How did they do that?

James K · March 15, 2007, 10:22 PM

I may have posted this before, but I don't think I did so here. So, for those who may be interested...

Most collectors and gunsmiths involved with the Model 1903, Model 1903A3, Model 1917 and British Pattern 1914 have seen the small lines on the receivers and barrels of those guns that line up with each other. Many people call them "draw lines" and assume they were put there to ensure that the barrel was installed properly so the sights would line up. That is true, but only part of the story.

In that type of barrel, the manufacturer has a problem, and it is not with the sights. Rather, it is the fact that the extractor cut has to be in a specific place when the barrel is installed, and there is no feasible way to make that cut once the barrel is installed. So, both the extractor cut and the front sight spline (or groove) are cut while the barrel is being made. But how? How can the manufacturer make those critical cuts before the barrel is installed on the receiver? How can they be made to line up later?

The answer is both simple and complex. When the receiver was made, its receiver ring threads were started at a specific place, then a mark was made in a fixed position on the receiver ring ("3 o'clock" on the Model 1903). After the barrel is drilled, rifled and straightened, the tenon threads are cut. Then a special gauge, a bit like a large nut with handles, is tightened down on the barrel threads. This gauge also has in it a square hole into which a small chisel is set. When the gauge is tightened down all the way, the chisel is struck, making a line on the barrel shoulder. Those two lines are called "draw lines." Barring some real problem, the barrel can be installed and will be drawn up with the proper tightness when the draw lines match.

But how does this help getting the extractor cut and the front sight spline in the right place? Easy. The barrel draw line is made before the machining is done in those areas. The barrel is set up in the machines so the draw line exactly matches a fixed indicator on the machine; this orients the barrel while the cuts are made. In the Model 1903, the sight spline is cut 90 degrees off the draw line, and the extractor cut is 180 degrees off. (This is not a rigid rule; the draw lines can be at the bottom, top, side or any other point on the barrel and receiver as long as everything agrees.)

With those operations completed, and the barrel finished, it can be installed with confidence that if the draw lines are lined up, the extractor cut and the front sight will line up, and the torque will be in tolerances. All right, so what about the rear sight base (fixed base)? It, too, is installed using the barrel draw line as a reference point, so both sights will line up.

The advantage of the system is that not only can the draw lines in assembly ensure that everything is properly aligned, but that spare barrels can be made that will install with a minimum of fitting, an important consideration in a military rifle.

Now having talked about the barrel draw line on the Model 1903, I can hear folks asking. "Where is it?" The answer is that when the fixed base was installed, it was covered up, and the small shelf on the side of the fixed base takes its place during installation. It can, however, be seen on the Model 1903A3.

I think I hear a chorus of voices informing me that Mauser rifles do not have draw lines, and that is true. The Mauser design does not require an extractor cut in the barrel. In the Mauser 1898, the extractor cut is in the fixed ring in the receiver; in the previous models there was no cut at all. So how did they get the sights to line up? Not having to worry about inside cuts, they first installed the barrel, then used the receiver as a reference point to install the sight bases. The Mauser rear sight base is soldered on, and the front sight base is attached with a screw, so there is nothing that has to be cut before the barrel is installed. It is a simpler system in terms of factory assembly, but more difficult when replacing a barrel, since replacement barrels have to be issued without sights.

Jim

Clemson · March 16, 2007, 09:12 AM

Wow! Thanks for that explanation, Jim. Do you have any idea why the Springfield arsenal chose to use square threads on the barrels?

By the way, there are a few Mausers that do have the extractor cuts -- the Yugo Model 48 and the 24/47's. I have no idea what sequence they used for machining the cuts. From the machine marks, the extractor grooves were cut with an end mill. When I rebarrel one, I chamber the barrel in the lathe, then finish ream it installed on the receiver, then mark the location of the extractor cut, pull the barrel, and cut the groove. I doubt that the military armorers want to go to all that trouble!

Clemson

Harry Bonar · March 16, 2007, 06:59 PM

Hey! Jim!
Great explanation!!!
So many "gunsmiths" don't know what you just explained.
Harry B.

James K · March 16, 2007, 07:48 PM

Thanks, Harry. I thought the folks might be interested.

Hi, Clemson,

I have no idea why Springfield used square barrel threads, but they did so from the Trapdoor right through the M14; I suppose they thought they would give a better hold. They weren't the only ones, the Savage 99 also used square threads, and they were common in machines in the late 19th and early 20th century.

You are correct. Those two actions are different from the regular 98 Mauser action in some other ways besides length. They do have a shallow extractor cut. When rebarrelling a M1903 or M1917 with a sporter barrel, a gunsmith has to do just what you do on the M48, but it is a PITA. The nice thing about draw lines is that not only can a replacement barrel be installed easily, but the sights will line up.

Another item of possible interest is that on British Lee action rifles, the Knox form (originally "Nock's form" - the flat spot on the top of the chamber area) serves the same purpose as the draw line on an M1903 barrel, as well as providing purchase for the barrel wrench.* The same techinque was used to mark the barrel, but the chisel mark was obliterated when the Knox form was cut and from that point, the Knox form served to align the barrel in installation and replacement.

(No, I didn't goof when I said barrel wrench; in factories and military depots, the practice is to fix the receiver and screw the barrel in. Most gunsmiths do the reverse.)

Jim

williamd · March 28, 2007, 03:09 PM

Thanx, Jim. That was worth a cut and paste for my records.

**Unclenick** · March 28, 2007, 08:20 PM

Jim,

Interesting information. I don't have any pre-Garand military barrels not in a receiver, so I can't look at the thread without pulling one. Are they truly square threads, or are they a low-profile Acme thread, like the Garand (29 degree included angle taper of the sides)?

Normally, square and acme threads are used for traversal mechnisms, of course; lead screws of one kind or another. They have less total frictional surface to rub than a triangular thread form, and apply longitudinal pressure more efficiently (sine of the angle; more torque is thus applied in line with the barrel, and less presses radially between barrel and receiver). They are alsp stronger at the thread peaks, just because they are thicker there. Acme threads are considered the strongest overall form, having no sharp corners, and are easier to machine than a true square thread. That may be why Garand chose them over the Whitworth thread form used in the Mausers?

I have a book of Garand drawings, and how they timed the contouring is complex. They apparently cut the shoulder, then milled in the threads so that the forward mid-side of the last turn is 0.540" +/- 0.001" from the shoulder at 6:00. Since the drawings call for the gas port to be drilled at 6:00 and right inbetween two lands, the rifling had to be timed relative to the start of the thread.

Nick

James K · March 30, 2007, 11:28 AM

Are the threads square? I have always called them square, but they might be as you describe; they look like the M1 threads, and I have little doubt they were cut on the same tooling. I don't have a loose barrel to check right now.

The M1 rifle, being made at a later time, used the technique you describe to get barrel indexing without using draw lines. Final barrel alignment was done in a fixture which used a pointer attached to the top gas cylinder spline cut.

Jim

March 15, 2007, 10:22 PM	#1
James K Member In Memoriam Join Date: March 17, 1999 Posts: 24,383	How did they do that? I may have posted this before, but I don't think I did so here. So, for those who may be interested... Most collectors and gunsmiths involved with the Model 1903, Model 1903A3, Model 1917 and British Pattern 1914 have seen the small lines on the receivers and barrels of those guns that line up with each other. Many people call them "draw lines" and assume they were put there to ensure that the barrel was installed properly so the sights would line up. That is true, but only part of the story. In that type of barrel, the manufacturer has a problem, and it is not with the sights. Rather, it is the fact that the extractor cut has to be in a specific place when the barrel is installed, and there is no feasible way to make that cut once the barrel is installed. So, both the extractor cut and the front sight spline (or groove) are cut while the barrel is being made. But how? How can the manufacturer make those critical cuts before the barrel is installed on the receiver? How can they be made to line up later? The answer is both simple and complex. When the receiver was made, its receiver ring threads were started at a specific place, then a mark was made in a fixed position on the receiver ring ("3 o'clock" on the Model 1903). After the barrel is drilled, rifled and straightened, the tenon threads are cut. Then a special gauge, a bit like a large nut with handles, is tightened down on the barrel threads. This gauge also has in it a square hole into which a small chisel is set. When the gauge is tightened down all the way, the chisel is struck, making a line on the barrel shoulder. Those two lines are called "draw lines." Barring some real problem, the barrel can be installed and will be drawn up with the proper tightness when the draw lines match. But how does this help getting the extractor cut and the front sight spline in the right place? Easy. The barrel draw line is made before the machining is done in those areas. The barrel is set up in the machines so the draw line exactly matches a fixed indicator on the machine; this orients the barrel while the cuts are made. In the Model 1903, the sight spline is cut 90 degrees off the draw line, and the extractor cut is 180 degrees off. (This is not a rigid rule; the draw lines can be at the bottom, top, side or any other point on the barrel and receiver as long as everything agrees.) With those operations completed, and the barrel finished, it can be installed with confidence that if the draw lines are lined up, the extractor cut and the front sight will line up, and the torque will be in tolerances. All right, so what about the rear sight base (fixed base)? It, too, is installed using the barrel draw line as a reference point, so both sights will line up. The advantage of the system is that not only can the draw lines in assembly ensure that everything is properly aligned, but that spare barrels can be made that will install with a minimum of fitting, an important consideration in a military rifle. Now having talked about the barrel draw line on the Model 1903, I can hear folks asking. "Where is it?" The answer is that when the fixed base was installed, it was covered up, and the small shelf on the side of the fixed base takes its place during installation. It can, however, be seen on the Model 1903A3. I think I hear a chorus of voices informing me that Mauser rifles do not have draw lines, and that is true. The Mauser design does not require an extractor cut in the barrel. In the Mauser 1898, the extractor cut is in the fixed ring in the receiver; in the previous models there was no cut at all. So how did they get the sights to line up? Not having to worry about inside cuts, they first installed the barrel, then used the receiver as a reference point to install the sight bases. The Mauser rear sight base is soldered on, and the front sight base is attached with a screw, so there is nothing that has to be cut before the barrel is installed. It is a simpler system in terms of factory assembly, but more difficult when replacing a barrel, since replacement barrels have to be issued without sights. Jim

March 16, 2007, 09:12 AM	#2
Clemson Senior Member Join Date: March 9, 2001 Location: Greenwood, SC Posts: 875	Wow! Thanks for that explanation, Jim. Do you have any idea why the Springfield arsenal chose to use square threads on the barrels? By the way, there are a few Mausers that do have the extractor cuts -- the Yugo Model 48 and the 24/47's. I have no idea what sequence they used for machining the cuts. From the machine marks, the extractor grooves were cut with an end mill. When I rebarrel one, I chamber the barrel in the lathe, then finish ream it installed on the receiver, then mark the location of the extractor cut, pull the barrel, and cut the groove. I doubt that the military armorers want to go to all that trouble! Clemson __________________ NRA Endowment Member, NRA Certified Instructor CWP Holder US Army veteran Gunsmith www.boltandbarrel.com

March 16, 2007, 06:59 PM	#3
Harry Bonar Senior Member Join Date: December 5, 2004 Location: In the Vincent, Ohio general area. Posts: 1,804	thread timing Hey! Jim! Great explanation!!! So many "gunsmiths" don't know what you just explained. Harry B.

March 28, 2007, 03:09 PM	#5
williamd Senior Member Join Date: March 13, 2007 Location: SoCal Posts: 801	Thanx, Jim. That was worth a cut and paste for my records. __________________ "Outlaw guns and only outlaws will have guns!" Unfortunately, we may be moving in that direction. NRA Benefactor, Conservative!, VN '64-'65. Never sell a gun or a car ... and retire rich!

March 28, 2007, 08:20 PM	#6
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,093	Jim, Interesting information. I don't have any pre-Garand military barrels not in a receiver, so I can't look at the thread without pulling one. Are they truly square threads, or are they a low-profile Acme thread, like the Garand (29 degree included angle taper of the sides)? Normally, square and acme threads are used for traversal mechnisms, of course; lead screws of one kind or another. They have less total frictional surface to rub than a triangular thread form, and apply longitudinal pressure more efficiently (sine of the angle; more torque is thus applied in line with the barrel, and less presses radially between barrel and receiver). They are alsp stronger at the thread peaks, just because they are thicker there. Acme threads are considered the strongest overall form, having no sharp corners, and are easier to machine than a true square thread. That may be why Garand chose them over the Whitworth thread form used in the Mausers? I have a book of Garand drawings, and how they timed the contouring is complex. They apparently cut the shoulder, then milled in the threads so that the forward mid-side of the last turn is 0.540" +/- 0.001" from the shoulder at 6:00. Since the drawings call for the gas port to be drilled at 6:00 and right inbetween two lands, the rifling had to be timed relative to the start of the thread. Nick __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle Last edited by Unclenick; March 28, 2007 at 10:28 PM.

March 16, 2007, 07:48 PM	#4
James K Member In Memoriam Join Date: March 17, 1999 Posts: 24,383	Thanks, Harry. I thought the folks might be interested. Hi, Clemson, I have no idea why Springfield used square barrel threads, but they did so from the Trapdoor right through the M14; I suppose they thought they would give a better hold. They weren't the only ones, the Savage 99 also used square threads, and they were common in machines in the late 19th and early 20th century. You are correct. Those two actions are different from the regular 98 Mauser action in some other ways besides length. They do have a shallow extractor cut. When rebarrelling a M1903 or M1917 with a sporter barrel, a gunsmith has to do just what you do on the M48, but it is a PITA. The nice thing about draw lines is that not only can a replacement barrel be installed easily, but the sights will line up. Another item of possible interest is that on British Lee action rifles, the Knox form (originally "Nock's form" - the flat spot on the top of the chamber area) serves the same purpose as the draw line on an M1903 barrel, as well as providing purchase for the barrel wrench.* The same techinque was used to mark the barrel, but the chisel mark was obliterated when the Knox form was cut and from that point, the Knox form served to align the barrel in installation and replacement. (No, I didn't goof when I said barrel wrench; in factories and military depots, the practice is to fix the receiver and screw the barrel in. Most gunsmiths do the reverse.) Jim

March 30, 2007, 11:28 AM	#7
James K Member In Memoriam Join Date: March 17, 1999 Posts: 24,383	Are the threads square? I have always called them square, but they might be as you describe; they look like the M1 threads, and I have little doubt they were cut on the same tooling. I don't have a loose barrel to check right now. The M1 rifle, being made at a later time, used the technique you describe to get barrel indexing without using draw lines. Final barrel alignment was done in a fixture which used a pointer attached to the top gas cylinder spline cut. Jim