determining leade (freebore)

[Jay Christopherson]

Not sure if those terms are exactly interchangeable, but I've heard it often enough that I put both in there...

Basically, let's say I have a chamber of known caliber, but unknown dimensions. I have all kinds of measuring tools, inserts, bushings, etc... and I want to figure out what the leade (freebore) is.

How do I determine that? If I look at a reamer drawing, it seems like I could simply measure a round that is touching the lands from base-to-ogive, and then subtract the length of the chamber from that measurement. A case will be a bit shorter than the chamber, so I can't just subtract that length.

Does that work or am I missing a critical part here?

 

[spclark]

Two ways:

First is buy (or make if you have the tooling) a gauge for measuring your chamber's real case maximum length. Sinclair sells 'em in sizes for most popular bullet diameters:

http://www.sinclairintl.com/mobile/aspx/search/product.aspx?pid=32925

That'll give you a baseline number. Then use any of a number of gauges out there (Stoney Pointe used to be one brand until Hornady bought them out; Sinclair sells several) for measuring a given bullet's seating depth when it's seated just enough to touch the lands in that chamber. That's number #2.

Now subtract your cartridge baseline from #2 & you'll know your chamber's freebore to a pretty fair degree. Only problem is bullets with different ogive profiles will give you different numbers, which may or may not matter for your needs.

Second way is to make a chamber casting then measure the area of interest.

Depending on your manual dexterity & ability to use the measurement instruments you have available, results from option 2 might be more precise than with option 1.

Technically your chamber's throat begins at the end of the portion at neck diameter and ends where the 'leade' or ramped portion of the lands ends, where the lands are first at full height. The leade is that portion of the lands which is tapered from 0 to full land height.

Freebore is the length of the chamber beginning where the neck diameter ends to where the leade (lands at their very start) begins. Throating reamers are used to cut custom throats to a certain length and do so by tapering the lands as the tool is advanced. They're made to a specific leade profile to produce the desired rate of taper and length of cut. How far they're inserted during use determines the ultimate length of the throat produced.

Either way will give you a better idea of what your throat is than what you have now.

 

[necchi]

Second way is to make a chamber casting then measure the area of interest.

+1
A Cerrosafe chamber casting of the existing chamber,,
Too easy,,
And you'll KNOW what you have
http://www.midwayusa.com/product/462291/cerrosafe-chamber-casting-alloy-1-2-lb

 

[BillPa]

+1
A Cerrosafe chamber casting of the existing chamber and you'll KNOW what you have

+2
Two examples.

Bill

 

[roundknob]

If I understand this with the stoney point method I could get the overall length with the stoney point,subtract the baseline length = freebore???
Steve

 

[spclark]

If I understand this with the stoney point method I could get the overall length with the stoney point,subtract the baseline length = freebore???

Correct, at least as far as the particular bullet you use sees it.

Looking at the picture in an earlier post, the dimension you'd be getting would be that from the extreme front of the neck portion of the chamber to where your particular bullet encounters the lands. This changes dependent on the shape of each bullet.

Technically 'freebore' is described as the distance from the case mouth to where the 'leade' or lands begin, which is really hard to measure unless you have access to the reamer used or a chamber cast (shown in that picture) of the chamber, and a means to measure that dimension with precision.

 

[Rustystud]

Throats are a combination of the leade and the throat.

Most factory rifles have a lead with a diameter approximately .002' larger than the bullet diameter. Example a .308 Winchaster factory lead may be .310. The leade on most target rifles is .0005" example 95 Palma has a .3085" lead. Throating reamers are cut with a specified throat angle, IE: 1, 1.5, 1.75, 2, and 3 degree taper. I don't completely agree with some of the following post. If you will go over to the JGS reamer web site ther is a blank form on line that correctly shows a print that clearly defines the starting and ending points for the lead and throat.

Throats can be cut for specified bullet length and Ogive.
Nat Lambeth

 

[roundknob]

Thanks guys, this clears up at least for me how to measure freebore. This has always remained a mystery as to how it was figured.
Thanks
Steve

 

[spclark]

This has always remained a mystery as to how it was figured.

If you have the actual reamer specs for your chamber, it's real easy to figure out. Problem comes when you're faced with a chamber for which you have no information. Casting a chamber is really easy, certainly for anyone who's at all experienced in reloading, once you can get the chamber free of the action. Bolt guns are easy compared to AR type actions....

Anyway, I prepared a little diagram this morning after I found a cast I'd made of a chamber last fall. Should answer any questions that might remain ....