Freebore

[Lucky Shooter]

This thread is started to not interfere with the thread by swadiver on zero freebore.

I have some zero freebore chambers and they've given me no problems. Its convenient
to be able to chase the throat with bullet seating instead of a partial barrel job----less
costly----less trouble.

This works well with the shorter bullets but some of the longer bullets need freebore
to keep the bullet out of the donut and powder area.

Dave Kiff has been very good at specifying freebore length for given bullets.
He once mentioned, without explanation, that some freebore was needed after I asked
about zero freebore----we specified .009" and the barrel was a fine shooter.

For some time, I've been thinking about freebore as a factor in bullet alignment as it goes
into and past the lead.

Does some amount of freebore actually contribute to accuracy via better bullet alignment ?

Most reamer prints I've seen call for a freebore diameter .0005" larger than nominal bullet
diameter. Is this truly the optimum freebore diameter ? Seems to be pretty much standard.

This is old stuff, well before Bill Gates, but I wonder if freebore serves an alignment purpose
in addition to seating depth accommodation.

Any words of wisdom ?

A. Weldy

 

[david kieffer]

This thread is started to not interfere with the thread by swadiver on zero freebore.

I have some zero freebore chambers and they've given me no problems. Its convenient
to be able to chase the throat with bullet seating instead of a partial barrel job----less
costly----less trouble.

This works well with the shorter bullets but some of the longer bullets need freebore
to keep the bullet out of the donut and powder area.

Dave Kiff has been very good at specifying freebore length for given bullets.
He once mentioned, without explanation, that some freebore was needed after I asked
about zero freebore----we specified .009" and the barrel was a fine shooter.

For some time, I've been thinking about freebore as a factor in bullet alignment as it goes
into and past the lead.

Does some amount of freebore actually contribute to accuracy via better bullet alignment ?

Most reamer prints I've seen call for a freebore diameter .0005" larger than nominal bullet
diameter. Is this truly the optimum freebore diameter ? Seems to be pretty much standard.

This is old stuff, well before Bill Gates, but I wonder if freebore serves an alignment purpose
in addition to seating depth accommodation.

Any words of wisdom ?

A. Weldy

freebore has no effect on bullet alignment in itself , jamming bullets may in some cases . freebore dia. must be larger then bullet dia. or it would not be freebore .hope this helps . dave

 

[Ned Ludd]

I have a slightly different take on the role of freebore than david. I have two .223 Rem F-TR rifles throated long for Berger's 90 VLD. I ordered both reamers from Kiff (identical, except they differ slightly in freebore length). The freebore diameter listed on the reamer prints is .2242". In other words, the freebore is only .0002" over bullet diameter within their tolerances. When using a Hornady OAL gauge, the freebore is tight enough that bullets will stop moving when they encounter the leading edge of the throat, and marked pressure must be applied to the plastic stick to get them moving again. In fact, the 90s are pretty well locked in by the long, tight freebore, so this effect may be a caliber-specific thing (i.e. caliber, freebore length, etc.)

Both of these rifles are ridiculously accurate/precise with 90 VLDs and I believe part of that is due to the tight freebore. First, with the no-turn neck chambers on these rifles, the tight freebore is essentially self-centering bullets with the bore in chambered rounds. In addition, it could also be argued that the tight freebore is somewhat analogous to seating bullets into the lands in terms of minimizing bullet yaw/pitch (i.e. balloting) as the bullet begins to traverse the bore. For this reason, IMO, the tight freebore also seems to minimize the effect of the sharp ogive/bearing surface transition on the secant ogive 90 when entering the lands, thus allowing them to be seated off the lands with no issues.

 

[Lucky Shooter]

Thank you David and Ned.

Ned, you've put some real world numbers on something I've been thinking about in general terms.

Have you guys just ignored the bullet's pressure ring diameter ? Some shooters think the freebore
diameter must be greater than the pressure ring diameter----following conventional wisdom. I wonder
if the pressure ring surface might be so small that it's presence on bullet drag might be insignificant.

I'm glad to hear you've been successful with such a tight freebore. Wonder if this is more common than
I thought.

Hope this brings about some more serious discussion.

A. Weldy

 

[Rdlningcltchdmpr]

Why is it that a 30AR reamer with (0) freebore still allows 50% of the bullet shank outside the case in order to touch the lands ?

 

[Zero333]

The only example I can think of about freebore diameter is 5.56nato and Wylde chambers. You know what I'm thinking,...
but since I have never used the same barrel chambered in both, my assumptions are not imperical.

 

[Ned Ludd]

Why is it that a 30AR reamer with (0) freebore still allows 50% of the bullet shank outside the case in order to touch the lands ?

That's because of specific bullet BTO length dimensions and the cartridge neck length. The zero freebore idea doesn't necessarily work well with other cartridge/bullet combinations.

I had a couple of discussions with Dave Kiff on this very topic a while back. My impression at the time was that he was also a proponent of the potential benefits of a tight freebore. Based on my prior experience with his .223 Rem reamers designed to cut very long tight freebores, I was trying to gain some insight about the possible merit of using a similar tight freebore in a .308 reamer I was about to have made. Unfortunately, I ultimately chickened out and went with a fairly standard .3085" freebore diameter, so I don't have any insight how something like a .3082" freebore might have worked in that scenario.

As far as the pressure ring, what I can tell you is that the 0.2242" freebore in my .223s is noticeably tight at the very front of the bearing surface, so I'd imagine it would be even tighter lower down at the pressure ring. The main problem for most people in terms of experimenting with this stuff is that if some new idea doesn't work, you're out the cost (and time) of having a reamer made, as well as lead time, cost, and chambering time for the barrel itself. Those are not insignificant considerations.

 

[Rdlningcltchdmpr]

That's because of specific bullet BTO length dimensions and the cartridge neck length. The zero freebore idea doesn't necessarily work well with other cartridge/bullet combinations.

I had a couple of discussions with Dave Kiff on this very topic a while back. My impression at the time was that he was also a proponent of the potential benefits of a tight freebore. Based on my prior experience with his .223 Rem reamers designed to cut very long tight freebores, I was trying to gain some insight about the possible merit of using a similar tight freebore in a .308 reamer I was about to have made. Unfortunately, I ultimately chickened out and went with a fairly standard .3085" freebore diameter, so I don't have any insight how something like a .3082" freebore might have worked in that scenario.

As far as the pressure ring, what I can tell you is that the 0.2242" freebore in my .223s is noticeably tight at the very front of the bearing surface, so I'd imagine it would be even tighter lower down at the pressure ring. The main problem for most people in terms of experimenting with this stuff is that if some new idea doesn't work, you're out the cost (and time) of having a reamer made, as well as lead time, cost, and chambering time for the barrel itself. Those are not insignificant considerations.

I do not know what you are saying ? A person would assume that a zero freebore chamber would require the bullet to be seated in very deep. Why is that not the case ?

 

[Ned Ludd]

^^^ You are correct. A rifle with zero freebore will usually require a longer/heavier bullet to be seated much deeper in the case/neck than is considered "optimal". In the example with 90 VLDs/.223 Rem that I used above, a rifle with zero freebore would put so much of the bullet shank down in the case/neck that you couldn't achieve reasonable velocity without significant pressure issues. A freebore of at least 0.150" is necessary to seat the long 90 VLD bullet such that the bullet boattail/bearing surface junction is positioned well above the case neck/shoulder junction. You can usually only seat a bullet into the lands around .025" or so (i.e. a "hard" jam) before chambering the round will force it to seat deeper.

The length of freebore required to seat a bullet in the typically desired region of the case neck depends on the bullet base-to-ogive length, and the length of the case neck. In cartridges such as the 30BR, where BR shooters are often using a relatively short (light) 30 cal bullet in a case with a relatively long neck, the point on the bullet ogive that first touches the lands does not need to be very far out from the case mouth in order to keep the bullet boattail/bearing surface junction above the case neck/shoulder junction, and above the doughnut region, for that matter. If you tried to seat a heavy 30 cal bullet such as the 200.20X that has a much longer boattail and bearing surface in a 30 BR case in a rifle chambered with zero freebore, the bullet would have to jammed into the rifling at least a couple hundred thousandths (which isn't feasible) in order to keep the bullet boattail/bearing surface junction above the case neck/shoulder junction. Therefore, the amount of freebore required to seat a given bullet optimally depends on the bullet base-to-ogive length, and the length of the case neck.

 

[1911nut]

If I was to measure freebore I would use my Sinclair chamber length gauge to measure from the case base to the end of the chamber neck.

Then I would seat a flat base bullet backwards in a case with a medium tension neck long enough so the base will be stopped by the first part of the rifling when the bolt is closed.


The difference in length is the freebore, aka leade


I did this for my Swift to find out I’m jumping 0.080” with a 55g Berger in the neck about 1/10”.


I can see the difference when comparing the chamber prints for the Swift and the 222 on the SAAMI site but the 222 print has lines I don’t know how to decipher.


Also, I don’t know what to do with a freebore expressed in a diameter?.?

 

[Lucky Shooter]

Is this tight freebore (.2242) more prone to collect fouling ?

Any other cleaning difficulties ?

You mentioned 2 different freebore lengths-----do you mind telling us
what these lengths are ?

I've thought about this for quite a while but somehow never put the money
up to do it.

This might be a good "next project" for me.

A. Weldy

 

[Ned Ludd]

Is this tight freebore (.2242) more prone to collect fouling ?

Any other cleaning difficulties ?

You mentioned 2 different freebore lengths-----do you mind telling us
what these lengths are ?

I've thought about this for quite a while but somehow never put the money
up to do it.

This might be a good "next project" for me.

A. Weldy

I have 2 PTG .223 Rem reamers with a "tight" freebore diameter (0.2242") designed specifically for Berger 90 VLDs. The only difference in their dimensions is the freebore length. One is PTG's "standard" .223 Rem ISSF reamer, which cuts 0.169" freebore length. The other cuts a slightly longer (0.220") freebore. The longer freebore chamber works "OK" with Sierra 95 SMKs and Hornady 88 gr ELDMs, but both of those bullets might benefit from an even longer freebore, something in the 0.250" to 0.275" range, perhaps even a tick longer.