...
Just re-asking it was all, so no worries.
 
Tool:
1 3/8 medium nickel steel (gun steel to approximate the barrel steel)
Bore one end 1.0015 x 1.5"
Duplicate the 1" diameter on the other end including the gullet and shoulder. Place this shoulder on a close order of 2" away from the end with the one inch bore.
Thread this 1" outside on a close fit to the barrel extension.

Clamp the snot out of the OD of the "tool" in too big a vice.
Use acceptable barrel extension tool to first "fist" tighten the barrel extension on the tool. Witness mark across the tool and barrel extension. Torque to acceptable value and mark across the extension to barrel parting line again.

Okay we've now established pre and post torque position in degrees for the barrel extension in a similar steel. We've also transferred the shoulder location 2" rearward of the barrel shoulder.

Through drill the tool for setscrews to clamp onto the 1" unthreaded barrel. Place copper or lead button under the setscrew.
Alternatively slit and clamp screw.

Place the gas port at TDC in the lathe (12:00 if you wish) and rotate the tool until flush with the barrel shoulder and TDC <less> the tightening torque in degrees. Mark the tool across the tool to barrel parting line.
Remove tool from barrel, place in vice, remove barrel extension and replace the tool back on the barrel to witness marks.

Now for the fiddly bit.
Engage the half nuts and take all the lash out of the lead screw in relation to the spindle while matching the thread cutting tool to the threads on the tool. The easiest way to do this is have a hand wheel on the off end of the lead screw to hold radial tension while the cutting tool is advanced into the tool's threads. With the spindle locked at TDC the jaws are "softened" until the barrel responds to gentle taps to get the cutting aligned, ie; timed to the threads on the tool. Once the cutting tool has found the root and by way of a dye check in only making contact with the breech, chuck, slope of the threads, remove the "tool" .

If all went well the indicated threads from the tool will be transferred to the barrel and the barrel extension will torque to TDC.

Some cautions

I'm going to test this several times in junk alloy then mild then 4150 to determine just how much my lathe flexes under load.
Chances are that no matter how much I hone 8% cobalt HSS or carbide insert, I'm going to have to understand machine flex and the effect of spring passes on the threads as the last few cuts are made.
Don't open the half nuts. I suppose it had to be said :)
Check the gibs.
Do not let the barrel turn in the chuck.
29 1/2 degree compound offset. Do I really have to explain?
(^^^ I will if I have to, it isn't a problem)
Make sure the cross dial is zeroed to begin with.
or
Just run the lathe in reverse after each thread pass.
>>> Come on now, anyone left that is still listening to this gibberish knows basic threading operations.
make sure the compound is zeroed to end with.


I suppose I owe a bit of an explanation for all the fires I'm jumping through.
I can drill the gas port with some accuracy and repeatability.
I don't have the bed length to chamber these tubes. :-(
I'm receiving them chambered because of that and my benefactor has the reamers, the lathe, and the expertise. He just has no interest in timing the threads to a gas port drilled in the groove.

In the end it's a bit of a puzzle where two of the pieces can't be moved.
The gas port
and
The timing of the threads in the barrel extension.

Okay, had enough?
 

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