Dimension Question

[Richard Jones]

On most, if not all, 6BR prints that I see, the 'throat' length dimension is taken at a point of .233 diameter. I don't understand this at all. Shouldn't this be the .237 bore size? Why .233? Won't the bullet first contact the lands at .237? Very confusing......

Thanks for the lesson.

 

[Richard Jones]

On most, if not all, 6BR prints that I see, the 'throat' length dimension is taken at a point of .233 diameter. I don't understand this at all. Shouldn't this be the .237 bore size? Why .233? Won't the bullet first contact the lands at .237? Very confusing......

Thanks for the lesson.

And I assume the .233 is the reamer nose dimension?

 

[hogpatrol]

And I assume the .233 is the reamer nose dimension?

.233 is the diameter of the lead close to the intersection of the freebore and rifling.

 

[Keith Glasscock]

On all of my reamers, the throat is simply a taper from the freebore to a diameter small enough that the rifling will have a smooth transition to the "normal" bore.

I think you are looking for the freebore to determine when the bullet will contact the rifling.

 

[geo.ulrich]

Reamer is ground to a smaller diameter behind bushing so there is no chance of a step in freebore...

 

[Keith Glasscock]

On my Dasher reamer, I have a cylindrical section of .2435 that is denoted as the freebore.

 

[Richard Jones]

.233 is the diameter of the lead at the intersection of the freebore and rifling.

OK, this makes no sense to me. How can there be a dimension smaller than the bore?

 

[JRS]

Read post #5

 

[Richard Jones]

Again, question is: why is the

Read post #5

Understand this, don't understand #3.

 

[Richard Jones]

On all of my reamers, the throat is simply a taper from the freebore to a diameter small enough that the rifling will have a smooth transition to the "normal" bore.

I think you are looking for the freebore to determine when the bullet will contact the rifling.

Keith,

Yes I am. So looking at a Shilen 222 print that I have, the lead is 1.725. Actual base to ogive measured at .237 is 1.774. So how can one determine actual base to ogive by looking at a reamer print?

 

[Richard Jones]

here's a pic of the Shilen 222 .250 nk reamer. Since I have a barrel utilizing this reamer, I wanted to compare what the print dimensions are to actual.

 

[hogpatrol]

There is a one and a half degree angle where the .243* freebore transitions to the rifling. The .233 dimension is taken at a datum line perpendicular to the bore on that angle. Assuming a bore size .236, the .233 dimension would not be exactly at the intersection but for demonstration purposes, close enough. We're splitting hairs here and it's of really no consequence. Depending on the angle of the lead and the ogive size and design, it would be easier to use the Stony Point tool or the stripped bolt method to determine actual bullet contact.

Mr. Ulrich stated the reason for why the reamer is ground in that fashion and the reamer at that point is not cutting anything.

 

[carlsbad]

here's a pic of the Shilen 222 .250 nk reamer. Since I have a barrel utilizing this reamer, I wanted to compare what the print dimensions are to actual. View attachment 1003212

Rich,
There is an error/typo on your drawing. U and V are shown the same dimension and they are clearly not. In fact, freebore is generally V-U on JGS drawings. Not usually marked on PTG drawings. --Jerry

 

[JRS]

All of my JGS prints list it as U & V

 

[hogpatrol]

Rich,
There is an error/typo on your drawing. U and V are shown the same dimension and they are clearly not. In fact, freebore is generally V-U on JGS drawings. Not usually marked on PTG drawings. --Jerry

With "a" showing "0", I read it as a zero freebore reamer. The 1.712 and .013 add up but I'm not sure if the rest of the numbers are correct. It is confusing.

 

[carlsbad]

Hogpatrol,
I think you're right. As an engineer I'm not used to drawings with two lines of different lengths being the same length. Indeed confusing. --Jerry

 

[hogpatrol]

Hogpatrol,
I think you're right. As an engineer I'm not used to drawings with two lines of different lengths being the same length. Indeed confusing. --Jerry

On the SAAAMI print there's a 1.727, then a 1.752 and then 1.797 at the beginning of the lead angle.

Unless this is throated for long bullets, the 1.868 doesn't make sense either.

 

[Richard Jones]

The actual base to .237 ogive measurement is perfect for a 52 FB, which is what I wanted.

Very confusing. I just want a dimension on a reamer print that says: "this is base to ogive dimension at .xxxx (hopefully bore) diameter." I'm not trying to make this more complicated, actually trying for easy............

Still learning, after all these years...........

 

[Richard Jones]

.....The .233 dimension is taken at a datum line perpendicular to the bore on that angle......

OK, that makes sense.

I tried to extrapolate some numbers using the lead & throat length dimensions and the .2435 and .233 diameters on the 6BR print using a spreadsheet. So I (or others similarly confused) could calculate, if I know those 4 dimensions, than I can find the base to ogive length at a particular diameter, in this case, .237.

I must be missing something simple. Just can't wrap my head around it yet and am waiting for the light to shine through..............be gentle with me..............

 

[geo.ulrich]

I must be missing something simple. Just can't wrap my head around it yet and am waiting for the light to shine through..............be gentle with me..............[/QUOTE]
you would still need radius of ogive and where radius starts and bullet dia.