Freebore Discussion

[VinceMule]

"CBTO do not 'grow'. They're set by your bullet seating"

Ogive Length is Fluid, and changes over time, due to firing the rifle.

I am referring to Growth in the leade the barrel is shot over hundreds of rounds, but some cartridges have their leade grow in 50 rounds or less. Throat dia grows over time also, and in combo with leade growth, there is a loss in velocity. How much of a loss is a factor of many things, including initial bore dia. Velocity is a huge factor on the harmonics of a barrel. Ever see groups turn into patterns when the weather drops 60*? Harmonics has changed the accuracy node with the velocity drop.

I am looking for extremely small groups on hunting rifles over the life of the barrel, and accuracy is a subjective thing, from one shooter to another.

Obviously, lots of ways to skin a cat.

 

[Mikecr]

I know what you mean, but you have your terms messed up.
Ogive, or ogive length, or entire length of a bullet nose, does not change. It's bullet build, independent of a barrel.
Cartridge Base To Ogive (CBTO) is set by cartridge build, and is independent of a barrel. Same with COAL.

Your seated distance to lands, land relationship, is a different attribute that really never matters to me.
The only thing I check in this regard is Max CBTO with a brand new barrel, just for a baseline of seating testing.
I don't really care what that comes out to, and it's rare that I ever check to see how much it changed later.

What matters is doing actual full seating testing (like Berger recommended) to find best CBTO, to go into powder development with.
While off the lands, it doesn't matter where best CBTO puts land relationship (which will constantly change), as best CBTO doesn't change.
It will hold for the accurate life of a barrel -for that bullet.

Your lands may erode 5-6" down the barrel as you go, but it doesn't matter. the gun may still shoot just fine.
Throat erosion is not what ends accurate barrel life.

 

[Rich Coyle]

What ends accurate barrel life?

 

[Mikecr]

What ends accurate barrel life?

An early constriction [carbon constriction].
Carbon diffusion into steel, at gas phase change points.

 

[nksmfamjp]

Carbon diffusion into steel, at gas phase change points.

Can you describe this better? Are you talking about carbon deposits which we clean out of the barrel or something else also, is carbon changing or product of combustion?

 

[Mikecr]

I'm not talking about a buildup, although any buildup(carbon, moly, copper) or late rough patch can kill accurate barrel life -if it's reached a point where fixing it would kill the barrel anyway.

I'm talking about carbon surface/subsurface fusing with metal, leading to a very small constriction some length down barrels.
This, causing extruding of passing bullets, then looser in bores.
I base this notion on testing performed by someone a long time ago. They used a 6PPC and an air gauge to see it.
While I haven't validated it myself (and pre-internet, no link to it), I have seen that it passes all tests otherwise. All of us can see it, and nobody has escaped it with standard barrel steel.

It's important to remind that this is not barrel life that I'm talking about, but accurate barrel life (big difference).
You can go right past accurate barrel life, if you never reached a peak accuracy to begin with, and never know it.
This is a condition behind so many wild claims of barrel life.
For instance, if your barrel is capable of 1/4moa, but your best has been 3/8 to 1/2moa and you're happy with that, you will not know when passing 1/4moa capable until passing your 1/2moa capability. That could be a long ways beyond accurate barrel life.
If content with 1moa, then I imagine some guns, like a once competitive 6PPC, could shoot at least that forever!

People adjust to extend accurate barrel life, but it never lasts, and the worsening accelerates.
We're not adjusting the root cause.

 

[nksmfamjp]

This, causing extruding of passing bullets, then looser in bores.
I base this notion on testing performed by someone a long time ago. They used a 6PPC and an air gauge to see it.

Thanks. Did they confirm the effect on the bullets? Seems like one of the abrasive bullet systems could help restore accurate barrel life?

 

[ARlife4me]

The creedmoor cartridges are designed for the boat-tail in the shoulder and still have a long freebore. The 22-250rem is almost the same as the 22creed in terms of case length (shoulder angle being greater), but the 250 has less freebore and built for varmints (light weight bullets per caliber). I don't remember the guy's name, but I believe he is somewhere in Indiana or near state. He discusses freebore/throat and chasing the lands. From his testing (say 20k off) and shooting X amount of times then remeasure throat and how it has changed/grown (say 22k off now). He does this several times and measures keeping track of growth. Then he test with (say 120k off) and goes thru the same test procedures. After the test the conclusion is that it's best to stay farther off that keep chasing the lands. you can shoot more without making so many changes. I'm not sure if it was from this forum or another that someone had posted? I don't remember if the person doing the test is a gunsmith/prs or just someone who has shooting classes?

 

[FEENIX]

I'm not sure about that, but I don't really know. A tight throat pushes bigger bullets or longer mono bullets deep in the case.

I prefer to seat a bullet where the base of the bearing surface is at the base of the neck and 0.030" off the lands….well, I can dream!

Agreed! Weatherby cartridges are known for long freebore and do not seem to have any issues throughout the years.

You can absolutely seat a bullet wherever you want to if you design the cartridge, seat a bullet where desired and have your own reamer made.

I've done it with 5 cartridges.

Indeed! And sometimes, you get lucky with the right load combination. I am loading my .257 WBY .011-.015" off the lands with 145 BH with excellent success.

 

[epoletna]

Just speculating here, but I wonder if your average WBY rifle gets fired fewer times than other common cartridges. For example, hunting vs. banging steel or shooting paper?

 

[Buckys]

I believe you are describing 'jump'.

Freebore is a cylindrical section of the throat between the chamber neck and the Leade. The bullet moves freely through it because it is (usually) slightly over bullet diameter.

When I measure distance to the lands using a Hornady gauge in a Weatherby chamber, and I am gentling tapping the projectile forward, it stops at the beginning of the freebore. If I tap a smidge harder, it keeps moving forward towards the lands.

This gives me a pretty good read point on the distance to the freebore and the delta distance to the lands (which is not precisely the same as the length of the freebore).

It also gives me a really good feel for how much wiggle room that freebore allows the projectile to lose concentricity with the chamber/barrel (it is really, really tight).

 

[Ray Gross]

When I measure distance to the lands using a Hornady gauge in a Weatherby chamber, and I am gentling tapping the projectile forward, it stops at the beginning of the freebore. If I tap a smidge harder, it keeps moving forward towards the lands.

This gives me a pretty good read point on the distance to the freebore and the delta distance to the lands (which is not precisely the same as the length of the freebore).

It also gives me a really good feel for how much wiggle room that freebore allows the projectile to lose concentricity with the chamber/barrel (it is really, really tight).

you may have a carbon ring constricting the throat that you are tapping past

 

[Buckys]

you may have a carbon ring constricting the throat that you are tapping past

Agree that that is a possibility. I don't think that is the case but I only have a cheapo amazon scope to peek inside there with.

It is a custom rifle and I'm the second owner so the other possibility is that the reamer was set towards the snug side of the specs.

But I suppose my point is that the tight tolerance of the freebore of the weatherby's has produced a lot of accurate guns. Can't (maybe I should not use that word, oh well) get to the lands but I don't lose any sleep that that freebore is costing me accuracy because the bullet is bouncing around.

 

[std7mag]

Too long of a freebore can be a negative at times.
Take the SAAMI 284 Win for example.
Due to the rifles it was originally chambered for the freebore is rather long.
It's a hit & miss (so to say) as to which bullets will shoot well from it.
Especially in the 120-160gr bullets that i shoot in all my other 7mm's.
I could jump up to the 180gr or heavier class, but i'd rather have a 30-06 for that.

Shooting the 140gr & 168gr Berger VLD just doesn't group well at all with the large jump.

At least i haven't been able to get them to do so.