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Freebore… how do you measure it?

This discussion has primarily been about chambers that have a cylindrical freebore running up to the leade, but one chamber drawing in this thread showed another configuration that some factory chambers have. Starting at the front of the 45 degree angle at the front of the chamber neck there is a continuous angle that starts at a larger diameter than a parallel freebore would and is continuous to the top of the lands. I have not seen this used much on custom reamers, probably because of the thought that a cylindrical freebore gives better bullet alignment. The only reason that I mention this is to point out the difference so that someone does not get confused trying to find features on that drawing that are not there. It is a different type of throat, that I believe the definition that Edd quoted was referring to.
 
BoydAllen said:
On the drawing the line that represents the leade angle stops well short of the dimension line for the front of the freebore. That is the gap I was referring to. It is not something that shows up in the given dimensions but if one took the drawing too literally, it might be confusing.
Click to expand...
So, are you talking about the actual "picture" or the stated dimensions? If the pic, then I agree and I can see where it could confuse someone.
 
RGRobinett said:
As stated, the free-bore length should be displayed on the reamer print - to understand variation, check the tolerances on the print. As Jackie pointed out, dimensions vary. Tolerances and potential "stacking" is the reason we cannot share "seating-depths" - not only can free-bore length vary, so can leade angle, which generally is +/- 1/4 of a degree . . .

The video shows one method of determining the contact point for a loaded-round using a specific bullet - related, but neither defines nor provides a measurement of free-bore.

Technically, free-bore is the un-rifled section of the barrel between the chamber mouth, and the origin of the lands, as defined by where the leade angle is tangent to the free-bore diameter. This section should be somewhat larger in diameter than the groove diameter - 0.0005" to 0.001" [larger] is a good range. Keep 'em ON the X! RG
Click to expand...
The unrifled section!!! Three words.
 
The OP asked a simple question that probably doesn't have
a simple answer----"How does one determine the exact free bore of their rifle?"

I take this to mean---How do you measure the free bore of an existing chamber,
with or without a reamer print.

How about a chamber cast ? This is something I've never done but it is taken
directly from the chamber in question. I'd think diameter measurements might
be fairly simple but wonder how difficult it would be to get reliable length measurements.

How about gage pins ? I tried this and came away with more doubts than answers.
In reality, this is measuring a very small cylinder, in a relatively deep hole----by feel.
The guy with the skills and tools for this doesn't live here.

How about a bore scope ? Its very easy to see the transitions from neck to free bore
to lands----but---I don't see anything to use for a comparison reference to estimate
free bore length. Wouldn't it be nice if such a reference could be built into borescopes.

If a frog carried a pistol, he wouldn't have to worry about snakes. If we could truly
believe that reamers were ground to print dimensions and tolerances, we wouldn't
need to ask about how to measure way down in that hole.

I hope somebody will tell us how to do this.

A. Weldy
 
On a relatively new bbl, I suppose one could use a method similar to what most smiths do while throating to a dummy round. Which is to run the short or zero(known length) freebore reamer in, measure the protrusion of the dummy round at "jam" point, then throat deeper by the amount of protrusion. It's not a super precise method but frankly, I always get tickled seeing freebore dimensions stated in tenths anyway. It's just not that critical of a dimension and the reamer makers themselves typically state length dimensions to only be within about .005"
So, if you had a dummy round that was loaded to "touch" for a known freebore bbl, you could get pretty close by long seating a bullet and pushing it into your chamber and measuring the difference in the two, the dummy and the long seated dummy round. There are far more important things than splitting hairs trying to attain a precise fb length. A PTG Uni-throater will give a consistent length once set, between barrels, which imo is more important to me than knowing an exact number. Ultimately, most are simply wanting to keep the shank or pressure ring of the bullet safely above the doughnut or neck/shoulder junction, with some available adjustment both ways for seating depth. Longer necks are obviously more forgiving than short ones, for obvious reasons, but still not what I'd call a critical dimension.
 
How about having a case long and untrim to ensure the mouth bottoms on the small chamfer noted on post 65 above. Inside diameter just large enough to hold a the bullet, then load a flat base bullet in reverse ( point inside the case) Load the round and push the bolt. The base of the bullet should stop before the rifling. Carefully remove the round, measure from the mouth of the casing to the base of the bullet and that should be your freebore dimension or close to it.
 
I generally just load a dummy round with a bullet I am serious about using, so that the rifling makes light marks. From there you can take a look at if it would fit in a magazine and where the bullet is in the case neck. This method will not give you your freebore number but it will give you useful information. If you are ordering a reamer and want a particular loaded length with a given bullet, just send your reamer maker a dummy round, with instruction that you want the FB such that the round would touch the rifling.
 
I'm not a gunsmith, I only play one on TV. But it seems to me that a SAAMI chamber drawing is not strictly interchangeable with a reamer print. In a Venn diagram depiction there's almost certainly some overlap, but that's little enough solace.
-
 
For an unknown chamber spec, (maybe a factory rifle, or 2nd hand, etc.) you could buy a single gauge pin that is the diameter of the bullet. Cut it to be of similar length to a bullet and stick the end that you cut into the modified case. Use the method in the video. Because the gauge pin has no taper, this will get you purdy durn close. Measure the difference between the mouth of the modified case and end of the gauge pin. Do some subtraction and call it a day.

A chamber cast would yield the most precise measurement but I’d rather not pour stuff in my rifle if I know what cartridge it shoots. Or if I’m having some weird bulging after firing... in the case of a chamber cast, just measure the areas mentioned in a few drawings posted more recently in the thread.

If this is purely academic, go for it. Otherwise there really isn’t a need in most situations. If you are looking to spec a reamer for a build, you can load a dummy cartridge and ship it to them. Also most quality gunsmiths for whatever type of shooting you do will have a few reamers with multiple freebores depending on the bullet you intend to shoot. Knowing the actual number isn’t all that important in the end, the right position of the correct bullet is all that matters.
 
What would one suppose would be the difference if you used the Hornady case that comes with the device, OR
A Fired Case Unresized from your rifle or a Resized case for your rifle's chamber?
 
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