Of the lands - Where to start. Newbie

H

HCT4ALL

Inactive
Hi, I am shooting a Model 7 7MM-08. The load she likes best is 139 SST Hornady's with 40 grains of IMR4064. They were loaded to 2.775 per the manual. I measured the lands with that bullet and it comes in at. 2.875. I measured it 3 times. Where do I go from here?

Thank you!

Rick
 
IMHO, start 0.020 off and go in/out 5-thou at a time.
Do NOT get closer that 5 thou off to avoid the god of
statistical variation.
;)
 
That's good advice, though, if you want to cover all the bases, you can reduce the charge 10% and start with the bullet touching the lands to see how that works for you. Then go back to full charge and try 0.030, then 0.060, then 0.090, then 0.120" off the lands. See what your gun actually likes best.
 
Most of the time you'll be limited to what will function in your magazine instead of what it takes to "kiss" the lands.
 
When I arrived at a load for my 6.5x47L I soft seated the bullets, and slowly added powder until it shot well. In this rifle .001" neck tension works well. The benefit as I see it to this method is that throat erosion doesn't have the same effect as loading to a specified distance off the lands... as the throat erodes, you don't have to change seating depth to chase the lands.

Drawback is sometimes when the bore is fouled, one cannot remove a loaded round without spilling powder everywhere, as the lands pull the bullet out of the case.

As another contributor stated, if you are shooting a magazine fed rifle, your limitation will be your magazine length. There are some bullets which are more tolerant of seating depth than others, typically the secant ogive types as opposed to the tangent variety. This means you can find an accurate load and still seat to magazine length.

Applied Ballistics website has some good reading on bullet seating depth using Berger bullets. Brian Litz posts over at Accurate Shooter on this subject sometimes. You might be surprised how well some bullets shoot seated waaaaay off the lands, according to Mr. Litz.
 
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I would redevelop at 0.030" using the OCW method, then fix that powder charge and vary the seating depth from -0.005 - 0.060 using:
-0.005, 0.010, 0.020, 0.030, and so on.
 
Thanks for the replies. I did check the cartridge at the lands and it will fit in the magazine. Have a great day.
 
HCT4ALL,

Let us know how it goes for you.


Nathan,

Take a look at Berger's tech note on finding a best seating depth for a secant ogive VLD design. They've found the seating depth steps can be larger than used to be thought. I'm still experimenting in this realm, but they do seem to dial you in pretty well. The problem is that changing seating depth with a fixed powder charge also changes the peak pressure and barrel time, so it's hard to tell when your change in seating depth is actually helping the bullet center in the bore better, and when it is just tweaking the barrel time same as adjusting a powder charge would do.


Stubbicat,

I think you said that backwards: the tangent ogives are more forgiving of seating position than the secant ogives. That's why Berger developed its hybrid ogive that starts out as tangent where the lands will make contact, then changes over to secant profile for a higher BC. The shallower angle of departure of the tangent ogive from the bearing surface line seems to help the bullet self-center better by engraving more gradually.
 
First, if off the lands is important I need to know where the lands are located in reference to the bolt face. The first thing I determine is the distance from the lands to the bolt face. I do not have to rediscover this measurement every new day.

After I discover the length from the beginning of the rifling to the bolt face I use a transfer to set up my seating die. I understand the two diameters of the barrel, I understand the diameter does not change quickly.

I am the fan of transfer and standards.

F. Guffey
 
F. Guffey. Thanks for the reply but you lost me. I have no idea what you just said.

Are you saying off the lands is not important?

What do you mean by transfer and standards.

Thank you
 
"...off the lands is not important?..." Isn't as important as everybody seems to think. What it's about is the slight differences in rifle chamber dimensions. Your rifle may like 20 thou off the lands or it may like 10. Whole thing is a trial and error thing. There is no formula.
Fiddling with the OAL is a load tweaking technique that's applies more to target rifles than hunting rifles. If you're happy with the accuracy of your hunting rifle, leave it as is. Bambi won't know or care.
And Hodgdon says 40.0 is the starting load and the OAL for a 139 is 2.800". Manuals will vary according to the rifle/receiver used in the tests.
 
T. O'Heir,

Follow the link I posted. It seems to depend on the bullet shape as to whether it matters to hunting accuracy or not.
 
Are you saying off the lands is not important?
Click to expand...

No, I am saying I am the fan of the running start, I want my bullets to have 'that jump'. I want to know how much jump, I want to know the distance from the beginning of the rifling to the bolt face.

I have tested chambers for free bore, a few had long throats, so long the bullet came out of the case before the bullet got to the lands. I have purchased new factory over the counter ammo that did not require tweaking.

And I have taken two new rifles to the range for testing. I used the same ammo in both rifles, one rifle shot one hole groups, the other shot like a shot gun, it shot patterns.

F. Guffey
 
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