Calculating OAL and jump?

[entoptics]

Thinking of a rebarrel, and wanted to get a rough idea of OAL, as my magazine is rather short for a long action. I plan to do this math for a few prospective bullets, and determine if my magazine box will accommodate all of them before deciding to proceed in the build.

Went to the SAAMI drawing for 264 mag (here, page 82). I believe the bolt face to rifling measurement is 2.7118 + 0.015 for a min of 2.7118" and a max of 2.7268".

I assume the bottom measurement in this drawing is the theoretical "bolt face to rifling" (different caliber, but you get the point)

I found some measurements posted online for the 147 ELD-M...

BTO = 0.795
OAL = 1.432

So...Assuming the chamber is properly cut to SAAMI, to calculate the maximum overall length I'd need to touch the lands in a loaded round, I simply need to add "ogive to tip" and "maximum bolt face to lands" right?

Ogive to tip, I subtract BTO from OAL (1.432 - 0.795) and get 0.637, add that to "max bolt face to lands (2.7268), and I arrive at 3.3638 cartridge overall length.

Sound right?

Problem is, I tried to check this with my 300 WM, and the numbers don't match up with reality. Same calculation gives me 3.696 for a 208 ELDM, but actual measuring of the OAL with a Sinclair gauge (and "mark the tip" and "will it close") gives me 3.666 currently. This barrel has over 1000 rounds through it (early OAL measurements were ~3.60), and has clearly lost a lot of throat.

I'm assuming I'm using the wrong dimension in my calculation, or missing something else, as it seems unlikely my Shilen was built ~0.1 shorter than SAAMI spec.

Thoughts?

 

[Mikecr]

Your thinking should get you close. It wouldn't approach exact until breaking out the rocket science of leade angle - vs- ogive radius.
Barrel finishing is also not exact to anything unless ordered to something specific. That's why your chamber is unlikely to match another (while possible). Headspacing is critical to chambering, always a focus, but there are limitless freebore & throating options, for all kinds of bullets in mind, and anticipation of throat erosion rates.

It's in an aftermarket barrel maker's best interest to boost barrel life(if only perceived) with a bit shorter freebore. We'd be able to endure this much more erosion before he hears concerns about customers unable to reach lands.
I've ordered specific freebores which were as short as possible while keeping intended bullet bearing short of neck-shoulder junction by 1/16".

 

[Edd]

Went to the SAAMI drawing for 264 mag (here, page 82). I believe the bolt face to rifling measurement is 2.7118 + 0.015 for a min of 2.7118" and a max of 2.7268".

I assume the bottom measurement in this drawing is the theoretical "bolt face to rifling" (different caliber, but you get the point)

2.7118" is the distance from the bolt face to bore diameter.

The distance from bolt face to bullet diameter is 2.5973".

 

[entoptics]

2.7118" is the distance from the bolt face to bore diameter.

The distance from bolt face to bullet diameter is 2.5973".

I'm not seeing that measurement on the diagram. How did you come about it?

 

[Edd]

I'm not seeing that measurement on the diagram. How did you come about it?

You have to calculate it using the information that "is" shown.
The distance it takes to go from a .268" diameter to a .264" diameter at a 2° angle is .0573".

Considering the current choice of 6.5 mm bullets, I think it would be a mistake to build a new 264 Winchester Mag to SAAMI specs.

 

[entoptics]

You have to calculate it using the information that "is" shown.
The distance it takes to go from a .268" diameter to a .264" diameter at a 2° angle is .0573".

Considering the current choice of 6.5 mm bullets, I think it would be a mistake to build a new 264 Winchester Mag to SAAMI specs.

Ahhh. A little trigonometry.

My mag box is only 3.5" in the Savage I'm considering rebarreling, so I want to make sure I can approach the lands at that OAL. It's currently a 300 WM Shilen, and mag length is over 1/8" from the lands for most bullets.

I haven't done the calculations for other 0.264 bullets yet, but based on the 147 ELDM, I shouldn't have an issue, so I may opt for a bit more throat if that holds up for other bullets.

 

[Edd]

Ahhh. A little trigonometry.

My mag box is only 3.5" in the Savage I'm considering rebarreling, so I want to make sure I can approach the lands at that OAL. It's currently a 300 WM Shilen, and mag length is over 1/8" from the lands for most bullets.

I haven't done the calculations for other 0.264 bullets yet, but based on the 147 ELDM, I shouldn't have an issue, so I may opt for a bit more throat if that holds up for other bullets.

By "more" I assume you mean longer. If you want it longer, you are going to need to change the throat style. Hornady has already figured this out. Just put a 6.5 PRC throat on the 264 chamber.

 

[Mikecr]

The SAAMI drawing doesn't make sense. My Pacific prints show chamber end - Leade - throat. The referenced drawing shows chamber end at 2.5400", and either leade only to 2.7118", or throat only to that(with zero leade).

But there has to be leade, because the SAAMI cartridge drawing above chamber has bullet bearing to 2.710", exceeding your proposed 2.5973".

I captured to post it here:

 

[entoptics]

The SAAMI drawing doesn't make sense. My Pacific prints show chamber end - Leade - throat. The referenced drawing shows chamber end at 2.5400", and either leade only to 2.7118", or throat only to that(with zero leade).

But there has to be leade, because the SAAMI cartridge drawing above chamber has bullet bearing to 2.710", exceeding your proposed 2.5973".

I captured to post it here:
View attachment 153390

I am confused here as well.

I did the trig on (0.268 - 0.264)/2 = 0.002 @ 2°, and got 0.0573" as well. The diagram shows that transition as 2.7118-2.5400 = 0.1718.

Am I correct that Throat = Freebore + Lead?

In the 264 SAAMI diagram, it appears this is all combined into one cone, not a cylinder (freebore) with a cone (lead). The math still isn't right though. I'm assuming the diagram should actually be drawn with the 2° lead starting from the lands/grooves at 2.7118 traveling backwards 0.0573, then becoming a straight horizontal line from 2.6545 (2.7118-0.0573) back to 2.5400.

In this scenario, bolt face to lands = 2.7118", throat is 0.1718", freebore is 0.1181", and lead is 0.0537".

Does that make sense?

I suppose this is all a little irrelevant to my OP, as using the longest dimension will ensure I can fit bullets in the mag box, and still approach the lands, as any other calculations mean shorter cartridge OAL.

I'd still like to sort it out though, as I'm just a curious sort...

 

[Mikecr]

I would drop SAAMI from consideration and focus on your barrel finisher's reamer.
Or, fill out a reamer print and order it how YOU WANT IT. That's what I do & I hold the reamers for every barrel finished for me.

 

[Edd]

The cone throat is common on Winchester cartridges. Some others use it too.

I would do what Mikecr says, get a reamer made the way you want it. Another option is to have someone that doesn't use SAAMI reamers, like Greg Tannel, chamber it.

 

[entoptics]

I would drop SAAMI from consideration and focus on your barrel finisher's reamer.
Or, fill out a reamer print and order it how YOU WANT IT. That's what I do & I hold the reamers for every barrel finished for me.

This is a "budget" build most likely, with a pre-fit Savage from someone, so if "SAAMI" will work for me, that's the ideal scenario. Going with a full custom reamer is probably not an option, though I know many makers will "throat to your dummies" for little or no cost.

The cone throat is common on Winchester cartridges. Some others use it too...

Still doesn't explain how the SAAMI geometry is so goofy though. A 2° slope for the distance they quote (0.1718") would give you a 0.256" bore.

Edit: I think I just solved my own question. The drawing has the "tube" as the bore diameter of 0.256 and not the lands diameter of 0.264...

So in the case of the "cone throat", the Throat = Freebore = Lead. I think I got it now.

 

[Mikecr]

"throat to your dummies" pretty much solves it, for that barrel.

I see you getting 2.7118-2.5400 = 0.1718, adding chamber end transition: 2.7118-2.524 = 0.1878
But look at exposed bullet bearing for the ammo above: 2.710-2.500 = 0.2100, if you seat deep to -0.020 tolerance the bearing is still 0.1900
No big deal, seat deep by 0.1900-0.1878 = 0.0222", but be sure to check your actual intended bullet. 26cal bullets are especially long.

That is a horrible chamber design IMO. The free bore is so much larger than cal, your bullets are just swinging in the wind on neck release. There will be a great deal of gas flowing around the bullet, cutting away at the throat and weakening bullet jacket and core connection integrity. Could end up with random bullet blow ups, and low barrel life.