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Throat length help needed

 
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  #1  
Old 08-20-2010, 07:26 PM
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Join Date: Jul 2010
Location: Oregon
Posts: 56
Throat length help needed

I decided it was time to get serious about long range shooting / hunting so I am ordering a new barrel for my rifle. I have never had a custom rifle or barrel before so this is all new to me.

It will be chambered in 300 Rem Ultra Mag. I want a custom throat cut for Nosler 200gr. AccuBond bullets. This is where I need help from the experts here. How do I go about determining the length of the throat to be cut?

First I want to build the cartridge then have the throat cut to it. I always used factory guns so I had to tailor the cartridge to fit the gun. From what I read, I am supposed to seat the bullet with the bearing surface to the shoulder neck junction.

BobbyL said in another thread “When i set one up i use a case and seat the bullet so the pressure ring is just starting to enter the neck. This gives us the best accuracy and allows for the most FPS by using more powder. The more room in the case the less the pressure will build with a specific charge. This has to be done carefully or you wont have enough bullet left in the neck to shoot it. Yes you want the gas ring or the boattail bearing surface junction right at or up in the neck of the case above the neck shoulder intersect”

With that in mind I used a Hornady bullet comparator to measure the bullet. I also emailed Nosler to get the exact dimensions of the bullet. I got two different set of numbers. I got base to ogive .848” and Nosler quoted me .770”
Boat tail length I got .093” and Nosler quoted me .110”
The neck of the case is .306”
I measured the comparator insert and it measured .294” Could that .006” be throwing me off that much?

So back to the throat. Using my measured numbers and a .030” jump to lands .848 - .093 - .306 + .030 = .479 throat.
Using Nosler’s numbers .770 - .110 - .306 +.030 = .384”

I already changed the throat three times. I think I may have wore out my welcome. Luckily it takes 12-16 weeks for a barrel to be made. I want to make sure I get it right the first time, but if I must miss then I rather be on the short side instead of long. You can always cut metal out but not cut it back in. Sorry for being long winded, but I wanted to be thorough. Any other advise would be welcome. Thanks for taking time to read and help.
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  #2  
Old 08-20-2010, 08:12 PM
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Join Date: Jan 2005
Location: OK
Posts: 2,145
Re: Throat length help needed

first off is this going to be a single shot, or reapeater.?

If it is going to be a repeater then load a dummie round to fit the mag box and have about .050" of room from bullet tip to front of box. Send that dummie to the smith and ask him to throat it so you are .030" aproximatly off the lands with that dummie. Not a smith in the world can give you an exact number to the thousandths. But with that dummie and a throating reamer he will get you real close. If you need more precise send it to ptg and have your own reamer made.

If it is a single shot I would throat it to the shorter of your two calculations. Once it's gone you can't put it back, and as the rifle gets long in the tooth you'll always gain on that number.

Typically guys get a bit wrapped up with the exact number, let your smith do the tough thinking.
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  #3  
Old 08-21-2010, 01:12 AM
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Join Date: Apr 2007
Location: ALASKA
Posts: 504
Re: Throat length help needed

+1 on Jim See's recommendations.

I would like to add, if it's a repeater application your building, ensure you look at other bullets (i.e. 208 Amax, 210 Berger) and make sure you don't throat your rifle to be a repeater for just one bullet. Your rifle may not end up liking 200 NABs as much as it likes 208 Amaxs or the 210 Berger or the Barnes or etc....

Since you've got that OAL tool...and are taking some measurements....take the longest bullet you'd want to shoot out of the rifle (if the 200 NABs didn't do what you want them to do), and seat it to working mag length, using the Jim See -.050 under mag box guidance. This is a dimension for two reasons...throat growth as the round count on the rifle increases....and it's a hunting rifle...things get contaminated, cold...you want it to work!

Now take a OAL ogive measurement.

Now seat your 200 NAB with the same OAL ogive dimension.

Then observe the amount of bullet intrusion into the powder charge, or vice versa, observe the amount of bullet bearing surface length in contact with the case neck....make sure those things make you happy with all your proposed bullets.

If necessary, reset your desired "go to" bullet you really want to use at a compromised seating depth.....now you have a dummy round that either your smith can use to throat your rifle or PTG can make a finish reamer for....and that throat/finish reamer will enable you to shoot (as a repeater) several bullets that've you've previously done the math for just in case your desired "go to" bullet and your new barrel have a disagreement

Barrels always win those arguments BTW

Good luck, and keep it fun! Your 300 UM is going to be a boomer I'm sure!
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  #4  
Old 08-22-2010, 02:49 AM
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Join Date: Jul 2010
Location: Oregon
Posts: 56
Re: Throat length help needed

I forgot to mention that this rifle will be a single shot so overall cartridge length does not matter.

I thought I had all this figured out before placing the barrel order. Then the second guessing kicked in and I began double and triple checking my numbers. I got caught up in the numbers game like Jim said. I think I will leave it at the .384” throat and see how it goes. And I agree with what you said about once it’s gone it’s gone so I rather be on the short side.

I know I am taking a gamble on cutting this barrel specifically for one bullet, especially one I have not tried before. I choose the NAB because they will be easier for me to get regularly. I can drive down to Nosler a couple times a year and load up on seconds.

I didn’t know that bullets and barrels could have a “disagreement.” I better start worrying that these two will get along. Maybe that will get my mind off the throat.

Thank you Jim and Autorotate for the advice.
I will post details of this thing when it is done. Hopefully it will be a shooter.
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  #5  
Old 08-22-2010, 05:00 AM
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Join Date: Jan 2007
Location: PA
Posts: 1,067
Re: Throat length help needed

Your first step should be to discuss this with your gunsmith. What does he recommend? Does he have a reamer that will cut the exact chamber you want? If not, will he rent one? If you want to buy a reamer call Dave Kiff at PTG and get his advice. You should not be jumping around trying to figure this out by yourself.
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  #6  
Old 08-22-2010, 09:00 AM
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Join Date: Mar 2010
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Re: Throat length help needed

Quote:
Originally Posted by el si View Post
I forgot to mention that this rifle will be a single shot so overall cartridge length does not matter.

I thought I had all this figured out before placing the barrel order. Then the second guessing kicked in and I began double and triple checking my numbers. I got caught up in the numbers game like Jim said. I think I will leave it at the .384” throat and see how it goes. And I agree with what you said about once it’s gone it’s gone so I rather be on the short side.

I know I am taking a gamble on cutting this barrel specifically for one bullet, especially one I have not tried before. I choose the NAB because they will be easier for me to get regularly. I can drive down to Nosler a couple times a year and load up on seconds.

I didn’t know that bullets and barrels could have a “disagreement.” I better start worrying that these two will get along. Maybe that will get my mind off the throat.

Thank you Jim and Autorotate for the advice.
I will post details of this thing when it is done. Hopefully it will be a shooter.
If I am doing all the work and worrying about throat length that you are, I'm not shooting any bullet makers 'seconds'.
__________________
"Shoots real good!": definition; it didn't blow-up in my face. Not everything can be fixed on an internet forum!
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  #7  
Old 08-22-2010, 09:12 AM
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Join Date: Apr 2007
Location: ALASKA
Posts: 504
Re: Throat length help needed

Quote:
Originally Posted by el si View Post
I think I will leave it at the .384” throat and see how it goes.
That is a bunch of freebore....Weatherby territory.

Here's a reamer print throated for 210 Bergers. If you're making a reamer specs off of Stoney Point/Hornady tools....you'd be much better off using a dummy round, and sending it off to PTG and having him make a finish reamer for the dummy you made.

JE Custom and Lone Wolf 30-338 Lapua Improved Weatherby Mark V
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