Freebore Discussion

[cajun]

Interesting article.
https://precisionrifleblog.com/2020/04/05/bullet-jump-load-development-data/

 

[bigngreen]

Some questions about this. Was finishing up a 300 WSM rifle build and debated about a SAAMI vs custom freebore. It seems the freebore physics is more complicated than simply pushing the bullet out gives more space for more powder. My question is have any of you fired the same powder, same bullet combo with a long freebore vs a SAAMI chamber and seen the difference?? Seems logical that there would be; however the only article I've ever seen testing the theory seemed to refute the adage.

Yes, I've played with the 270 and 300 WSM and throating, their original sammi design is too short and makes loading more finicky and you'll see spikey performance, this is the reason most run a longer throated reamer, PTG uses a WSM II reamer just adding enough to let it mellow out a bit. I would never build a WSM with it's SAMMI throat.

 

[nksmfamjp]

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.

That doesn't fix the mag length issue. 6.8 western is the fix, but velocities are so so. Or 6.5 PRC. Better, but small.

Meh! Just go long action 280 ai, 300 Sherman or 7 PRC at that point!

 

[VinceMule]

Maybe but I have 2 rifles that have a cavernous freebore, yet they shoot 1/4 & 1/2" groups.

My 257 Wby has .375" of freebore and I don't even know how much my .308 has. I seat 175 VLDS at 2.950 and still have .055" to the lands.

We have to remember that freebore grows from the first shot fired, in some cases more than others. Say you work up a load when the barrel is new. 500 rounds later, the leade has grown over .100. With the long freebore, you can not keep seating the bullet out due to mag length. When the barrel was new, you worked up a load, you can chronograph that load. As the freebore wears, you lose velocity, then bump up the load to the same velocity as it was when the barrel was new. Often, you can gain back accuracy, but to what degree is unknown.

Experienced long-range rifle smiths can offer great advice on this issue. Best to know the bullet you intend to shoot. The one twist and one throat to do it all from elephants to coyotes is a poor approach, indeed.

 

[Muddyboots]

Freebore for .270Win or 30-06 built in LA with sufficient magazine COAL are much different cartridges as well. Rem700LA's have really good magazine COAL 3.65" so longer heavier higher BC bullets can be used with some of the newer powders. Monos can really benefit from freebore.

This is .270 with 156HH that barrel got 0.290 freebore added. Shoots well with RL26. You can do whatever you want with freebore to maximize a specific bullet performance or range of similar bullets. Why wouldn't you max out the rifle's capacity based on magazine COAL?

 

[hemiford]

Question about terminology:
As I understand it, "freebore" is simply the distance the projectile moves out of the casing before it hits the lands.
When it hits the lands, the base of the projectile is usually still in the casing.
But this is not always the case. A scenario can exist in which the projectile is totally out of the casing
but not yet reached the rifling. There is still a "freebore" distance, but is there an additional term for
this scenario, or this bullet travel distance ? "Free Floating" or something ?

 

[nksmfamjp]

Why wouldn't you max out the rifle's capacity based on magazine COAL?

I've had guns that didn't shoot well with the bullet too far down the neck. Those were usually hunting rounds.

My 6 PPC likes to be way out in the throat, not sure why…

 

[Zen Archery]

I didn't read what end result was. But out of a different caliber I did the test you mentioned. Running one bullet .015 off lands and SAAMI. Accuracy remained under .65 MOA. Velocity and pressure obviously increased with SAAMI case specs.

If pushing caliber and bullets maximum distance it's worth fine tuning. For my purpose hunting under 500 yards I quit using all the long range loading methods. Keeps things a lot simpler, save a ton of components, and accurate enough to hunt critters in my range.

 

[Mike D Texas]

Yes and yes.

I have used two different reamers for the 300wsm one is .280fb the other is .215fb....the .280fb makes more velocity with a .025 jump and takes more powder than the .215fb jumping .005 using a Berger 215hybrid.

I also used two different reamers in a 284Shehane....one was a bore rider with .323fb for the 190Atips the other was a .235fb for the 180/184 Bergers. I ran the 184's in both chambers. The bore rider with the long freebore obviously made more velocity with less pressure but more powder. No way could I run the same powder charge I ran in the .323fb in the .235fb it would have locked the gun up.

A great example of this is Weatherby chambers. Other than the RPM cartridges all of them have a lot of freebore and typically run higher velocities than traditional freebore lengths.

 

[Ray Gross]

Question about terminology:
As I understand it, "freebore" is simply the distance the projectile moves out of the casing before it hits the lands.
…

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.

 

[Mikecr]

As the freebore wears, you lose velocity, then bump up the load to the same velocity as it was when the barrel was new. Often, you can gain back accuracy, but to what degree is unknown.

I have not found this to be the case, given certain qualifiers.
With small/mid capacity cartridges I find that MV with initial seating at least 10thou OTL does not change with land erosion.
So If my tested best CBTO had me 10thou OTL initially, that CBTO remains best for the accurate life of the barrel, regardless of changing land relationship.

Given this, I do not prefer seating inside 10thou off.
I'll choose an initial seating that does not lock my powder load into dependency of higher starting pressure (from closer to land seating).
I just don't & won't chase lands.

 

[VinceMule]

Ok, on a new barrel, let's say you have a Cartridge base to Ogive at 2.000. As the barrel is fired hundreds of rounds, let's say now your CBTO has grown .070 at .2070. While you can not measure it, the throat dia has also grown for some distance down the bore. The use of a Grizzley rod with a set of reamer pilots that vary .0002 in dia is an easy tool to measure throat/barrel dia with. The dia will grow over the life of the barrel for 10-14 inches depending on the cartridge and the heat index of the powder used.

As the barrel wears, the bore does not seal the bullet in the barrel as tightly/quickly as it did when the barrel was new, there is a loss of pressure and velocity. Sometimes, You can add more powder in some cases to bring up the pressure to where it was when the bbl was new, and maintain that velocity where the barrel liked the harmonics...this is critical in your thinking. Bores vary in diameter, and I wish gunsmiths would stamp on the outside of the barrel which dia reamer pilot they found "fit" the barrel when they were chambering.

If you found the barrel to be the most accurate with a bullet jump of .010 when new, accuracy can be maintained by seating the bullet out maintaining that .010 jump, for as long as you can with the barrel being serviceable with Mag length. Some bullets may like a .120 jump when new, you never know. Often, you may be forced into using the rifle as a single shot to get that 2" accuracy at 600 yards.

It is amazing at how many Remington 700's in 7 Mag, 7/08, and 280 Rem shoot very tiny 3-shot groups, less than 3/8", with the bullet just barely kissing the lands. Bullets have to be sorted by ogive length, and Nosler 120,140, and 150g Ballistic tips shoot extremely tiny groups, so do the 175g Nosler long-range accubonds .003 off the lands.

I have Wyatt's extended mag boxes installed in all of my Remingtons which gives me another .125 in length, and they do offer a 4.000-length mag box. The action has to be milled out on the bottom and the bolt stop cut back to accommodate the extra bolt travel to pick up the back of the cartridge case.

The Heat Index of the powder you shoot is a major factor in how the bore Wears in dia. and land height taper. We are used to seeing pics from a Teslong on bore pitting, seeing a picture of bore dia wear, graduated in .0002 is a game changer. With Teslong, create a movie or pics of a barrel, save that barrel's pics on a single memory stick, and watch the erosion over the number of documented shots...barrel history.

With some cartridges, the Base to ogive is so long that they can not touch the lands on a brand-new barrel while using the magazine... and this is one heck of a mess to deal with using VLD type of bullets...sometimes you get lucky.

I have my own 300 yd rifle range, and shoot out of a heated AC house, reloading on the spot.

 

[VinceMule]

Muddyboots, your question:

"Why wouldn't you max out the rifle's capacity based on magazine COAL?"

COAL grows from the first shot fired, if you max out the mag length on a new barrel, you are in effect cutting your barrel life.

Depends on how much you shoot, throats do wear. If you only shoot 20 rounds a year hunting and checking zero, then barrel life will be a very long time, perhaps a lifetime given proper maintenance.

Hammer Hunters will have their own characteristics on how they will shoot sub 1/2" either jumping or demanding to be seated near the lands. Extending the throat to the length you have, I would certainly buy up enough bullets for the barrels' life.

 

[Mikecr]

let's say now your CBTO has grown

COAL grows from the first shot fired,

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

Our seating testing leaves us at some CBTO, which happens to be at some land relationship(at the moment), but if well off the lands, it doesn't mater what that relationship is, was, or will be. There is no need to log or even know land relationship. Just log tested best CBTO.

You can choose to seat in the lands, and your powder load might actually need it, but that's a choice.
Doing so could lock you into chasing the lands for results, which is pretty close to chasing tail with a barrel burner.
Otherwise, you can test for best CBTO, log it, never change it.