Help..Throat length / Freebore / bullet jump

[Colin78]

Hey guys, I'm building a 7 SAUM on a XM length Defiance action so my max mag length is 3.2". I am wanting to take full advantage of that mag length. Now trying to decide on freebore and needing some clarification and guidance. I know typically you pick one bullet and build off it but I don't know what bullet I want to use. With today's market not being able to get anything reliably I want to have options. 4 of the top bullets I'm looking at are listed below with the COAL and CBTO measurements:
175 ABLR COAL 3.101" CBTO 2.445"
175 Elite Hunter COAL 3.101" CBTO 2.382"
180 Hybrid COAL 3.100" CBTO 2.382"
145 LRX COAL 3.100" CBTO 2.455"


So looks like the 175 ABLR and the 145 LRX are very similar in CBTO and COAL. And the 175 EH and 180 Hybrid are very similar.

Question if you set the freebore on the rifle to give zero jump (jammed into lands) on a bullet with a CBTO of 2.382" would all bullets with a CBTO of 2.382" also be jammed or zero jump? And would that mean that a different bullet with a CBTO of 2.445" not fit in the chamber without seating it 63 thousandths deeper? Or does CBTO measurement not directly correlate to freebore?

In my situation with the bullets listed above and the different CBTO's and wanting to take full advantage of the max mag length of 3.2" how would you set the freebore on this rifle and please explain why. I'm trying to understand all this.

Thanks.

 

[L.Sherm]

How are you measuring CBTO on each bullet? I've seen as much as .020 difference in same mfg and bullet between lots.

 

[Colin78]

How are you measuring CBTO on each bullet? I've seen as much as .020 difference in same mfg and bullet between lots.

Hornady bullet comparator.

 

[L.Sherm]

I would stay atleast .020 shorter than max inside mag box length, you dont want bullet tip slamming against front of mag box or a tip binding in mag box

 

[L.Sherm]

Hornady bullet comparator.

They measure way up on the ogive, so its gonna be different in relation to how far off the lands each bullet will be.

 

[Colin78]

I would stay atleast .020 shorter than max inside mag box length, you dont want bullet tip slamming against front of mag box or a tip binding in mag box

I am. Max mag length is 3.2" I'm seating them COAL 3.1"

This thread is more on freebore and CBTO.

 

[L.Sherm]

A smith who has the reamer should have some insight or JGS would be alot of help.

 

[Colin78]

They measure way up on the ogive, so its gonna be different in relation to how far off the lands each bullet will be.

Ok. Do you have any recommendations on how you would set the freebore on this rifle with the given bullets / measurements/ and a max mag length of 3.2"?

 

[Colin78]

A smith who has the reamer should have some insight or JGS would be alot of help.

Yep, my Smith is looking into it. Just thought I'd ask on here. I know there are lots of people on here with a ton of knowledge and experience.

 

[L.Sherm]

Yep, my Smith is looking into it. Just thought I'd ask on here. I know there are lots of people on here with a ton of knowledge and experience.

Its gonna be a tough one guessing for 4 different bullets. I know berger has bullet dimensions on there website so with those it could help.
JGS is a wealth of information they may be able to tell you chances are they have built a reamer for all or most of your bullets

 

[L.Sherm]

Lance Kenyon would be one to P.M he has shot alot of the 175's and 180's in the 7mms.
My guess would be in the neighborhood of .180 freebore for the 175-180's

 

[Colin78]

Lance Kenyon would be one to P.M he has shot alot of the 175's and 180's in the 7mms.
My guess would be in the neighborhood of .180 freebore for the 175-180's

Thanks.

 

[bigngreen]

With the bullets you chosen the Bergers will shoot better closer to the lands and the LRAB and the LRX typically need a good bit of jump, I would consider that. I always just set up a couple dummy cases then send to ADG and when they get back to me with measurements based on the throat diameter and lead angle then I choose a freebore to cut.

 

[L.Sherm]

You will be way past the case nk- shoulder junction on any of the bullets you mention at 3.1. Seating at the bearing surface boatil junction.
According to bergers dimension with there 180 VLD at boatail bearing surface- nk- shoulder junction on brass your OAL is 2.987 with the 175 Elite hunter its 3.02.
I know the 180 VLD isn't the hybrid but thats all they listed in there dimensions, I couldn't imagine it much longer than the 175 elite