Freebore - how to determine

[Kings X]

Is Freebore a wild guess?
I have neck length and bullet bearing surface length of my Hybrid bullet. And I don't want to seat into the donut.
Is it bearing surface length minus neck length plus desired jump ?

 

[Kings X]

Bump

 

[dixieppc]

Is Freebore a wild guess?
I have neck length and bullet bearing surface length of my Hybrid bullet. And I don't want to seat into the donut.
Is it bearing surface length minus neck length plus desired jump ?

If you are shooting a 6BR and the hybrid that you're talking about is the 105 grain hybrid, then I can send you a picture showing you the results of a .120 free bore with the hybrid touching off on the lands. FYI, the boattail is stuffed pretty good distance down into the case but the bearing surface is coming nowhere near where the donut would be. Let me know and I'll make the photo up and post it here.

Regards,

 

[dixieppc]

I decided to go on and post the pictures. If it is not your setup or not what you're asking maybe the pictures can help someone else.

The two pictures are based on a .120 freebore of a .268 necked 6BR chamber. There are two pictures showing an unseated bullet laying next to a seated bullet in the case. In each picture I tried my best to get the points of the bullets even and in each picture you will notice that I put a magic marker mark on the boat tail end of the unseated bullet showing where the bearing surface begins.

The first picture is of a bullet seated to just touching the lands while the second picture is with the bullet seated with a .040 jump. You will notice that even with a .040 jump, the bearing surface of the 105g hybrid is not coming anywhere near to where the donut would be.

Hope this helps you or somebody else.

Regards,

TOUCHING LANDS

.040 JUMP

 

[noload]

Is Freebore a wild guess?
I have neck length and bullet bearing surface length of my Hybrid bullet. And I don't want to seat into the donut.
Is it bearing surface length minus neck length plus desired jump ?

Its not a wild guess, you can look and a reamer dwg, (print) and it will show you what the freebore is and how it relates to your chamber.
It ( basically) is the dimension from the end of your case neck to the ogive of the bullet,when you load a round with the bullet just touching the lands, and that freebore will get longer as it erodes the lands of your barrel.
The term Freebore has nothing to do with bearing surface and neck length and jump. Freebore is a dimension that the reamer cuts when chambering your barrel.

 

[cbratcliffe]

Dixieppc, thanks a bunch for the pictures and the detailed explanation. I have also wondered about how much freebore a specific bullet would need to keep the bearing surface away from the dreaded donut. I'm getting ready to have another barrel made for my 6mm BR and I want to have it chambered to handle the burger 105 and possibly 108 VLDs and or hybrids. By reading these message forums here and on other websites it's pretty much a given that these bullets will like to be jumped, possibly up to .040. Of course the more the jump the deeper the bullet seats into the case making the bearing surface closer to the dreaded donut and that the determining factor that keeps the bullet's bearing surface away from the dreaded donut will be how much freebore you have. From your pictures and your explanation it would appear that .120 would be a good freebore for these long VLDs and Hybrids.

Charles

 

[Kings X]

No Load - this is about ordering a reamer - not after the fact. Wondering how the tool grinder determines the FB amount.
Let's take Dixieppc example using information from the Berger Reloading Manual and the Berger Quick Reference sheet found on their website:
The 6 BR Norma case is 1.5598" long with a shoulder dimension of 1.2378". That means neck length is .322" long.
The 105 Hybrid has a Bearing Surface length of .381".
So if you want to seat the bottom of the bearing surface at the neck/shoulder junction you would have .38"1-.322"= .059" of bearing surface sticking out of the neck. Which is the point at which the bullet diameter COULD be touching the lead in the throat.
Now if I want to jump .015" my freebore would be .015"+.059"=.065"
So with the .120" freebore the bullet is really seated .055" above the neck/shoulder junction to get within .015" of the throat.
Now add Charles's two other bullets with different bearing surface lengths - 108 VLD (Berger only list the 108 BT target) = .409" and the 105 VLD = .389".
So from the NUMBERS stand point - the difference from the upper bearing contact point to the lead is a wild guess.
The only drawback with extended Freebore is - less neck used to align the bullet.
I have always believed that I would rather "shoot it out than cut it out.
COMMENTS ?

 

[mao0720]

The only thing I see that you are missing, and this is the part that really makes things complicated, is the lead angle. The lead angle will change where your bullet will contact the rifling, and it is much harder to account for the lead angle mathematically. I still have not figured out a way to account for it in a mathematical solution to determining optimal freebore for a chamber.

 

[dixieppc]

KingsX, I appreciate your detailed mathmatical model. That helped me see it in another way. When I was building my BR, the decision on freebore length came up and I had no idea. I knew of a couple of bullets that I would be wanting to use and like Charles, I was aware that I wanted to avoid contact with the donut and that the laundry list of people shooting these bullets were finding the sweet spots at a jump distance. I was also aware of the inevitable and initial erosion that would come into play with a new barrel during the first rounds fired. Then a suggestion was offered that just cleared the whole thing right up. Living within 10 miles of me is a gentleman that has shot many records with the combination that I was wanting to build, so we just used his reamer which happen to have a .120 freebore. Hey, a real scientific approach, ie. if it works for him, it should work for me, right?

You finished by saying, "I have always believed that I would rather shoot it out than cut it out".

I would never do an inside neck reaming for any reason. No scientific reason, I'm just weird that way. Once I have my necks turned to give me the loaded round clearance I want, usually .002, they are never touched again beyond normal cleaning, annealing, sizing and trimming. Actually I have really never had a problem of donut interference with seated bullets. Even though my PPCs shoot flat base bullets they are never seated that deep into the neck to reach where the donut might live. And it looks like I will never have that problem with this .120 freebore 6BR either. And anyway, I don't want my seated bullets any where near a possible donut restriction because I am extremely anal about bullet seating tension and use many methods and assembly time to make sure that seating tension is consistent, especially with this long-range 6BR. I used to think that seating tension didn't matter that much with the point blank benchrest PPC's but I have found that it does.

I apologize. I got off of the subject there. Didn't mean to. I'm finding that the older I get the more I ramble.

Anyway KingX, thanks again for the math lesson concerning freebore setup. I really appreciate that.

Best Regards,
Dixie PPC

 

[Kings X]

The only thing I see that you are missing, and this is the part that really makes things complicated, is the lead angle. The lead angle will change where your bullet will contact the rifling, and it is much harder to account for the lead angle mathematically. I still have not figured out a way to account for it in a mathematical solution to determining optimal freebore for a chamber.

If you think about the bearing surface diameter is the largest of the bullet - in this example - .243". And the lead angle widest diameter is .243". So the .243" bullet touches the .243" lead angle is the freebore length. Look at a reamer print and see where the FB measurement starts - that is where the bullet touches.
But then on the other hand, I do not have the cad program to match the 1-1/2 degree angle to the ogive form of the bullet. So your point might be correct - as hard to measure.
I wouldn't have thought about the distance until Berger released the bullet dimensions. Now I can truly compare bearing surface to neck length. And the question came to mind. I hoped I could get an expert to come forward with a explanation so I could understand.
I F-class some folks are looking at using the 230 or the 215s. There is .070" difference in bearing surface length. Be aware that if you throat for 230s - you lose a lot of neck alignment when you switch to 215s.
As far as donuts go - I had them in 6.5-284, but not in 6BRDX and 7mm Walker.

 

[noload]

Kingsx, I have shot many 6br barrels with a .104 FB, I find you may have a donut problem with a 105VLD, especially if jumping. I do not jump and only shoot 105 VLDs. I just tested my throat erosion after 980 rounds shot, it moved forward .028", which now makes my FB .132". At that point I am well above any donut problems, bearing on touching and not trying to jump bullets. You will also find regardless of the FB reamer you use. If you find a just touch dimension, Your bullet will out of the case another .010" if the barrel bore is .237 instead of .236 bore.This is what I have found to be true since I use both .237 and .236 bore 6mm barrels reamed with the same reamer.

 

[Kings X]

Noload, thanks for the "hands- on" information. I am not saying that what people use won't work. Just trying to figure out how they arrived at that number.
My thinking is that someone seats a bullet in a case until it "looks right". Then sends it to the reamer manufacture to grind a reamer to match. Then that becomes a "standard" because it shot well.
I am just trying to understand where we get the initial FB dimension. Like where did your .104" come from.
Berger list your 105 VLD bearing surface to be .389". Your neck length is listed to be .322".
So if your bullet is seated to neck/shoulder junction - it sticks out .067". So you have .037" to play with before you get to the .243" diameter of the lead.
Good info about bore .001" difference in bore diameter make .010" difference in touch point.
And as Mason O pointed out - the ogive might be touching the lead before the .243 bearing surface does. That would mean the FB should be shorter to locate the bullet at the N/S junction.
I am getting to hung up on that junction point. What is important is the JAM/JUMP distance. Where the bullet sets in the neck is not important unless you think the neck aligns the bullet down the bore. Less bullet in the neck - the better chance for misalignment and uneven release.
Looking at all the reamer prints I have verses the Berger dimension - I think the FB lengths are getting too long. Everyone trying to get away from the donut and add more and more length and get less and less bullet in the neck.
This all started with a reamer design for a 300WSM. The customer want to throat for the 230 Hybrids but MIGHT shoot the 215 Hybrids. There is .070" difference in bearing surface length. So the 215s are ready .070" up the neck. The if the FB dimension was "long" for the 230s then there is even less bullet seated in the case with 215s. An expert told him he needed a .280" FB for the 230s. The WSM has a .300" long neck. The 215 has a bearing length of .450". That is .150" sticking out. To get to .280" FB - move the bullet out another .130". So now you have .170" in the neck. It helps to "jump" as that much more is pushed back into the neck.

 

[noload]

I think I understand what your original FB Guess is meaning now. The tool grinder will do what you want when it comes to grinding the reamer. I do not think so much credence should be given to alignment of bullet to bore by the neck. I believe the neck even if formed within .001 TIR is fine, but a bullet is aligned more by the diameter of the throat than the neck. Especially during the jumping of a bullet.That throat diameter is a number I would be as well concerned about. I do not get too caught up it the mathematics to the thousandths of inch with free bore dimensions. That dimension will all fall apart when the lands burnout so enough. Chances are there will be a reamer with a FB already in print form close to your needs. As long as you have enough bullet in the neck and are out of the donut zone you should be good.

 

[fm1947]

Leade
There is a short section of the chamber in front of the throat where the rifling begins. Rather than have an abrupt step from freebore to fully rifled, there is a conical transition. This is known as the leade. You'll see a few standard angles for this - often it's 1.5 degrees. Honestly, I don't know how much this matters. The way I see it, this is the very first part of the barrel to erode, and the angle will be gone soon enough anyhow, so how much can it matter? I am more than willing to be proven wrong here, but I've never seen any evidence that the leade angle matters much for accuracy, and most shooters I know do not seem all that concerned about it.

All in all, your rifle's chamber is something that is important to understand. Knowledge of your chamber's dimensions and why they matter will help you load ammunition that is more accurate in your rifle, and it will make your brass last longer.

http://bisonballistics.com/articles/an-introduction-to-rifle-chambers

 

[spclark]

Freebore is the distance from the end of the extreme forward end of the chamber (where the diameter decreases from that giving desired clearance to the neck upon firing) and where the bore diameter then begins to decrease once again at the very beginning of the leade angle cut into the lands.

Freebore doesn't include the leade. Freebore can be specified as a greater diameter than that as measured across the grooves, but can't be any less.

Leade angle will affect pressure curves: steeper means greater force is needed to 'swage' bullet jackets to the shape of the lands and grooves and may erode faster than more moderate.

Shallow (longer leade, lower slope; < 1.5 degrees) isn't commonly used for reasons I've never explored.