Freebore
- Thread starter v7wsm
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FREEBORE..In your rifle chamber where it ends (end of case neck) to where the lands just start to engrave on the bullet is a measurement called free bore. THIS IS NOT THE SAME AS JUMP. This free bore is used to determine how far the bullet ogive has to move befor contacting the start of the lands. So 0.025 of lands would be a number to use for setting bullet depth in case..MOST factory rifles have VERY excess free fore for legal reasons.0.125 or more..That is why we speck. our custom reamers..some have 0.000 freebore, 30br using BIB 118gr. as an example..You will get other replys on this topic from great shooters..
Freebore is that portion of a barrel's chamber that starts just where the case neck portion ends and continues to where the rifling begins.
Different chamber reamer profiles allow for various slopes where the rifling is to begin, the degree of which will affect bullet seating depth: the shallower the transition, the farther into the rifling any given bullet will have to be seated before encountering the lands. A chamber's throat is that portion of the chamber extending from the case shoulder up to the point where the rifling lead ends.
Throating reamers can be used independently of chamber reamers to cut a specific throat depth for a particular purpose.
Where this all matters is in selection of a finished chamber profile for a specific purpose or bullet design. In single-round rifle chambers, freebore can be customized (if the shooter feels it's warranted by experience with that bullet and shooting discipline) by starting with a short freebore then extending it as performance is evaluated.
As for headspace, that should be held to a minimum safe dimension owing to the need for cartridges to chamber without undue force (using a rifle's bolt and chamber as a sizing die is NOT recommended!) along with the tendency of brass cases to stretch to excess if headspace is greater than the minimum safe dimension. Greater stretch leads to overworked brass which weakens with every subsequent firing until cases fails.
Excessive headspace can cause failure-to-fire because a case will move forward when the firing pin drops, rather than being held in place with all the force of the firing pin going towards ignition of the primer. Something else to avoid.
Hope this helps.
Different chamber reamer profiles allow for various slopes where the rifling is to begin, the degree of which will affect bullet seating depth: the shallower the transition, the farther into the rifling any given bullet will have to be seated before encountering the lands. A chamber's throat is that portion of the chamber extending from the case shoulder up to the point where the rifling lead ends.
Throating reamers can be used independently of chamber reamers to cut a specific throat depth for a particular purpose.
Where this all matters is in selection of a finished chamber profile for a specific purpose or bullet design. In single-round rifle chambers, freebore can be customized (if the shooter feels it's warranted by experience with that bullet and shooting discipline) by starting with a short freebore then extending it as performance is evaluated.
As for headspace, that should be held to a minimum safe dimension owing to the need for cartridges to chamber without undue force (using a rifle's bolt and chamber as a sizing die is NOT recommended!) along with the tendency of brass cases to stretch to excess if headspace is greater than the minimum safe dimension. Greater stretch leads to overworked brass which weakens with every subsequent firing until cases fails.
Excessive headspace can cause failure-to-fire because a case will move forward when the firing pin drops, rather than being held in place with all the force of the firing pin going towards ignition of the primer. Something else to avoid.
Hope this helps.
Freebore is the unshaded portion in the drawing here, courtesy of John Holliger & JGS reamers. Specified as .1170" in this particular design, intended for use in semi-auto spaceguns, it's fairly generous by contemporary standards but works great with bullets in the 105-108 grain range when loaded for 600 yards & beyond.
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The main reason that people request special freebores is to shoot specific bullets in that chambering. A vld bullet will require less freebore then a tangent ogive bullet because the area of contact on the bullet in the chamber is farther back on the bullet. What the goal is of most special freebores is to get the base of the bullet up farther in the case as to not limit yourself on case cap and to get the extra MV some may need by being able to load more because of that. Some br shooters will tell you that the gas ring on the bullet should be started up the neck of the case. They will say this helps gain extra accuracy. But as a warning if your freebore is to deep then you wont have much bullet left in the neck of the case when loaded. Throats will move a bunch in the hotter calibers so this must always be considered unless you are nitriding the barrel or something similar. Then the throats move very little.
Now when it comes to headspace some people will tell you to keep it really tight but i believe that this only causes problems when you go to reload. .001-.002" is minimum when i chamber one of my guns. I never want the virgin brass to be to tight i would rather use that first firing to form them the extra couple of thou rather then deal with having to bump shoulders on the virgin brass. If you get more then .005" of extra headspace you may run into problems with the cases not staying up against the bolt face. The brass will move forward in the chamber and not come all the way back causing the primers to back out of the case head. I know that i like to set them up so im getting .002" of shoulder bump when i FL size my brass. My gun shoots better and i never have the heavy bolt lift or jamming problems.
Now when it comes to headspace some people will tell you to keep it really tight but i believe that this only causes problems when you go to reload. .001-.002" is minimum when i chamber one of my guns. I never want the virgin brass to be to tight i would rather use that first firing to form them the extra couple of thou rather then deal with having to bump shoulders on the virgin brass. If you get more then .005" of extra headspace you may run into problems with the cases not staying up against the bolt face. The brass will move forward in the chamber and not come all the way back causing the primers to back out of the case head. I know that i like to set them up so im getting .002" of shoulder bump when i FL size my brass. My gun shoots better and i never have the heavy bolt lift or jamming problems.
[quote author=Notaguru . . . A vld bullet will require less freebore then a tangent ogive bullet because the area of contact on the bullet in the chamber is farther back on the bullet. . . [/quote]
guru
It's my experience that VLD bullets require more freebore because of their long length and the desire to keep the boattail out of the shoulder area. My three VLD chambers have freebores of .300, .253, and .121. Far more than conventional chambers.
Ray
Why does my post look like this ?????
guru
It's my experience that VLD bullets require more freebore because of their long length and the desire to keep the boattail out of the shoulder area. My three VLD chambers have freebores of .300, .253, and .121. Far more than conventional chambers.
Ray
Why does my post look like this ?????
You may be right for a certain situation. But i know when i was just setting up a 6.5 for a 130 berger vld i was using about .06" less freebore then i used in the other chamberings set up for the heavy sierras. The bore diam is where they will hit which lets say a .300 for a .308 sized bullet. You have to take your measurements off of that when doing it. It just completely depends on what you setting up for. But if you take the heavy seirra's and compared the freebore to the vld bergers they will have shorter throats most of the time.
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