Rechambering question for you experienced machinists

In the realm of poking a reaming die into an already existing chamber and rechambering it to something new;
Q. If the new reamer (metaphorical wildcat chambering) has the same case head dimensions as the existing chamber, is this an issue to not take out any metal to 'clean up' or align a new chamber? If the new reamer (metaphorical wildcat chambering) is only longer than the existing chamber or has a different profile around the neck or shoulder, are there any inherent problems that can occur?
I understand that reaming an existing chamber with an Ackley Improved reamer is as common as a common thing, but what if the case length or freebore is also being changed?
I'm sure its a simple question to folk who've done it, I'm just not entirely sure on these points.

Yes yes I know.. 'go ask your gunsmith' I hear you say. I will, I will.. but there are some right here too.

Before you ask 'But what are you trying to actually do?'... I'm conducting an exercise in theoretical design and learn some points on chamber design, not actually trying to do anything as such.

Re-reaming can lengthen freebore with the appropriate reamer but it would have to be done by hand to avoid increasing the headspace. Such jobs require 'feel' which only comes from experience. Setting the barrel back 1 turn and completely re-doing it would be a better option. Normally custom leed length is done with a specialist throating reamer which cannot alter headspace. Chamber design is already well laid down in the SAAMI specifications and based on over 100 years of 'best practice'. Anything other than a round wheel is going to give problems!

With ackley improved chambers, they have to set the barrel back a little bit in order to get the crush fit on non-AI brass at the neck-shoulder junction.

I have heard that if a barrel has had too many rounds down it, the steel can be work hardening which makes opening up a chamber to something else more risky on tooling/less precise.

Case head isn't the only important dimension. The angle of the case wall is critical because if the new reamer has more slope, it would have to be run in a long way to tidy it up. Which means setting the barrel back lots. Or it being impossible or dangerous because there isn't enough meat left on the tenon.

As this is a metaphorical/theoretical exercise in wildcatting, SAAMI specs would not apply. Obvious parameters such as pressure and headspace are a given though.
This was more directed at the area of the chamber were new reamer dimensions may be very close to or the same as the existing chamber, such as around the case head. But gundoc quite rightly pointed out the obvious regarding barrel setback, and that would give the re-chambering job some metal to shave off and clean the new chamber. I was aware of barrel setback being a common practice, but didn't connect the dots there.

In my theoretical exercise I am designing a wildcat with a longer chamber than the existing one, so headspace is not an issue, new brass would be created for the new chamber.

I am not a gun smith......so take advice as worth what you have payed for it.

If reaming an existing chamber any existing eccentricity could be followed by the reamer.
If any eccentricity (chamber is on the piss to the bore) does exist it could be bored to true to the bore then reamed (an extra step and time)......providing the eccentricity is less than both the increase in lenght and diameter needed to rechamber.
A decent margin above size would be needed if any eccentricity is present as surface hardening is a real possibility in any machine worked steel particularly and particuarly S/S of the types used in firearms.

You could have a perfectly straight chamber already, smith pokes reamer in and you live happily ever after.
Or not.

You would be surprised at the level of excentricity of certain factory chambers and on how well they sometime can still shoot.

Go to the SAAMI pages and compare the differences between maximum cartridge dimensions and minimum chamber dimensions and you will get the tolerances you need for designing chamber and loading die reamers. The die reamers are smaller diameters to compensate for brass spring-back when resizing, and will be .001"-.002" below maximum cartridge specs. I would also advise having a thorough search of existing wildcats as the range is vast and covers every conceivable variation. The advantage of wildcats is that the time and money involved in developing them keeps you both fully occupied and poor so you have little opportunity to get into mischief before you discover a factory round that pretty much duplicates the results you seek.;)

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Originally Posted by veitnamcam

I am not a gun smith......so take advice as worth what you have payed for it.

If reaming an existing chamber any existing eccentricity could be followed by the reamer.
If any eccentricity (chamber is on the piss to the bore) does exist it could be bored to true to the bore then reamed (an extra step and time)......providing the eccentricity is less than both the increase in lenght and diameter needed to rechamber.
A decent margin above size would be needed if any eccentricity is present as surface hardening is a real possibility in any machine worked steel particularly and particuarly S/S of the types used in firearms.

You could have a perfectly straight chamber already, smith pokes reamer in and you live happily ever after.
Or not.

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And don't forget the other option here, the chamber is straight but the bore is on the piss relative to the outside of the barrel. More common than most people realise I think especially on the lower end of the quality spectrum.

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Originally Posted by gundoc

Go to the SAAMI pages and compare the differences between maximum cartridge dimensions and minimum chamber dimensions and you will get the tolerances you need for designing chamber and loading die reamers. The die reamers are smaller diameters to compensate for brass spring-back when resizing, and will be .001"-.002" below maximum cartridge specs. I would also advise having a thorough search of existing wildcats as the range is vast and covers every conceivable variation. The advantage of wildcats is that the time and money involved in developing them keeps you both fully occupied and poor so you have little opportunity to get into mischief before you discover a factory round that pretty much duplicates the results you seek.;)

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Yep understand those points. If I was to actually go from thinking to doing, the thing wouldn’t involve custom sizing dies. Creating anything distinctly new or different wouldn’t be a requirement of the endeavour. It would be for the process not the end result.

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Originally Posted by No.3

And don't forget the other option here, the chamber is straight but the bore is on the piss relative to the outside of the barrel. More common than most people realise I think especially on the lower end of the quality spectrum.

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That's pretty much a given with cheaper rifles.....my jw15 was miles out once cut....still shot great for a 22 but one could fairly expect a decent quality centerfire to have the bore in the middle of the barrel.