You don't want anything longer than .180" on that XM action.
Kimber's FB will be around .140" which really is ideal to me.
Kimber's FB will be around .140" which really is ideal to me.
Help..Throat length / Freebore / bullet jump
- Thread starter Colin78
- Start date
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Here's my math with the 175 elite and 7 saum.
Base to nk shoulder junction 1.724
OAL BULLET. 1.502
Boatail. - .207
= 1.295
OAL cartridge. 1.724
+ 1.295
= 3.019
Freebore to fit
Base to ogive. .762
Boatail. - .207
= .555
Neck length. - .324
Freebore past nk = .231
So with .231 freebore your OAL would be 3.019 with 175 Elite hunter.
Base to nk shoulder junction 1.724
OAL BULLET. 1.502
Boatail. - .207
= 1.295
OAL cartridge. 1.724
+ 1.295
= 3.019
Freebore to fit
Base to ogive. .762
Boatail. - .207
= .555
Neck length. - .324
Freebore past nk = .231
So with .231 freebore your OAL would be 3.019 with 175 Elite hunter.
Rhett Crider
Well-Known Member
As Longtine said, you will gain free bore over time shooting. I always give my smith dummy loads when chambering. If you plan on loading Berger hybrid ogives and Nosler ABLRs, then I would go with the shorter free bore. 0.070 bearing surface behind the junction is not critical enough to me. Besides that, your smith can always add free bore/ throat but they can't take it away. Just make sure it's 1.5* leade angle on the reamer. I also usually find Bergers hybrid sweet spot around 0.020-0.025 jump. Hunting rifles, not comp but achieve sub 1/2 MOA also.
Are you having a reamer made or are you having the gunsmith cut the throat to length based on your freebore wishes?
You should just make some dummy rounds with all of the bullets you plan to potentially shoot and give them to the gunsmith to work off of or send them to the reamer manufacturer. They should know what to do with all of that.
For what its worth I just finished load development on virgin brass with the 180 hybrid and I settled on 2.355 as my cbto using my hornady ogive comparator. Your ogive comparator and my ogive comparator will not reliably give the same measurement unless you and I have compared our comparators against the same standard, ie. the same loaded case.
My cbto on those rounds was ranging between 3.088-3.095 from the same box of bullets. Im hoping to work on increasing velocity with further load development, however I had my reamer freebore set to what my loaded round was plus allowing for a specified 0.02 jump. It ended up being 0.221. That reamer is set to shoot 180 eldm's at the neck shoulder junction and has an extra .02 added for the 2 thou jump everyone always talks about. The 180 berger hybrids shoot bugholes and are jumping a really long ways. The eldm measures 2.402 cbto + .02 = 2.422. That means on paper the hybrid is jumping circa 0.067.
However a caveat is that measuring bullet cbto to either jam or touch on your lands will not be the same actual measured distance to the lands on paper. The shape of the ogive will determine how the bullet fits and a stumpy shaped bullet will not touch at the same point as measured compared to a sleeker bullet. Case in point the 180 eldm is fatter up higher on the nose at the ogive vs the slender sleeker nose of the 180 hybrid, yet they are very near the same in bullet oal length compared to their bto measurements. Think of it this way, if you have a 2 inch hole and you measure how far down into that hole a tennis ball will sit compared to a beach ball, the tennis ball will sit deeper than the beach ball due to the size of the balls. I hope that made sense.
I guess my point is .02 bullet jump is not gospel. It works a lot, but its not the only way. You can always tune seating depth regardless of the amount of jump a bullet has to make, even if it is a lot. The 180 eldm and the 180 hybrid I mocked up will both fit within your 3.2 mag box. Dont be afraid to cut a longer throat than your magazine can accommodate, but your seating depth may need to be reduced slightly in order for you to tune your load to your barrel either way you do it.
Here is a picture of the 3 shot group I shot with the 180 hybrid yesterday from my 7 saum at 100 yards. I think that flyer on the top was my fault.
You should just make some dummy rounds with all of the bullets you plan to potentially shoot and give them to the gunsmith to work off of or send them to the reamer manufacturer. They should know what to do with all of that.
For what its worth I just finished load development on virgin brass with the 180 hybrid and I settled on 2.355 as my cbto using my hornady ogive comparator. Your ogive comparator and my ogive comparator will not reliably give the same measurement unless you and I have compared our comparators against the same standard, ie. the same loaded case.
My cbto on those rounds was ranging between 3.088-3.095 from the same box of bullets. Im hoping to work on increasing velocity with further load development, however I had my reamer freebore set to what my loaded round was plus allowing for a specified 0.02 jump. It ended up being 0.221. That reamer is set to shoot 180 eldm's at the neck shoulder junction and has an extra .02 added for the 2 thou jump everyone always talks about. The 180 berger hybrids shoot bugholes and are jumping a really long ways. The eldm measures 2.402 cbto + .02 = 2.422. That means on paper the hybrid is jumping circa 0.067.
However a caveat is that measuring bullet cbto to either jam or touch on your lands will not be the same actual measured distance to the lands on paper. The shape of the ogive will determine how the bullet fits and a stumpy shaped bullet will not touch at the same point as measured compared to a sleeker bullet. Case in point the 180 eldm is fatter up higher on the nose at the ogive vs the slender sleeker nose of the 180 hybrid, yet they are very near the same in bullet oal length compared to their bto measurements. Think of it this way, if you have a 2 inch hole and you measure how far down into that hole a tennis ball will sit compared to a beach ball, the tennis ball will sit deeper than the beach ball due to the size of the balls. I hope that made sense.
I guess my point is .02 bullet jump is not gospel. It works a lot, but its not the only way. You can always tune seating depth regardless of the amount of jump a bullet has to make, even if it is a lot. The 180 eldm and the 180 hybrid I mocked up will both fit within your 3.2 mag box. Dont be afraid to cut a longer throat than your magazine can accommodate, but your seating depth may need to be reduced slightly in order for you to tune your load to your barrel either way you do it.
Here is a picture of the 3 shot group I shot with the 180 hybrid yesterday from my 7 saum at 100 yards. I think that flyer on the top was my fault.
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Im not saying you are wrong on bullet jump tendencies and where they like to be, but for information relative to the discussion, my brothers 7saum has a .180 freebore and his most accurate cbto using the 145 LRX is 2.310 and my .221 freebore shoots the 180 hybrid at 2.355 cbto.
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I don't doubt this a bit. But your brothers cbto may be the 1st accuracy node, 2nd, etc. unless one starts from touching or jamb everytime you don't know where it is. There's fining to be a accuracy node every so often. And those nodes that are the deepest seated will generally be the most consistent and broadest node. LRX bullets want to be seated deep I found when they first came out. Deeper ther better. Even found it was better to crimp them also.
When I said typically they like close to the lands it was first node off lands. There will be a few nodes further out into the lands also but your getting into the danger zone.
I like the one furthest out without touching lands for powder purposes.
Hawk in WY
Well-Known Member
I went through this exercise with a 7 SAUM.
Once I factored in minimum bullet length in the case, higher velocities from shorter COAL, and optimized load data at shorter and longer lengths, I decided it wasn't worth rethroating for longer COAL even though I had room in the magazine box.
My contribution to this discussion is longer is not always better.
Once I factored in minimum bullet length in the case, higher velocities from shorter COAL, and optimized load data at shorter and longer lengths, I decided it wasn't worth rethroating for longer COAL even though I had room in the magazine box.
My contribution to this discussion is longer is not always better.
Yup. I just wanted to point out that there are more nodes to work with on seating depth than just the .02, etc... and not to pull the plug on setting up to shoot one bullet over the other because there is a freebore conflict with another.
I just went through this process on a 7-08 build. I gave 3 bullets to the Smith and we came up with what was best. Many of the bullets with a Hybrid ogive like to jump and don't need to be seated at max length. I run the 144 out of my 65PRC at .070 off.
While that may generally be true not in all cases. Ive got them that will shoot best at
.010 off and some at .050 off.
Every gun and barrel is different, what i want is the option to have a bullet at the lands or jump. There is a happy medium.
Agree, went through this process also with a 65 PRC AW2 built on a defiance long action. Decided on the 144 Berger after numerous testing also with 131 Hammer Hunter and the 156 Berger. I found good accuracy at .020-.030 off but always had pressure spikes quickly and was disappointed with MV. Slowly backed off the lands and found great accuracy with the Bergers and decided eventually on the 144 Berger. Best result showed up about .070 off the lands and no pressure. Running around 2975 out of a 22" 7.5 twist Proof, sub 1/2 MOA. For me, lately, I've had my best results with jumping further. Also had a friend recently hitting pressure fast but having great accuracy with the 143 ELDX at .020 off. Started backing off and in .005 increments and found a great combination between .045-.050 off, pressure gone and increase powder charge with increased MV. Then I have my 65 CM I run the 140 Berger .010 off and it's sub 1/2 MOA. So I completely agree with you about every barrel and gun is different. Was just sharing my recent experiences with the hybrid Ogive.
I can attest. Seating depth nodes are wider as you get further off the lands. Throat erosion has less affect on chamber pressure further off the lands as well. I will say though, the absolute best accuracy is usually found with 10k jam to 30k jump. This node will likely be only 003 thou wide, and it takes a lot of notes and tuning to keep it shooting there. But these are hunting guns. Half moa accuracy at distance will get the job done. No real need to persue 1/4 moa unless you desire to do so. Also bench accuracy and field accuracy are usually two different things, make sure you're half moa in field positions with proper recoil management to spot your shot for a follow up correction if needed. Anymore I throat to long freebore and jump my bullets a fair amount for consistency and forgiveness in reloading.
