throat diameter

V

vinconco

Guest
I am trying to sort out what is most important in getting the bullet aligned with the bore. It seems that most of the emphasis is put on using the case neck dimensions to achieve alignment. Wouldn't having the throat diameter as close to bullet diameter and then having the throat concentric with the bore do more to achieve this? I have done searches on this but most of the topics are more about throat length than diameter. Other “throat” topics jump directly into neck diameter discussions apparently confusing the two. Any input on this subject would be appreciated

Thanks
Steve
 
Steve

There are 3 seperate parts to the chamber in front of the shoulder. The neck, the lead or freebore, and the throat. All 3 need to be perfectly aligned.

The freebore is generally .0005 over bullet diameter and the length will vary depending on the bullet being used. Long VLD bullets require a long freebore while some point blank shooters may cut their chamber with little or no freebore.

The throat is the part that tapers from the freebore diameter to bore diameter. Length of the throat depends on the angle.

I cannot emphasize enough - all 3 have to be perfectly aligned.

Ray
 
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Thanks for the reply Ray, I guess my next question is if the freebore is .0005 over bullet diameter why is it so important to have a "tight neck" where the chamber dimension will still be .003 over case neck/bullet diameter. How could this align the bullet any better than the freebore?
 
Throat Fit

Ray, I would suggest the article in the Sept 2006 Precision Shooting by James Boatright.
I had Dave Kiff make a reamer for my new BR LR Rifle based on the article. Dave used the three new cases with bullet set to my length.
What is really the point here is in- bore yaw, caused by jump and free bore etc.
hope this will help out the fellow.
And by the way, this comes from "Beautifull Down Town Chico"
Cheers
 
"Freebore" means "clearance," or "doesn't touch." If you use it to "align" the bullet, it isn't freebore anymore.

As to "tight necked chambers," if you look at long-range practice with big cases, most people use .002 to .004 clearance. The necks are even, though.

BTW, The notion of a "tight necked" chamber came about in the old days of the .222 & the Wasp. Some cases were bad enough that when you put a round in the chamber, the lands would engrave on one side only. Obviously, the bullet was out of alignment in such a case. More important than "tight" is "even."

This got taken to crazy extremes. But even the guys who ran .0005 total in their PPC would usually allow that .002 likely wouldn't make any difference.

As to Mr. Boatrights' theories, they are just that. If you go over to the 1,000 yard forum right now, you'll see that the people who have been testing the new 210 Berger VLDs are getting the best accuracy when they jump then .020 to .030.

* * *
I'll bet Ray can tell some good stories about the bad brass that had to be used for the Wasp & .222 about the time Paul Marquart came up with his neck-turning tool.
 
I beg to differ regarding the definition of freebore. Freebore is smoothbore or bore free from rifling in my opinion.


some quickie cites, easy to copy ;)


Wiki ------ "The part of the barrel between where the bullet exits the cartridge, and engages the rifling, is called the "throat", and the length of the throat is the "freebore". In some firearms, the freebore is all but nonexistent."

Hornady-------"The section ahead of a rifle's chamber just before the rifling starts (above) is called the "leade," throat, or "freebore" and is really the bore of the barrel with the rifling reamed away."

Guntec -------" Definition for "leade" : A short distance of unrifled bore area immediately in front of the chamber. Also called 'freebore'."

It is often .0005 over bullet diameter in a custom chamber and the bullet definitely touches it in any case. If it were .005 oversized the bullet would STILL touch it because this cannot be avoided.


At .0005 over it has been argued that not only does the bullet touch but that it obturates enough to seal all the way around. I am of this opinion.


al
 
I have always preferred Al's definition of free bore ...

Free bore is smooth bore or bore free from rifling ...

...and this is different than bullet jump ...
 
Al, Well, the rifling is reamed away, that's for sure, but in a barrel with .308 across the grooves, a typical .3085 bullet would have an interference fit. And .3085 is small across the pressure ring, some are greater.

I've always set my freebore diameter as .001 over the bullet's nominal diameter, e.g., .309 for a .308.
 
Throat diameter

Charles E
We seem to have a good discussion going here, which is good.
Unless you can ride the bullet from box to the target, all is "theory". My point, is tight chamber, tight neck, and on the lands to assure concentric entry, to avoid 'In Bore Yaw".
Good to hear from all

Cheers:)
 
Looking at three different 6mm PPC drawings I have, they show freebore diameter being 0.2435. Since the nominal diameter of a 6mm benchrest bullet is 0.2433, then the clearance in these chambers would be 0.0002".
 
Regarding inbore yaw;

The inbore yaw CANNOT be less than the amount of clearance used because the bullet simply cannot be crammed int the lands with 600lb measured force and somehow magically "float" nice and straight without slamming a sidewall. The firing event is violent with the lead core acting nearly liquid and the bullet jacked being subjected to enormous compressive and then radial force. To think that a typical bullet can take a 600lb blow to the butt without expanding a half a tenth is ludicrous. In Charles' instance of using .001 clearance my SWAG is that the bullet still obturates to seal and then is swaged back down as it enters the bore. This may well be a good thing.


This inability of a bullet to "float" the gap accounts for much of the inability of any "SAAMI Spec" or factory chamber to really shoot well. A perfectly smithed rifle with a custom name-brand barrel WILL NOT shoot to capacity with an oversized neck and an oversized and overlong freebore. Add .020-.040-.060 jump and you've got bullets skidding into the lands like Formula One cars on a foggy turn.


In my anecdotal testing at my own range I've run neck clearance and freebore tolerance clear down to interference fit with throats freebored out to as much as .160 (and in my case with bullets always jammed into the lands) and never run into anything in the way of "dangerous pressures". The guys who tell you that "touching the lands or running your necks too tight can blow up your rifle!!" have simply never done it. NOW, if you're running like 60thou of jump and a hot load and you decide to just change to a deep jam you will get a pressure spike that MIGHT make an ejector mark or leave you with a sticky bolt but the result is nowhere near as bad as loading to max in 50degree weather and then going out to the 80-90degree dogpatch and shooting the heated up rounds. Changing from 60thou jump to deep jam will likely give you 50fps..........far from "dangerous" unless you're already tipped well over the edge.


Yup Larry, this is fun stuff :) and me'n Charles have been hacking this subject back and forth now for what 6-8yrs???


But in that same timeframe agg's have halved in longrange shooting........ many techniques have changed from when "Precision Shootin at 1000yds" came out.


I was just at a little 600yd match a week ago and the winner agg'd right at .275 moa and the next five shooters were in the .3's...... I remember not so long ago when anybody shooting under 1/2" @100 was in the running. Back in the early 90's I had 5yr go at trying to make the .243AI shoot because it was a serious contender...... Half-inch 6-.284's and hopped up 6MM's and .243's were he order of the day.


Then came the 6BR


And NOW comes the 6X47L ....... YOWEEEEE!!!!!


It's a good time to be alive :)



LOL



al
 
I'm sure obturation exists and can be demonstrated but i have not been able to do so with conventional loadings.
I once shot a bunch of IVI 7.62 bullets through a 303 british barrel. This barrel was .303 bore diameter and .312 groove. The throat was the typical 303 long funnel. The bullets measured from .3064 up to .3072. I loaded them ahead of 44 grains of 4895. Recovered bullets showed no signs of having expanded at all. In fact, they hadn't even really engraved. The bullets had essentially ridden on top of the lands and bore five shiny streaks. They were also slightly pentagonal. Surprisingly, they shot remarkably well. 10 shots formed a nice, round group measuring 1.1 inches. This at 100 yds.
A 300 Weatherby featured the usual Weatherby freebore of the time. This was about 3/4 inch long and 3115 in diameter. The barrel was a little big also at nearly .309. Hornady 180 grain bullets shot well (1 1/4 inch or less at 100) but also showed no sign of expansion. Recovered bullets had grooves from the rifle lands but there was no evidence they had even contacted in the rifle grooves. 180 Sierra boattails shot horribly (8 to 10 inches at 100 yds). Recovered bullets showed definite evidence of having travelled down the bore in a tipped attitude. Marks from the grooves showed on one side at the front of the bullet and on the other side at the heel. Why the Sierra tipped so badly while the Hornady did not, I can't really say.
By the way, later Weatherbys with throats of near bullet diameter handled Sierras just fine.
This same sort of thing was noticable in the aforementioned .303. Sierra 180 flat based 30 caliber bullets shot just fine. With 180 boattails I couldn't garantee a hit on a 5 gallon pail at 100 yds. Regards, Bill
 
JerrySharrett said:
Looking at three different 6mm PPC drawings I have, they show freebore diameter being 0.2435. Since the nominal diameter of a 6mm benchrest bullet is 0.2433, then the clearance in these chambers would be 0.0002".
Click to expand...
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Sounds good but with my reloading tool/technique/components I've never been able to achieve this concentricity with any big bore ammo.

Shoot well
Peter
 
Alinwa

I've never knew that you vere so technical. a Formula One?? What about a Pick up truck and a froggy night?

LOL

Shoot well
Peter
 
Hornardy may have had a stronger jacket and construction that resisted the potential deformation of yaw better.
 
PPP MMM said:
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

Sounds good but with my reloading tool/technique/components I've never been able to achieve this concentricity with any big bore ammo.

Shoot well
Peter
Click to expand...
Freebore diameter vs bullet diameter has nothing whatsoever to do with concentricity of loaded ammo. Tighter fit within the chamber-neck/loaded cartridge neck will tend to correct some of the inaccuracies of the loaded round.
 
JerrySharrett said:
Freebore diameter vs bullet diameter has nothing whatsoever to do with concentricity of loaded ammo. Tighter fit within the chamber-neck/loaded cartridge neck will tend to correct some of the inaccuracies of the loaded round.
Click to expand...
,,,,,,,,,,,,,,,,,,

100% Agreed.

Shoot well
Peter
 
alinwa said:
Regarding inbore yaw;
In my anecdotal testing at my own range I've run neck clearance and freebore tolerance clear down to interference fit with throats freebored out to as much as .160 al
Click to expand...

Did you notice improvement with the tighter fit?
 
I did a little experiment where I had a round that had about .007 runout and then chambered it in a factory PSS and re-measured the runout to .004. Now it seems that the bullet was brought into a little better alignment by the somewhat closer tolerances of the freebore/throat. The bullet was seated about .040 (mag length) off the lands. If a factory freebore/throat of questionable tolerances could align the bullet in the case that much, why couldn't a closer tolerance do better?

I understand the importance of the whole package being concentric but I'm not sure why using a tight throat/freebore as a final alignment tool isn’t being given more consideration. It would seem more positive and tolerant of ammo that wasn’t perfect.

I am new to precision reloading so I don’t have a dog in any particular fight, just trying to make sense of all this before I spend a lot of time and money chasing my tail all over the place.
Having been a competitive shooter in many other disciplines through the years I don’t have enough tail left to chase anymore.
 
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