Measuring leade angle

Intheshop

Well-Known Member
For you folks that pound cast,or cerrosafe chamber cast....

How do you measure the leade angle?

And no,this isn't a loaded question. I can understand measuring it on a reamer.... just trying to wrap my pea brain around how to go about measuring a chamber cast.....
 

Ian

Well-Known Member
That's actually a very good question I haven't seen asked before, and am glad you brought it up.

What I generally do is use a mic with cone anvils, or reasonable facsimile and plot the slug measurements at increments on graph paper. Difficult to if the lands/grooves are an odd number, in that case (one rifle I have with 5R) I used a utility knife to peel away lead between land engraves and then measure the engrave flats in increments with normal mic until I reached the parallel bore diameter. Then a little Arctangent calc.
 

DougGuy

New Member
Lay the casting down, hold a straightedge in one of the grooves made by the leade, take a good clear photo of it , blow the image up, take a piece of wire, bend it so it matches both the straight side and the straight edge. Google protractor or compass until you get an image with degrees on it you can see, hold the wire up to the screen and measure the angle, or print the compass out and lay the wire on the image until you can read the degrees.

When I needed a crowning tool, I used video of a 1911 firing in slow motion. When the boolit exited the muzzle, and the gas cloud followed it, I stopped the video and used a piece of wire, I bent the wire until both sides were parallel with the edges of the gas cloud, measured the degrees and divided it in half, came up with 22.5 degrees on each side, for a 45 degree included angle. I found a carbide NeWay valve seat cutter for a Yamaha engine was 20 degrees. I ordered this cutter, made a sleeve to fit over the Manson mandrels that index off the lands for concentricity, and I was in business.

I chose 20 degrees for the angle to use for the crown, if you look at that gas cloud, having the crown parallel to the exiting gas provides the least amount of deflection of the boolit from the pressures of the gas, there is no surface to introduce turbulence, there is no surface for the gases to push against that could possibly upset the boolit, and I can clean up most factory crowns with this without losing barrel length. It just made sense to me that both the angle of the crown and the angle of the gas cloud needed to be as close as possible to the same.

Now I am telling my trade secrets, but I included this info to illustrate how to use images and a simple piece of wire bent in the middle to transfer angles from a computer screen or photograph to a compass, so that they can be measured.
 
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popper

Well-Known Member
Other than not making a step transition from throat to bore, does it make any difference in cast bullet design? It's a couple thou. max. dia. transition. Does affect seating depth for a no-taper drive band.
 

Ian

Well-Known Member
One of the complications that will arise when measuring throats is that when a throat wears....the angle turns into a parabolic arc. Does it really make any difference to cast bullet design? You bet it does.
 

Intheshop

Well-Known Member
Thanks for the brain jog guys! Got a cpl protractors out,the one on the right gets used a lot... early 30's Brown & Sharpe,fore runner to the 493#(both of these were purchased new,and passed on to me by an uncle).8374Headed out to a bow shoot this weekend so will be doing some head scratching and reading.
 

Ian

Well-Known Member
Heeeeyyyyyy, speaking of brain jogs, just clamp it in your mill vise level and with one if the engraves at 12:00, put a test indicator in the chuck and tram along the whole throat.
 

Intheshop

Well-Known Member
That's gonna be my vote Ian for the quickest setup..... which "should" equate to higher measurement accuracy because of simplicity. In a,reduce the problem to the lowest number of obstacles.
 
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Intheshop

Well-Known Member
Here's a glowing difference in over the counter, JB's and CB's.

We get to vary the diameter and radius of the ogive waaay more quick/efficient than either changing barrels (to change throat angle) or making new swage dies. Although you could argue that swaging cast takes resources.... but there's a big difference in a set of JB swage dies and bumping noses. Which is one of the reasons for the inquiry. Can I bump a specific bullet for summer vs winter which will increase bite or traction as air density increases.
 

Intheshop

Well-Known Member
Look at "industry" reamer prints at the leade angles. A 270 is less than 1* .... sumthin like 47 minutes. Then look at a 222.... which is over 2*.
 

Ian

Well-Known Member
Years back I thought the worst (out of ignorance/inexperience) when looking at the pound cast I'd just pulled out of my .270, that throat looked like it must have been an inch long! Best part was more than 2/3 of it had visible land marks. Turns out that's normal. Soooo, I set off to shoot cast in it and got the RCBS 150 second-hand for lack of options at the time. You all can guess how well that worked out, even at 1300 fps.
 

fiver

Well-Known Member
I use a gauge quite similar to the one on the right in ITS's pic.
it's actually for setting driveline angles and is a bit smaller.