Reading bullet jump from reamer print... ***UPDATE***

[thefitter]

Just making sure that I'm understanding all the info available on a reamer print.

"Throat datum .336 x 2.845 from bolt face." This is the point where the bullet ogive at .366 diameter will just touch the lands on a new throat.

So assuming that my comparator reads right at the .336 point on the bullet, a measurement of 2.835 equals 10 thousandths jump, right?

Thanks

 

[thefitter]

Re: Reading bullet jump from reamer print...

.338 LM with a Berger 300gr OTM was the dummy round the reamer was made off of.

"Throat datum .336 x 2.845 from bolt face." was in the notes at the bottom.

It is a JGS reamer print.

Thanks

 

[Alex Wheeler]

Re: Reading bullet jump from reamer print...

Can't help you with your question, but if your trying to determine seating depth with out actually checking it your gonna have problems, you can get different touch lengths in different barrels with the same reamer.

 

[thefitter]

Re: Reading bullet jump from reamer print...

Yes .336 NOT .366

I will take a measurement and see how close it is.

As far as there being differences between the print and the ground reamer it better be damn miniscule at $250 a reamer!!

My thoughts were that looking at a print will give me a easier starting point than measuring the chamber. I always have to do 6 measurements and then average. Even then I don't feel dead on like I would like...it's the OCD.

 

[thefitter]

Re: Reading bullet jump from reamer print...

Can't help you with your question, but if your trying to determine seating depth with out actually checking it your gonna have problems, you can get different touch lengths in different barrels with the same reamer.

How does that happen? What size variances?

Thanks

 

[Alex Wheeler]

Re: Reading bullet jump from reamer print...

Bore size. Touches different part of the ogive. I dont think you will be close trying to do what your doing.

 

[dmoran]

Re: Reading bullet jump from reamer print...

As far as there being differences between the print and the ground reamer it better be damn miniscule at $250 a reamer!!

In a perfect world, yes....
But in our reality world, add in the tolerances given on the print and pray that's all. But often enough there is more.
Then add in the run-out aspects from when the barrel was chambered. And the actual bore diameter and land heights of the barrel.
All these aspects combined/added together is why there is a good chance your actual chamber will not be identical to the reamer print.
- A chamber cast will answer most all these questions.
- An optical-comparator is how you can measure a reamer.

 

[thefitter]

Re: Reading bullet jump from reamer print...

Well I'll take some measurements and see what we have.

Thanks

 

[Erik Cortina]

Re: Reading bullet jump from reamer print...

Direct measurements are always the most precise.

Too much tolerance stacking between reamer print and chamber.

 

[thefitter]

Re: Reading bullet jump from reamer print...

I finally got to take some measurements on where the lands actually start on this rifle. I used the burnished bullet method that Dwight Scott and another member taught me.

After multiple checks I have decided that I'm just kissing the lands at 2.933

What I have discovered in this whole process is that my comparator, even though sold by Sinclair as a .338 comparetor does not actually contact the bullet at the point the lands do. My comparator actually contacts the bullet at the .326 diameter mark. Now maybe all you already knew this but I was surprised...and yes maybe a bit embarrassed that I did not think of this prior. I never really looked closely and thought about where the comparator was contacting on the bullet, but now I understand that it's merely a reference point.

Now here is where it gets interesting...the lands just contact the bullet at the .336 mark. So I measured from the .336 mark back to the bolt face and...wait for it...got damn near dead on what the print says 2.845!

Now since my comparator does not contact the bullet in the same spot as the print measurement I can not use it blindly. But if I had a comparator that contacted right at .336 I would feel pretty confident going off a JGS print.

I have also discovered that I have been unknowingly jaming the bullets all this time. I have been getting less than .5 MOA at times but I'm thinking that it's just been luck and incredible shooting skill. I need to back off and see what a jump will do.

Thanks for all the helpful replies.

 

[fguffey]

Just making sure that I'm understanding all the info available on a reamer print.

"Throat datum .336 x 2.845 from bolt face." This is the point where the bullet ogive at .366 diameter will just touch the lands on a new throat.

So assuming that my comparator reads right at the .336 point on the bullet, a measurement of 2.835 equals 10 thousandths jump, right?

Thanks

Datum: Measured from.

F. Guffey

 

[fguffey]

http://www.saami.org/PubResources/CC_Drawings/Rifle/30-30%20Winchester.pdf

I furnished a typical link to SAAMI, for the 30/06. The barrel has two diameters. one is .308”, the other is .300”, the diameter you are looking for is between .308” and .300”, the diameter of the barrel between the two diameters is variable because it is round and has a tapper, and that locks up everyone.

F. Guffey