How is Freebore defined?

[NICKNICK]

I see that chamber reamers often have the distance to the (rifling) lands measured as a distance from the case base to the lands. For example, I believe the Palma 95 reamer for 308 Win has that distance as 2.159"

How is freebore measured/defined? When somebody says his rifle has .150" of freebore, it's .150" from the lands to what?

It seems like freebore and throat depth are talked about a lot, but I don't see a good definition.

Case base to lands makes sense, though, but I don't see that used as often.

 

[Trigger Time 23]

Freebore= distance from bullet ogive to lands of rifling.

 

[korykhunter]

^^^what he said or Weatherby

 

[ChrisGarrett]

Leade: n. That portion of a firearm's barrel immediately in front of the chamber where the bullet travels prior to contacting the rifling.

Chris

 

[damoncali]

Different people define it differently. It's the space ahead of the neck in the chamber before you get to the full lands and grooves. Specifics depend on who you talk to.

 

[NICKNICK]

Freebore= distance from bullet ogive to lands of rifling.

That's tricky because it would vary with seating depth or bullet choice and not be something constant and measurable in the rifle itself. Maybe that's how I got confused.

 

[NICKNICK]

In this reamer print it looks like the throat is .096" [reference dimension]

Am I reading this correctly that the leade begins at 2.041 from the bolt face and ends at 2.137"? The throat begins where the leade ends (2.137") and goes until 2.3374"?

Does that mean that the rifling doesn't begin until 2.3374?

So with a case base to ogive length of 2.072 (Berger 185 Juggernaut loaded to 2.865" COAL) there would be 0.265" Jump from the ogive to the lands?

 

[ChrisGarrett]

In this reamer print it looks like the throat is .096" [reference dimension]

Am I reading this correctly that the leade begins at 2.041 from the bolt face and ends at 2.137"? The throat begins where the leade ends (2.137") and goes until 2.3374"?

Does that mean that the rifling doesn't begin until 2.3374?

So with a case base to ogive length of 2.072 (Berger 185 Juggernaut loaded to 2.865" COAL) there would be 0.265" Jump from the ogive to the lands?

The leade starts at the first .3085" and ends at .2980", where the rifling begins. You can see that 1'30* note and that's the beginning of the rifling.

You can see right before the throat starts, the chamber portion of the neck area, angles inwards, like the shoulder portion.

The .3420" mark is where the neck portion ends and freebore/leade begins, until the rifling starts. The ogive of the bullet will hit that rifling and without force, will stop. That's the exact spot that you would call '0' and backing off of, or moving forward past that mark, would indicate whether you're off the lands, or into them.

Chris

 

[NICKNICK]

The leade starts at the first .3085" and ends at .2980", where the rifling begins. You can see that 1'30* note and that's the beginning of the rifling.

You can see right before the throat starts, the chamber portion of the neck area, angles inwards, like the shoulder portion.

The .3420" mark is where the neck portion ends and freebore/leade begins, until the rifling starts. The ogive of the bullet will hit that rifling and without force, will stop. That's the exact spot that you would call '0' and backing off of, or moving forward past that mark, would indicate whether you're off the lands, or into them.

Chris

That makes sense. I'm surprised at how long the distance is to the rifling. With this reamer it looks like you'd have about a .250" or more jump to the lands with most rounds loaded to magazine length. Maybe this is one of those reamers for FTR that has a long throat for people loading up 208 AMAXs and 215 Berger Hybrids.

 

[ChrisGarrett]

That makes sense. I'm surprised at how long the distance is to the rifling. With this reamer it looks like you'd have about a .250" or more jump to the lands with most rounds loaded to magazine length. Maybe this is one of those reamers for FTR that has a long throat for people loading up 208 AMAXs and 215 Berger Hybrids.

Just from your picture, I'm going to say that it's a Palma match chamber. Palma shooters in the past, had settled on the 155gr HPBTs, so no...that chamber isn't meant for the longer, heavier .30 cal bullets, although it might accomodate them.

Unless you're Remington, or fond of shooting short 130gr SMKs, a little bit of free bore is a good thing. As long as you can get a caliber's worth of bullet shank seated into the case necks, you're golden and having a little bit of wiggle room allows people to play with different brands/types of bullets.

I always laugh at the guy that has his rifle tightly throated to accept something like a 175gr SMK just touching the lands at an overall length of 2.815" lol.

Talk about a one trick pony.

Chris

 

[NICKNICK]

Just from your picture, I'm going to say that it's a Palma match chamber. Palma shooters in the past, had settled on the 155gr HPBTs, so no...that chamber isn't meant for the longer, heavier .30 cal bullets, although it might accomodate them.

I always laugh at the guy that has his rifle tightly throated to accept something like a 175gr SMK just touching the lands at an overall length of 2.815" lol.

Talk about a one trick pony.

Chris

What you mention about the one trick pony is what I want to avoid and what got me looking into this in the first place. What I want is a 308 that can shoot 175s and 185 Berger Juggernauts at magazine length (appx 2.865 COAL) but still be able to load 215 Hybrids for single-feed FTR.

It looks like the 175s and 185s would have on the order of .100" jump to the lands if the rifle was throated to have the 215 hybrid .020" off the lands with the end of the bearing surface lining up with the neck/shoulder junction.

What concerns me is that my calculations for a 185 at 2.865" put the case base to ogive length at 2.072" (Berger lists the nose length at .793".) If you're right that the pictured reamer is for a Palma chamber and not longer bullets, and its 2.337" case base to lands would require a .265" jump with a Berger 185 at magazine length, just how much more jump would be needed for a chamber throated for longer bullets?

I want to experiment and play with longer bullets, but not at the expense of losing magazine functionality with normal weight bullets.

 

[BillCh]

My one trick pony is not set up to shoot anything bigger than the Berger Juggernaut. Most of the time I shoot the 175 MK.

175 Matchking, OAL to the lands, 2.821". Bullet shank seated into the case neck .270".
185 Juggernaut, OAL to the lands, 2.898". Bullet shank seated into the case neck .265".
155.5 Fullbore, OAL to the lands, 2.935". Bullet shank seated into the case neck .169".

Just because the freebore may be a little short for the 175 MK doesn't mean you can't shoot the Juggernaut.

As a suggestion, you could get your hands on a 215 hybrid and compare it against what you have now. That would give you a relative difference. You could decide from there how much of a compromise you are willing to make.

B

 

[NICKNICK]

My one trick pony is not set up to shoot anything bigger than the Berger Juggernaut. Most of the time I shoot the 175 MK.

175 Matchking, OAL to the lands, 2.821". Bullet shank seated into the case neck .270".
185 Juggernaut, OAL to the lands, 2.898". Bullet shank seated into the case neck .265".
155.5 Fullbore, OAL to the lands, 2.935". Bullet shank seated into the case neck .169".

Just because the freebore may be a little short for the 175 MK doesn't mean you can't shoot the Juggernaut.

As a suggestion, you could get your hands on a 215 hybrid and compare it against what you have now. That would give you a relative difference. You could decide from there how much of a compromise you are willing to make.

B

BillCh--
If you've got a 185 touching the lands at an OAL of 2.898", and the tip to ogive of that bullet is .793", I'm figuring your lands are located 2.105" off the bolt face. Does that sound right/match what you expected from your reamer?

When you had your barrel chambered, what did you ask your gunsmith for? Did you give a spec for throat length, did you say you wanted a specific bolt face to land distance, did you just say you wanted magazine length 175SMK rounds to touch the lands?

 

[BillCh]

Here is how I remember it.

(Referencing the 308 Palma Match print above.)

Freebore is the 'free length' of grove diameter forward of the transition from the neck diameter to said grove diameter. This begins at 2.041 and ends at 2.137.

At 2.137 the 'throat' begins. The 'throat' is the transition from the freebore diameter to the diameter of the lands. It is an angle cut. At 1 degree, 30 minutes, it happens to correspond to a length of .2004. The bullet begins to engage the lands somewhere in this transition. The ogive of the bullet is a factor here. I think that to describe the freebore as the length to the lands is a misnomer. That length could change if the freebore remained the same and the angle were different, and/or the bullet ogive were different.

My chamber was cut with the 308 Fullbore 2011 reamer from PT&G. I have the PT&G Palma '95 too but chose the Fullbore - after a coin flip. Both have .050 freebore and a 1 deg., 45' throat angle.

I didn't ask my gunsmith. I told him. I brought my reamer and barrel blank to him and told him that I need to have it chambered and threaded in two weeks. Done and done.

I single load only, into a Savage target action. It works for me. I've thought about throating it a little deeper but with a 12 twist I don't think I'd gain anything.

So that's my story and I'm stickin' to it. Hope this helps.

B

 

[NICKNICK]

Here is how I remember it.



My chamber was cut with the 308 Fullbore 2011 reamer from PT&G. I have the PT&G Palma '95 too but chose the Fullbore - after a coin flip. Both have .050 freebore and a 1 deg., 45' throat angle.



B

Just to make sure I'm on the same page, can you tell me what your bolt face to rifling (or rather, bolt face to end of throat) measurement is for your chamber? I want to double check that I'm making sense of all the numbers.

Thanks!

 

[BillCh]

Here is a link to the

308 Fullbore 2011 reamer print.


B