Another column B question

C

Charles E

curmudgeon
Conventional wisdom holds that the freebore diameter of the throat has to be larger than the pressure ring of the bullet.

Why is this? Once the bullet enters in the bore proper, the pressure ring has to conform. What bad things happen if the freebore diameter is, say, .3083 and the pressure ring measures .3086?

Does any of this change if you can take possible misalignment out of the picture (e.g., heaven forbid, but there is enough clearance in the chamber for the neck to be a few ten-thousandths off center w/ the bore)?
 
Charles, the freebores in my .30's measure .3085. With my main point die, shank diameters on my bullets are .3084 with pressure ring diameters of .3086-.3087 (depending on how I hold my tongue). I've got another die (The Wide Body) that will make .3088 shank diameters with .3092 pressure rings.

Just sayin'...... -Al
 
I appreciate the information Al. When you're about to try something that goes against conventional wisdom, it is good to review why it came about, but really helpful to find others who have had success ignoring it.
 
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Charles: A freebore diameter .0005 (half a thou.) over the groove dia. seems to be the baseline. I kow that in the .30's, freebore dimensions of .3087-.3088 still show good accuracy. Larger than that, and you're likely not in Kansas anymore........

I know of a couple of instances where the freebore diameter was too small (.3075-.3080ish). Barrel makers were blamed, bullet makers denounced, scopes were changed, bedding jobs redone, nuns spat upon and bulging envelopes stuffed into Sunday collection plates. :eek: ;) :D

Another plus for having the throat done as a seperate operation. -Al
 
This might be a silly question but how do you know exactly what your freebore is? i assume from the reamer print, but i dont have one for my 30br Robinett chamber. im sure i could get one if i would ask. im not real sure what i would do with this info but i do enjoy knowing these kinds of things.
I have noticed that my 30br is shooting better with the bullets that have a smaller pressure ring as compared to those bib bullets i have with a bit larger pressure ring. I wonder why this is? I dont have a caliper that measures four decimal points so i dont have any hard evidence of what i am saying, but it appears so. My rifle shoots the knight bullets better than the bibs. I kinda wish it were the other way around but it doesnt appear so. On the other hand, my shooting pard claims that the Brady knight bullets i am shooting have a large pressure ring?? I guess truth be told, i really dont know. lee
 
I'm really loading the deck with this one Al. Trying to cover .308 bullets with jackets as short as 1.150, and as long as 1.400. Well, 1.150 short for long range . . . I expect some bullets to be tried will be Yellow Box, right on .3080. And some, without any doubt, will come from Madrid, Iowa. Count on a .3086 pressure ring here.

The equivalent question, I suppose, is what diameter freebore would you use for the Wolf Pup when specking for your fatties? A .3088 shank trying to fit in the .3085 freebore seems like a bad idea. But sounds like you feel the .3084 shank/.3086 PR might be OK in a .3085?
 
Freebore

Al Nyhus said:
I know of a couple of instances where the freebore diameter was too small (.3075-.3080ish). Barrel makers were blamed, bullet makers denounced, scopes were changed, bedding jobs redone, nuns spat upon and bulging envelopes stuffed into Sunday collection plates. :eek: ;) :D

Not sure why a .3075-.3080 freebore would pose any problem.................just means the reamer would have zero freebore length, and we have seen plenty of zero length freebore reamers shoot competitively although with reduced case capacity...........................................Don
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Hi, I have messed around with this some, I use the skinnies in a cold gun , fatties in a hot gun . Just what I have found, heat the bigger the bore the more it grows.
John
 
Don said:
Not sure why a .3075-.3080 freebore would pose any problem.................just means the reamer would have zero freebore length, and we have seen plenty of zero length freebore reamers shoot competitively although with reduced case capacity...........................................Don
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Well, sort of, kind of. Take .3075. First, the bullet will be squeezed to that diameter, then forced to open up to whatever the bore was -- usually .308. I'll allow it's not much change. But all of a sudden the length of the freebore would be an issue, no? You'd be "jumping" whatever that was. If freebore was .100 long before the forcing one, you'd be jumping that sucker a fair bit. Or maybe jumping takes on a different meaning? Interesting notion.

(@ 1Shooter -- I'm interested in specifying reamers, rather than dealing with the results after the fact.)
 
Charles E said:
Well, sort of, kind of. Take .3075. First, the bullet will be squeezed to that diameter, then forced to open up to whatever the bore was -- usually .308. I'll allow it's not much change. But all of a sudden the length of the freebore would be an issue, no? You'd be "jumping" whatever that was. If freebore was .100 long before the forcing one, you'd be jumping that sucker a fair bit. Or maybe jumping takes on a different meaning? Interesting notion.

(@ 1Shooter -- I'm interested in specifying reamers, rather than dealing with the results after the fact.)
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Hi Charles, I may be assuming too much here, or we are using different terminology..................my assumption was that you were referring to a .3080" diameter gun barrel bore therefore the freebore could not "squeeze" a bullet down to .3075" diameter because a reamer with a .3075" diameter freebore cannot cut a .3080" diameter bore past the .3080" diameter dimension, hence the statement about the zero freebore length.

In other words, you would have .000250" land profiles where the normally smooth freebore area of the chamber would normaly be located, hence zero freebore.............Don
 
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And what happens when you put a super fat bullet in a minimum dia.throat. I
have seen this where a complete circle was on the bullet just in front of the neck. It
still shoot well.
 
Don, you're right. I shouldn't try to think after 9:00 pm -- and here I am doing it again.

Questions of shank interference aside, I'm still more interested in the original question, where freebore diameter does offer clearance for the bullet shank, but not the pressure ring.

For example, most of the BIBs bullets in 6mm or 30 have a pressure ring larger than .0005 over nominal. I've got some .2437s from Randy, and .3086s. Times past, I'd spec freebore anywhere from .0008 to .001 over nominal bore diameter, to clear that. But my .30-BR reamer is .3085, and if we can manage to cut a minimum chamber, that would mean interference on the PR. And I'm about to get another .30 reamer -- the bigger BIBs line will certainly be tested in it.

I'll allow that a reamer with a .3085 freebore is probably going to make a slightly larger hole. Is that all we're talking about?
 
Imagine how the lands look (.3000 dimension) when a .3075 freebore is machined in a .3080 (bore dimension) barrel. The height of the lands would not be completely removed...for whatever the length of the freebore section.

In a well done freebore/throat, you can look in there and see the symmetrical 'scallops' between the lands. -Al
 
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