Custom throat?

[TxAggie94]

If you have the option, do you have a chamber custom throated for a hunting rifle? And if so, what do you specify?

I'm trying to learn here, so bear with me

I am in the process of planning for a 28 nosler build. I have not decided what bullet I want to try to use, so I will talk generically for now.

My thoughts are that I want to seat the top of the boat tail approximately at the base of the case neck (i.e., the shoulder-to-neck junction). I am assuming this gives maximum contact between case neck and bullet for better alignment. At the same time, this should maximize case capacity.

Is my reasoning sound thus far?

From there, how much jump do you plan for? Does it depend on what you think the selected bullet likes? Do you try to err on the side of being either too long or too short? If so, why?

Would you throat to touch the lands with the bullet seated as described above and then give up case capacity to find the jump the bullet likes? This is what I am leaning towards. Is there a better way to decide on throat length?

Or am I thinking too hard? Do I just have it chambered to the standard spec and be done with it?

FWIW, the magazine box I am looking at will support loading longer than the above length for all bullets I'm considering, so I should not be limited by mag length.

Thanks for the help.

 

[MudRunner2005]

I would never have a rifle custom throated for a specific bullet. That really limits your long-term abilities with that rifle.

All of mine have standard SAAMI throats.

 

[Garycrow]

My thoughts are that I want to seat the top of the boat tail approximately at the base of the case neck (i.e., the shoulder-to-neck junction). I am assuming this gives maximum contact between case neck and bullet for better alignment. At the same time, this should maximize case capacity.

Is my reasoning sound thus far?

From there, how much jump do you plan for? Does it depend on what you think the selected bullet likes? Do you try to err on the side of being either too long or too short? If so, why?

Would you throat to touch the lands with the bullet seated as described above and then give up case capacity to find the jump the bullet likes? This is what I am leaning towards. Is there a better way to decide on throat length?

I send a dummy round with my chosen bullet seated as you describe, with the top of the boattail at the neck shoulder junction. I tell the gunsmith to throat it to touch the lands at that length, this allows for throat wear so I still have room to seat the bullet out farther as it lengthens. I prefer to err on the too short side, a gunsmith can always lengthen the throat with a throating reamer but he can't put metal back. Most SAAMI chambers have the throat too long so it's tough to touch the lands and still have full neck contact, or they won't fit in the magazine. It's especially critical to pay attention to throating if you plan on shooting VLD's, they often have different requirements than other bullets. I've even gone so far as to have my own reamer made with the correct freebore for the bullet I wanted to shoot. Most good gunsmiths will have a reamer with relatively short freebore and finish the throat with a throating reamer so they can custom throat it. In any case I always want to be able to touch the lands with nearly full bullet to neck contact. Some will tell you that their rifles shoot great with a jump, but I haven't always found that to be the case and it limits your options if the throat is too long.

It's a custom rifle, the whole point of custom is to get something you can't get off the shelf.

 

[TOM H]

If you have the option, do you have a chamber custom throated for a hunting rifle? And if so, what do you specify?

I'm trying to learn here, so bear with me

I am in the process of planning for a 28 nosler build. I have not decided what bullet I want to try to use, so I will talk generically for now.

My thoughts are that I want to seat the top of the boat tail approximately at the base of the case neck (i.e., the shoulder-to-neck junction). I am assuming this gives maximum contact between case neck and bullet for better alignment. At the same time, this should maximize case capacity.

Is my reasoning sound thus far?

From there, how much jump do you plan for? Does it depend on what you think the selected bullet likes? Do you try to err on the side of being either too long or too short? If so, why?

Would you throat to touch the lands with the bullet seated as described above and then give up case capacity to find the jump the bullet likes? This is what I am leaning towards. Is there a better way to decide on throat length?

Or am I thinking too hard? Do I just have it chambered to the standard spec and be done with it?

FWIW, the magazine box I am looking at will support loading longer than the above length for all bullets I'm considering, so I should not be limited by mag length.

Thanks for the help.

I agree with Greycrow. The post #2 about 270Wby I had build but didn't go into detail about having it done.

http://www.longrangehunting.com/forums/f19/257-weatherby-bullet-depth-181583/

 

[TxAggie94]

So, whether or not I ultimately decide to ask for a custom length, I do want to make sure I understand. First step is to figure out where the ogive would fall relative to the SAAMI spec chamber if seated for full neck contact.

If I look at the SAAMI chamber dimensions, it looks to me like it is nominally 0.491" from the shoulder/neck junction of the chamber to where the lands start....and 0.5626" from the junction to where there would be full engagement.

I know I am ignoring tolerances, brass thickness, junction radius and any gap between case shoulder and chamber.

Next, I would determine the length of the bearing surface for the chosen bullet and compare that to one of the neck-to-lands measurements above. If the bullet bearing surface is longer, then I have to seat deep or have the rifle throated long. If the bearing surface is shorter, then I have to load long and give up neck contact or have the rifle throated short.

Unfortunately, all the uncertainties now come in to play. Which length do I use? The 0.491 to where the freebore first gives way to the lands? or the 0.5626"

Further, depending on how you measure or determine bearing surface length of the bullet, the distance from shoulder/neck junction to the bullet ogive for the 3 bullets I am considering are:
* 195 Berger: 0.524
* 175 ABLR (posted): 0.623
* 175 ABLR (measured): 0.664
* 175 ELD-X: 0.57

Of course, these numebrs are all from different sources, so I can't really do a direct comparison. The Berger value came from Berger.com. The 175 ABLR (posted) and ELD-X values were posted on LRH. The measured ABLR value (0.664) I measured with the hornady gage. I don't know which values to believe for what I am trying to do. If I already had the rifle chambered, I would seat a bullet deep in a fired case, load it, and push it forward (through a drilled-out flash hole) until I feel resistance. Then I know where the lands are. Trying to figure it theoretically leaves me with a lot of doubts.

I guess Garycrow is right...the way to do this is simply pick a bullet and seat it as desired and send it in to have the chamber throated to match.

I guess the reason I started this exercise is what MudRunner alluded to: if I have it throated specifically for one of the bullets and then that one doesn't shoot for some reason at any length, it may leave me having to load long for one of the others in order to get it to shoot.
I'm probably overthinking. I just can't afford to do this twice, so I want it right the first time.

 

[Engineering101]

I have done a couple of custom throats in WSMs and got amazing results by pushing the long bullets way out. In a 7mm WSM I gained a bunch of extra powder capacity that was very useful to significantly boost performance. However, I have a 28 Nosler and it does not need a custom throat to perform. There is a ton of capacity in that case that you won't even use so go SAAMI on that one.

 

[elkaholic]

It seems you have a pretty good grasp of things! I would send a dummy round with the longest, or most likely bullet you will be using, seated at. or just above the neck shoulder junction. Another thing that is often overlooked is what powder you will be using, or would like to be using. Sometimes just a LITTLE more throat will allow a slower powder to be used which can reap good velocity gains. I have a project going on right now where that comes into play. Good Luck.......Rich

 

[AZShooter]

I prefer to buy a reamer to my specs. I would encourage you to do the same. You can't depend on the gunsmith to have the reamer that matches your requirements.

Other considerations:

Throat angle
In addition to the length of the throat there is the angle to consider. Most of mine use a 1 1/2 degree throat angle.

Neck clearance
Most Sammi spec chambers have generous chamber neck dimensions. I understand their reasoning, because the rifle may encounter different brands of brass with different neck thicknesses. Heck even the same manufacturer can produce different thicknesses with the same case. Look at the issue Gunwerks had with the 7 LRM brass supplied from Hornady.

I would suggest you acquire your brass and measure the thickness of the neck then calculate what you need for the chamber neck dimension. I generally use .006" total clearance between loaded round and chamber neck with hunting rifles. This way the necks are not over worked when sized (provided a bushing die is used) extending the brass's useful life.