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
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