Measuring case taper - in/in vs degrees

[H3R3T1K]

Reading the article on the 5.8x42mm Chinese case in Cartridges of the World I came across a reference to case taper stated in degrees (one). Is it possible to translate one degree of case taper to the more commonly used in/in? Is this a metric thing?

 

[rsrocket1]

If I recall my trig correctly, the taper is related to the tangent of the angle. So a 1 degree taper means 0.01745" change for every 1". If you translate that into diameter, 1" up from the initial point of measurement should be about 35 mils smaller in diameter (2 x the tangent). Check that out with a set of calipers to see if it's correct or way off.

Hope that helps.

 

[H3R3T1K]

We didn't do those calculations back in highschool in Germany I think or I'd know about it I'd like to use an example in order for me to understand the principle. The 7.62x39 has a 11.35mm base diameter, the length of the case body is 27.3mm from base to shoulder. How do you calculate the shoulder diameter if the case was to have a one degree taper?

7.62x39 dimensions: http://upload.wikimedia.org/wikipedia/commons/7/75/7.62x39mm.jpg

 

[rsrocket1]

According to the dimensions, the case tapers from 11.35mm to 10.07mm over a span of 30.5-3.20 = 27.3mm. We want to look at only one side of the case so we take the change in radius instead of the diameter.

(11.35-10.07)= 1.28 then divide by two = 0.64mm

0.64mm/27.3mm = 0.02344 <- this is the tangent of the case taper angle.

To get the angle, you need to take the arc-tangent of 0.02344 which calculates out to about 1.34 degrees.

If it had a 1 degree taper, the tangent of 1 degree is 0.017455 so, 27.3mm x 0.017455 = .4765mm. Multiply this by 2 to get the change in diameter which calculates out to a change of 0.953mm. 11.35mm - 0.953mm = 10.40mm

I learned this in high school in California back in the 70's.

Nowadays in the California schools, the only math they teach kids is 10+1= "Assault clip" and don't defend yourself, 911 will solve all your problems.

 

[Naterater]

I am in high school nowadays, and we still do learn this. Rsrocket is correct. Education is degrading today, but those who still strive to learn can still do very well and learn a lot.

 

[rsrocket1]

Good for you Nate. If you want to learn, yes, the opportunities are still there and those who go for it rather than expect to have it spoon fed to them will succeed now and in the future.

 

[H3R3T1K]

So if a case is said to have a one degree taper, it means one degree PER SIDE?

 

[Jeff H]

So if a case is said to have a one degree taper, it means one degree PER SIDE?

Yes, unless you know of a case that wasn't symmetrical.


The math described above is pretty basic Trigonometry. I would have hoped that most High Schools still teach this.

 

[brickeyee]

I would have hoped that most High Schools still teach this.

Most high schools cover algebra I and geometry.

Trig is for the college bound.

 

[H3R3T1K]

Thanks for the help guys. I feel I gotta defend the German educational system a little even though it sure isn't the best. We did at least basic trig, I just forgot about it 'cause I never used it since. So we have different types of schools in Germany with just one allowing you to attend college upon successful completion of a final exam. I don't know if they teach trig in the other schools.