Hypothetically, if one has a chamber that is 1.570inches in total length and has a cartridge that is 1.550 and his reamer freebore is a nominal .025"; does that mean that he really has .045 " of actual FB ?
True Freebore ?
- Thread starter jamesdmock
- Start date
I’d have to think so James, at least until I could make then measure a chamber cast of that finished chamber.
“Nominal” is a loaded adjective. I’ve never had to fall back on a ‘money back if it’s not right’ guarantee from a reamer maker. So whether what you get for the $$$ you hand them cuts a chamber exACTly true to spec dimension every time? That’s a NASA-level expectation IMHO.
If true freebore length’s that critical I’d spec a reamer w/short throat then use throater to carefully approach final length.
“Nominal” is a loaded adjective. I’ve never had to fall back on a ‘money back if it’s not right’ guarantee from a reamer maker. So whether what you get for the $$$ you hand them cuts a chamber exACTly true to spec dimension every time? That’s a NASA-level expectation IMHO.
If true freebore length’s that critical I’d spec a reamer w/short throat then use throater to carefully approach final length.
Alex Wheeler
Site $$ Sponsor
The term freebore is used to describe the section cut into the barrel in front of the neck. We dont use the term to describe how much bullet is exposed from the case neck. So no, technically you have .025" freebore. But you are correct in noting that the un-used part of the chamber neck as well as the chamfer at the end of the neck do ad to the oal of a cartridge.
well for bench rest or target rifles I spec a very short neck "growth" length like 0.007 not .020 typically seen. I just track and trim all the time. no place for a carbon ring
ridgeway
Silver $$ Contributor
The FB dimension is referenced off the case length. So to answer your question, it's still .025 FB. You will simply have less bearing surface in the case.
Let's say you get a reamer with a 1.550 chamber length with a .025 FB. The OAL length of a seated bullet touching the lands will VARY by .020". Given they are accurately ground.
Freebore is not understood by many and its very important to match it to the bullet you intend to use. It's hard to do that just by doing the math because of different ogive. Best way is to get a short FB reamer and throat reamer. Cut to a dummy round where you want the bullet.
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I guess it boils down to semantics. In other words, the "effective" freebore in my example is .045 " and the technical FB is .025. Many years ago while ordering a reamer from Hugh Henriksen, he tried to explain this to me.
ridgeway
Silver $$ Contributor
No. FB will not change by using a shorter case
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I did not say that. The effective freebore will be altered by a LONGER chamber.
ridgeway
Silver $$ Contributor
I am not sure you fully understand. If you change your chamber length on your reamer and specify .025 FB, it will still be a 25 thou freebore no matter how you look at it. Only way you change the freebore is to cut it or built into reamer. I'm really confused as what you are questioning. Can you elaborate more on your terminology of "effective" FB? Nominal to me is a number in the middle of a tolerance range.
Whether the FB is called freebore or whether the neck portion of the chamber is longer than that of the cartridge, both will cause the distance the bullet must be seated out to touch the lands and that distance is the effective freebore. It may not be CALLED freebore but it serves the same purpose. Hugh Henriksen is one of our premier reamer makers and he went to great detail to explain why freebore is often misunderstood. I understand what you are saying about FB and it amounts to the nomenclature used. But the reality of the matter is that the length that the neck portion is cut does IN EFFECT alter the REAL FB. Good shooting....James
I agree with what you're explaining, with the caveat that a chamber's freebore is a machined-in dimension: once cut, that's what it is until you do something further with a cutter to alter it.
I think what James is getting at is: if one sets up a case that's shorter in OAL than a specified case around which a chamber it's to be fired in has been cut, then the effective freebore - the distance from the end of the shortened case's neck opening to where the throat's lands begin - will be different from what a case with an OAL for the chamber as designed will experience.
The distance from case mouth to the lead's beginning is what freebore describes, and that's not going to change what a reamer print specification is for a given chamber. But in practice, if one chooses to bring the case mouth back from what that 'design-length' case OAL is then yes, effective freebore will grow by an equal amount.
As far as 'nominal' goes, if you consider the lowly 2x4 dimensional stick of lumber, you'd have a hard time finding one anywhere but at a sawmill these days that measures a true 2"x4".
Nominal acquired that "somewhere close" meaning in engineering-speak, which is arguably appropriate here. But it doesn't mean any particular deviation from what's specified, it merely suggests that "what you see isn't going to be what you get" once the part is created.
But it ought to be close enough to still work.
What you're thinking of for the meaning of nominal I think would be better described by the word median; https://en.wikipedia.org/wiki/Median
ridgeway
Silver $$ Contributor
Makes no sense to me...you shorten the case OAL and you call that a change in "effective" freebore? Freebore is freebore. Case trimming in no way shape or form alters freebore. I've been doing this for a long time and never heard anyone use "effective FB", "real FB" "actual FB".
The way I was referring to nominal is in the machining world pertaining to dimensions and tolerancing on prints.
Educate me... I'm always looking to learn new things...
The way I was referring to nominal is in the machining world pertaining to dimensions and tolerancing on prints.
Educate me... I'm always looking to learn new things...
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Alex Wheeler
Site $$ Sponsor
"Effective" freebore can be debated. The end of the neck and chamfer reduce how deep a bullet is seated. But it does not guide or align a bullet like an actual freebore does. So if a freebore's only purpose is to reduce the amount of bullet in the neck, then yes a longer chamber length and the chamfer increase effective freebore, but if we expect a freebore to guide or align a bullet then no it doesnt. I think most are mainly concerned with bullet in the neck. In that case when a dummy is made to send to a reamer manufacturer, you should specify the chamber length at that time. A short chamber length or one tight the the end of the neck is something I avoid and none of my reamers are set up that way. I like room there. My 6BRA and dashers are a 1.570" chamber length on purpose, I am fully aware of the shortening that occurs and in the past I shortened up the chamber length, not any more.
ridgeway
Silver $$ Contributor
Please explain in further detail. To me freebore is freebore. It's cut...no changing it. If you trim your case and call it the "real freebore" and saying it effects the freebore cut, that's beyond me. I'm just not understanding the point here.
Let's say I have a 1.560 chamber length with a freebore of .150. I trim my case to 1.550. Now from how I understand this, it altered my effective freebore by .010?
No you have not!
I deal with this all the time trying fit bullets to a customer’s chamber.
For example two reamers for a 6PPC made by the same manufacturer.
Reamer 1 has .045 Freebore and case length of 1.500
Reamer 2 has .045 Freebore and a case length of 1.520
In order to get to the rifling with Reamer 2 the bullet will have to be seated .020 further out of the case than Reamer 1.
Another way to think of it is Reamer 2’s Freebore will be cut .020 deeper into the barrel.
Hope this helps!
Bart
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ridgeway
Silver $$ Contributor
Yes, that is my point from the beginning. The freebore length dimension comes off the chamber length. I explained the identical thing early on regarding two reamers with same freebore but different chamber lengths. The seated OAL to lands is different. Giving the reamer is cut correctly.
I'm just lost in regard to changing the case length on the brass changes the freebore thing. This is being confused early on with different terminology. Only way freebore is going to change is if you cut it or erosion, period.
Actual FB, effective FB, nominal FB, real FB, true FB is way too loosely used to and I've never heard them referred to as they are here. I have Henriksen reamers and spoke to him numerous times.
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