Bullet Size Relationship to Free-Bore Diameter

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  • Last Post 22 April 2023
Wm Cook posted this 20 April 2023

Since the CBA covers black powder to unlimited BR competition this is a pretty broad question but I though I would give it a try. 

Does the freebore diameter have any sway on the choice you make for what you size your bullet to?

Up until a couple years ago I was only shooting military or old lever guns and my practice was what I had read and that was to size the bullet as little as possible.  So in all cases I ignored the freebore diameter and would size as big as I could.  Heck at the time I didn't even know what terms like throat, free-bore and leade were.  So 30 cals were sized .312 or even .313.  Looking back I'm guessing that the free-bore was so generous on the old rifles that it was a loose fit even with a bullet sized big. 

But when I started loading for new production rifles I had a problem.  I don't know if it was by accident or by design but the last three new rifles had tighter free-bore diameters than my old rifles. I found that if I were sizing bullets a couple thousands greater than the free-bore I would get a scraping of alloy on the face of the free-bore and it was a real bear to get out. 

I was hoping someone could comment on this.  Thanks, Bill C.

A “Measured Response” is as effective as tongue lashing a stuck door.

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RicinYakima posted this 20 April 2023

Yes, I think your observation is correct, newer rifles have smaller free bores than older ones. The one exception that I can think of is Winchester 94 30/30's, since they had no free bore at all. The leade started at the end of the chamber neck cut. 

My opinion is that a lot of this was cost of tooling, make the reamer to the maximum dimensions and keep sharpening it till it is the minimum. Today cutting tools are so much better that isn't required anymore. 

I have done chamber casts or pound casts for years and size to the free bore dimension. 

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Wilderness posted this 20 April 2023

 Bill

I can contribute an experience on this.

I had an early Mannlicher Schonauer M1905 9x56 carbine. The barrel slugged .353", so I set out to match the bullet mould to that diameter. The choice was Lyman #350457HP, casting .354". Results were disappointing.

The next mould was #3589HP, casting .360" and sized to .354". Again, nothing special. Out of curiosity I shot some bullets unsized. Bingo! With 40 gns 3031 it shot 1.25" groups at 50 meters (and shot to the sights). From then on I lubed and gas checked in a .360" die and asked no further questions.

Much later I did a chamber cast and found that the rather long throat started at .359". I also got to chronograph the load - 1920 fps.

This experience convinced me that as well as filling the barrel, the bullet has to fill the throat.

Rick mentions .30-30s, and their lack of freebore/throat. My solution there is to fill the case neck, using it to guide the bullet into the barrel - another accidental discovery. My full power loads use .3125" bullets. In all but a few instances (some Winchester cases) these chamber OK in my Savage and Marlin rifles. To be on the safe side I ONT the cases to .010" max neck thickness, just enough to rub off the high spots and catch any thick necks. I suspect they shoot a bit better after the ONT also, given that the case neck is being used to keep the bullets true to the barrel. With these dimensions a fired case has the same outside neck diameter as a loaded cartridge - clearance is covered by springback after firing.

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Wm Cook posted this 21 April 2023

Ric, Bill; thanks for the feedback.  That kinda calibrates with what I was thinking.

While we're talking about freebores, is there such a thing as a tapered freebore?

Thanks again for the help.  Bill C.

A “Measured Response” is as effective as tongue lashing a stuck door.

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45 2.1 posted this 21 April 2023

Ric, Bill; thanks for the feedback.  That kinda calibrates with what I was thinking.

While we're talking about freebores, is there such a thing as a tapered freebore?

Thanks again for the help.  Bill C.

 

All rifles acquire that condition after being shot enough. It's not a straight taper either.

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Wilderness posted this 21 April 2023

Is there such a thing as a tapered freebore?

The freebore in my MS was a long taper, starting at .359" and presumably running down to .353". This was quite a long freebore, from memory something like 3/4".

 

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Ross Smith posted this 22 April 2023

This raises the question of what is throating and what is freebore. In my benchrest rifle there is a long tapered throat and the bullet is bumped to match that taper and then breach seated. I think you still have to match the freebore and bullet diameter as in my douglas barreled 30-06.

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