I called x caliber barrels and asked them if I could get a chamber cut for a 204 ruger to have the 39 gr bullet to be 30k off the lands when seated to manufacturer oal.. He said their 204 has a 30k free bore.. What does that mean and for bullet seating depth also.. Thanks. Dan
free bore
- Thread starter dan158
- Start date
Varminterror
New member
Freebore in a chamber isn't the same as bullet jump. Effectively, on a chamber print, the freebore is a straight portion of chamber forward of the case neck (and forward of the angular "front edge" of the neck), before the angular portion of the lands entering into the rifling. Clear as mud?
On most loaded cartridges, the bearing surface of the bullet extends forward of the case neck, so the bullet will extend a little into the freebore. The jump is the remaining distance the ogive-to-bearing surface union has to travel the rest of the distance of the freebore plus the angular portion of the rifling leade.
In the pic below, the yellow brackets represent the true free bore of these chambers - but most folks include the angular portion between it and the case neck as well in their manner of thinking. You can see one of these chambers has a small free bore, whereas the other is around ~5x longer.
Also, there's typically some portion of the "beveled" section of the rifling the bullet will jump before making contact. So if your bullet bearing surface is flush with the end of the neck, and the end of the neck is flush with the end of the chamber neck, then the jump would be the angular length of the front of the neck, the true free bore, then the angular entrance of the rifling. But again - typically the ogive-to-bearing-surface union sticks out of the neck some distance, eating up that length.
So for you wanting a 30thou jump - unless you seat the bullet such that the ogive-to-bearing union exactly to the neck, with the X-caliber barrel, you will NOT have a 30thou jump. It only has 30thou freebore. You'll need a longer freebore to give you that much jump with the bullet NOT seated clear to the ogive.
On most loaded cartridges, the bearing surface of the bullet extends forward of the case neck, so the bullet will extend a little into the freebore. The jump is the remaining distance the ogive-to-bearing surface union has to travel the rest of the distance of the freebore plus the angular portion of the rifling leade.
In the pic below, the yellow brackets represent the true free bore of these chambers - but most folks include the angular portion between it and the case neck as well in their manner of thinking. You can see one of these chambers has a small free bore, whereas the other is around ~5x longer.
Also, there's typically some portion of the "beveled" section of the rifling the bullet will jump before making contact. So if your bullet bearing surface is flush with the end of the neck, and the end of the neck is flush with the end of the chamber neck, then the jump would be the angular length of the front of the neck, the true free bore, then the angular entrance of the rifling. But again - typically the ogive-to-bearing-surface union sticks out of the neck some distance, eating up that length.
So for you wanting a 30thou jump - unless you seat the bullet such that the ogive-to-bearing union exactly to the neck, with the X-caliber barrel, you will NOT have a 30thou jump. It only has 30thou freebore. You'll need a longer freebore to give you that much jump with the bullet NOT seated clear to the ogive.
Last edited:
I generally specify zero freebore for light bullets in varmint chamberings. My .20-250 and .20 Dasher, both slow twists for light bullets, both have zero freebore. I'd add that the leade angle on them is only 1.5*, which is tremendously shallower than that shown in the illustration above. Guess I will also throw in that I don't agree with the terminology in that illustration either. It seems to be saying that leade, freebore and throat all mean the same thing. They don't. Reamer prints separate the freebore and the leade (which doesn't start until after the freebore, if there is any), both freebore and leade together are what is generally referred to as the throat.
- DAA
- DAA
Originally Posted By: VarminterrorSo for you wanting a 30thou jump - unless you seat the bullet such that the ogive-to-bearing union exactly to the neck, with the X-caliber barrel, you will NOT have a 30thou jump. It only has 30thou freebore. You'll need a longer freebore to give you that much jump with the bullet NOT seated clear to the ogive.
This is simply incorrect. I think you're forgetting about the leade. It's in front of the freebore.
Yes, you most definitely would be able to get .030 jump with that much freebore, if you want it. And more. Without seating the ogive clear down to the neck.
At exactly what COAL though, who knows. If you know the leade angle, which is also expressed as TPI on the reamer print (.5327 TPI, for instance), you could perhaps calculate how far forward of the chamber neck a given bullet diameter would be. And work backwards from that. All that would need to assume some very exact machining at every stage from reamer grinding to bore lapping to chambering though. In the real world, things don't ever seem to come out quite that precise...
But, for dang sure, you'll be able to jump a good bit more than .030, with .030 freebore. If you want to.
- DAA
This is simply incorrect. I think you're forgetting about the leade. It's in front of the freebore.
Yes, you most definitely would be able to get .030 jump with that much freebore, if you want it. And more. Without seating the ogive clear down to the neck.
At exactly what COAL though, who knows. If you know the leade angle, which is also expressed as TPI on the reamer print (.5327 TPI, for instance), you could perhaps calculate how far forward of the chamber neck a given bullet diameter would be. And work backwards from that. All that would need to assume some very exact machining at every stage from reamer grinding to bore lapping to chambering though. In the real world, things don't ever seem to come out quite that precise...
But, for dang sure, you'll be able to jump a good bit more than .030, with .030 freebore. If you want to.
- DAA
Of no help to the poster, as he's looking at a pre-chamber apparently. But some 'smiths can do necking and throating separately and can, indeed, throat a rifle to the exactly jump you want for a dummy round.
But for a traditional reamer with throat and neck already set, you just get whatever it is. Which is part of the reason guys end up having their own reamers made. That and good chamber brass fit.
I just wish reamers would always turn out exactly how you spec them in the print. My experience has been that too often, they don't. And it's a real pizzer...
- DAA
But for a traditional reamer with throat and neck already set, you just get whatever it is. Which is part of the reason guys end up having their own reamers made. That and good chamber brass fit.
I just wish reamers would always turn out exactly how you spec them in the print. My experience has been that too often, they don't. And it's a real pizzer...
- DAA
Varminterror
New member
I try to be very careful with vernacular. For example, the image I linked uses throat, leade, and freebore interchangeably. I personally consider the leade to be the angled entrance of the rifling, but the gunsmith I worked under when I started used to call the freebore the leade, and then actually called the neck of the chamber the throat.
The angle of the leade - as I mentioned in my first post - if we take leade to mean the conical entrance of the rifling, makes the biggest difference in this equation. From the front of the neck, through the freebore, and into the contact diameter of the leade is the length you really need to know. I tend to call the sum of those distances, as mentioned in my first post, the total freebore, then I use "throat" to describe the cylindrical or slightly conical "freebore" depicted between the yellow brackets in the image above.
If Xcaliber uses a short leade angle and/or a tight throat, and only 30thou freebore - in the vernacular of the graphic above - then it may not fit. But like DAA said - all you need is a dummy cartridge sized the way you want it to determine the necessary throat depth (freebore). Then you can compare that to the reamer print to see how whether the leade buys you the jump you want, or not.
The angle of the leade - as I mentioned in my first post - if we take leade to mean the conical entrance of the rifling, makes the biggest difference in this equation. From the front of the neck, through the freebore, and into the contact diameter of the leade is the length you really need to know. I tend to call the sum of those distances, as mentioned in my first post, the total freebore, then I use "throat" to describe the cylindrical or slightly conical "freebore" depicted between the yellow brackets in the image above.
If Xcaliber uses a short leade angle and/or a tight throat, and only 30thou freebore - in the vernacular of the graphic above - then it may not fit. But like DAA said - all you need is a dummy cartridge sized the way you want it to determine the necessary throat depth (freebore). Then you can compare that to the reamer print to see how whether the leade buys you the jump you want, or not.
coyotezapper
Member
Originally Posted By: DAAI generally specify zero freebore for light bullets in varmint chamberings. My .20-250 and .20 Dasher, both slow twists for light bullets, both have zero freebore. I'd add that the leade angle on them is only 1.5*, which is tremendously shallower than that shown in the illustration above. Guess I will also throw in that I don't agree with the terminology in that illustration either. It seems to be saying that leade, freebore and throat all mean the same thing. They don't. Reamer prints separate the freebore and the leade (which doesn't start until after the freebore, if there is any), both freebore and leade together are what is generally referred to as the throat.
- DAA
I agree with Dave. I think you would be better to have a 0 freebore in the 204. My 17 Tac built by Gre'tan is 0 freebore and works perfectly. When I built my 17 Tac I asked Dave his opinion on this matter and this is what I went with and have been more than pleased with it.
- DAA
I agree with Dave. I think you would be better to have a 0 freebore in the 204. My 17 Tac built by Gre'tan is 0 freebore and works perfectly. When I built my 17 Tac I asked Dave his opinion on this matter and this is what I went with and have been more than pleased with it.
coyotezapper
Member
Originally Posted By: dan158Thank you both.. X caliber has 25% off the fluting also...So I should be able to get the bullet seated 30k to the lands? Dan
I would not waste your time with fluting. Recent study done by Bryan Litz showed that fluted barrels actually and I quote "showed measurably more POI shift compared to barrels of the same contour that weren't fluted." And weight savings is minimal. His newest book really dispelled a lot of gun industry claims when it comes to a lot of things and they have actual test data to prove it.
I would not waste your time with fluting. Recent study done by Bryan Litz showed that fluted barrels actually and I quote "showed measurably more POI shift compared to barrels of the same contour that weren't fluted." And weight savings is minimal. His newest book really dispelled a lot of gun industry claims when it comes to a lot of things and they have actual test data to prove it.
gene10pntr
New member
Kind of off the subject,but other than some weight loss...isn't fluting suppose to help cool the barrel a little faster? Great info though on the original question!
Thanks!!!
Thanks!!!
steve garrett
Active member
the best thing to do is have your gunsmith chamber to a dummy round. If your wanting to have X amount before you hit the lands just send him a dummy round with the bullet seated out a bit longer than you intend to load.
most gunsmiths don't setup a chamber like that though. they just run a reamer through it. if you want something custom you have to have the reamer made. Greg tannel is the only gunsmith I know of that lets you spec out everything you want, chamber to dummy round and you can pick the neck diameter you want.
but good luck its over a year wait these days.
most gunsmiths don't setup a chamber like that though. they just run a reamer through it. if you want something custom you have to have the reamer made. Greg tannel is the only gunsmith I know of that lets you spec out everything you want, chamber to dummy round and you can pick the neck diameter you want.
but good luck its over a year wait these days.
To clarify some misconceptions. Most gunsmiths order a reamer with xyz amount of freebore. Few, and I mean very few order reamers with zero freebore and throat according to what the customer wants, and they may charge you a tad to throat.
Many worry about the base of the bullet being below the shoulder neck junction due to the formation of doughnuts. I prefer to seat the bullet deeper with a zero freebore, and monitor for doughnuts, remove the doughnuts with a K & M carbide cutting mandrel if/when they form. By having shorter freebore, you can get more barrel life.
On the flip side, some cases will exhibit a "pressure spike" if the bullet is seated very deep in the case; a perfect example of this is a 223 round loaded with 80g A max with .040 freebore. Freebore on this round should be much longer to avoid the pressure spike. IN order to avoid the pressure spike in the 223, often you have to lower the powder charge and experience a velocity loss.
So, the 204 Reamer in question has shorter than std SAAMII freebore, but we often like less. As Dave has explained you will have a tad bit more "freebore" as there is usually about .020-.024 that comes off the chamfer angle on the case mouth.
You are in a quandary of sorts trying to find a gunsmith that has a zero freebore reamer on a 204, and speaking of such may leade to frustration. If you want a zero freebore chamber, then order a reamer with such, and I would also change the neck dia at the same time. You can split a case neck with a thin dremel cut off tool, seat a sierra 39g blitz king to where the shank of the bullet is right at the shoulder neck junction, have JGS make you a reamer with the bullet seated at that depth or just tell them you want a zero freebore reamer. Get a print from them, and check out the neck dia and change according to what you want. No use in having a big sloppy neck.
Common practice is to deduct $50 from what you pay for the reamer after use(minus postage), and sell the reamer, it will go fast.
Many worry about the base of the bullet being below the shoulder neck junction due to the formation of doughnuts. I prefer to seat the bullet deeper with a zero freebore, and monitor for doughnuts, remove the doughnuts with a K & M carbide cutting mandrel if/when they form. By having shorter freebore, you can get more barrel life.
On the flip side, some cases will exhibit a "pressure spike" if the bullet is seated very deep in the case; a perfect example of this is a 223 round loaded with 80g A max with .040 freebore. Freebore on this round should be much longer to avoid the pressure spike. IN order to avoid the pressure spike in the 223, often you have to lower the powder charge and experience a velocity loss.
So, the 204 Reamer in question has shorter than std SAAMII freebore, but we often like less. As Dave has explained you will have a tad bit more "freebore" as there is usually about .020-.024 that comes off the chamfer angle on the case mouth.
You are in a quandary of sorts trying to find a gunsmith that has a zero freebore reamer on a 204, and speaking of such may leade to frustration. If you want a zero freebore chamber, then order a reamer with such, and I would also change the neck dia at the same time. You can split a case neck with a thin dremel cut off tool, seat a sierra 39g blitz king to where the shank of the bullet is right at the shoulder neck junction, have JGS make you a reamer with the bullet seated at that depth or just tell them you want a zero freebore reamer. Get a print from them, and check out the neck dia and change according to what you want. No use in having a big sloppy neck.
Common practice is to deduct $50 from what you pay for the reamer after use(minus postage), and sell the reamer, it will go fast.
I had a 22k hornet that had lots of freebore. If I wanted to be flush with the lands the base of the bullet would be a quarter of an inch from the case mouth. I talked to Cooper about it and they didn't have a issue with it. Every thing I loaded had whatever OAL was recommended and it shot great. Wish I hadn't got rid of it. Chris