Basics: The Leade Angle
by Germán A. Salazar
The letter below from our friend Lance poses a question that many shooters have: "Just what is the leade angle?" We'll cover the basics of the question as well as some additional information to help understand this rarely seen and inadequately discussed element of a rifle's performance. - GAS -
There is one technical term, I often read in the discussion of rifle chamber dimensions. I have an idea of what it is but am not real solid on its function and its importance to performance. The term I am referring to is: leade angle which I always see expressed in degrees and minutes of angle. Here's a quote from your article on reloading for the 6XC:
"I ordered a reamer from PTG with a 1°30" leade angle and a 0.271" neck diameter (see Figure 2). The standard Henriksen reamer is made with a 0°45" leade angle and a 0.276" neck diameter. While the shallower leade angle seems to be gaining some popularity in various Highpower oriented reamers, I have yet to see a cogent explanation of why it might be better."
I have been reloading for about 10 years now but have never heard (sought out) an explanation of leade angle. I guess I could always Google it but you have always provided cogent answers.
You always have good questions!
First let's nail down some terms: as you've noted the leade angle is expressed in degrees and minutes. Second, let's bear in mind that the leade angle is a "per side" measure, which may become more apparent in a moment. Finally, despite frequent misspellings, it is properly called the leade with an "e" at the end (you got it right, others sometimes don't).
Simply stated, the leade angle is the angle cut by the reamer on the very end of the lands. This allows the bullet to begin its engraving into the lands on a relatively gentle angle rather than against an abrupt 90 degree "wall".
The pictures below, taken at high magnification through a borescope show the leade angle cut into the lands of a barrel by the reamer. These pictures are from an earlier article by Mario Favaron on throating reamers, a whole different topic, but they illustrate the leade angle quite nicely.
The first picture (above) shows the end of the leade angle as the land transitions to full height. On most cut-rifled barrels, by the way, the lands are about 0.004" high, less on a button-rifled barrel.
The second picture shows the beginning of the leade angle, a little further back in the chamber, closer to the neck. At the far left of the picture you see a sharp transition, marked by the first vertical white line - that's where the case neck ends. Then there is an area where all the rifling has been cut away by the reamer, that's the leade. Then the throat begins, that is the part where the leade angle cut into the rifling gives the bullet has a smooth transition into the rifling so that the engraving force is applied over a period of time, not abruptly, which would cause a severe pressure spike.
Let's have a look at a reamer print to get some harder numbers on the subject. By some strange coincidence, I happen to have a .30-06 chamber print handy, so we'll use that. As always, you enlarge any of these images by clicking on them.
As you look at the chamber print, let's identify a few elements. As you look at the neck, you'll see a box labeled "45" just below and indicating an angle at the end of the neck. That is the aptly named "mean little shoulder" which marks the end of the neck and is what your brass will run into if you don't keep it trimmed. It makes a very effective (if unwanted) crimp and pressure will really skyrocket if the brass gets in there.
Next in line is the leade which you can see is 0.086" long; there is no rifling left here after the reamer has made its cut. Also notice that the diameter here is 0.3085", this is another element of accuracy; a large leade diameter does nothing good for accuracy, but too tight can cause pressure problems as well as difficult chambering.
Now look at the dimension box furthest to the right on the print, it says 1 - 30, shorthand for 1 degree, 30 minutes, or 1.5 degrees. That is the leade angle and the throat is the length from where the leade angle begins to where it ends and the lands assume full height. The print can be a little confusing to look at, since it almost appears that the leade angle extends past the point marked as the end of the throat, but in reality, it doesn't (see the first picture above).
This last picture needs careful attention. It shows a case with a cast of the inside of the barrel stuck into the case mouth. Because you're looking at bore cast, the lands and grooves look opposite of what they really are. The lands look like grooves and vice-versa. Understanding that, look at the lands as marked and you'll see that they taper into full depth - that taper is the leade angle. Now just reverse things in your mind and you'll have a perfect image.
Over the years there has been a trend towards shallower leade angles. The .30-06 was often cut with a 3 degree leade angle but the 1.5 degree seems to shoot more accurately and is more commonly encountered today. Looking at my SAAMI manual, it calls for a leade angle of 1 degree 22 minutes for the .30-06. I don't know if that's a change in the modern era or if it's always been specified that way, but I tend to think it's a change. There are leade angles for different cartridges from 6 degrees per side (25-35 Win.) to plenty in the 1.5 degree area. As a general rule, cartridges standardized by Remington tend to have 3 degree leade angles and those standardized by Winchester tend to have 1.5 degree leade angles (although there are exceptions to both).
My .308 chambers are cut with a reamer with a 0.5 degree leade angle. I can't say for sure that they shoot any better than the old chamber that had a 1.5 degree leade angle. When I specified the 6XC reamer you mentioned above, I hadn't yet tried this shallower angle .308 reamer so I stuck to the "standard" 1.5 degree leade angle for the 6XC.
The real question in my mind is, how close are these dimensions after a few hundred or a few thousand rounds have been fired and some erosion is taking place? I can see in the borescope that the leade angle is still there, but I certainly can't tell if it's at the original angle and I also don't know if the shallower angles wear more quickly than the 1.5 degree angle.
The ultimate effect of the leade angle on accuracy and barrel accuracy life is beyond my ability to test and quantify. I probably won't change the .308 reamer because the rifles are shooting well, but I also don't think I'm in any hurry to change the other reamers to an angle shallower than 1.5 degrees - nor steeper. For reasons of simplicity, standardization and drawing on the useful collective experience of the shooting community, I'll stick to 1.5 degrees as my Goldilocks "just right" leade angle.