Help with headspace measurement

[Dacam]

Hey guys,
Im a noob here trying to figure out what my headspace measurements are on my 1x fired brass.
I have an LMT Mws 7.62x51 with 20" stainless barrel.

I use the Hornady headspace bushing insert (.400) on a digital caliper. My fired brass measures 1.623 headspace, this seems way too small
compared to some of the other LMT's out there. I even thought it was the bushing that was out of spec and went and bought a new one only to
find it was the same measurement. I measure new factory ammo (168g FGMM) and it is 1.624 with the Hornady gauge.
Could my chamber be that small?
A good member here (Elfster1234) has been helping me figure this one out. Does anyone have any ideas? Am I doing something wrong?

Thanks,
Dacam

 

[Temp9]

If you slowly chamber a factory cartridge (1.624"), does it feel like it just chambers a bit tight? If the larger factory case chambers easily, it might just be that the fired case isn't yet fully fireformed to the chamber. This happens sometimes, especially when you shoot lighter loads. If you FL size the casing, but make sure the die is adjusted to not bump the shoulders, sometimes you can actually get the case headspace to grow. Then slowly bump the shoulder until it just fits, and that would give a more accurate measurement of headspace.

 

[waste_knot]

You really need to calibrate a Hornady gauge before use if you want to measure anything but fired cases for setting up a FL die.
The 0.400 bore will read undersize for headspace if placed on a 1.630 "GO" gauge. This base was bored to 0.396 and reads closer to to actual length.

 

[Suasponte]

The Hornday bushing is the reason you are getting a small reading and the below diagrams
that I made show you why. The Caliper is being zeroed to the "FACE" of the insert, however there
is a chamfer on the inside of the lip that changes the actual datum point inward from the face of the tool.
Thus you are not getting a good reading based upon the 1.630" headspace that everyone references to.
What you can do is insert a "GO GAGE" into your setup and see what the difference is from the 1.630".
That will be you setback dimension for the chamfer on the inside lip of your tool. Back off that dimension
from the face of your tool and re-zero your tool and I bet the number will be more acceptable for what you
expect to see.






Terry

 

[Suasponte]

Here is a video I made describing the Hornady Headspace gage bevel.
The only way to know what your true headspace number is in reference to the SAMMI spec
is to check your gage with a "GO" gage and subtract the diffrence, then add that number back into
your dimensions you are taking. H

This however will have no affect on setting up your dies. You are referencing to the same datum and that is
all that really matters.



Terry

 

[elfster1234]

being that he has a LMT with a 7.62nato chamber and not .308 chamber, wouldn't you need a 7.62nato GO GAGE of 1.6355" ???

all of this headspace stuff and trying to get a number locked down to what your bumped back brass datum length should be even drives me bat shit crazy..... its like why even sell the hornady headspace gage if it is all f#cked up from the get go.


7.62x51nato GO 1.6355"
.308 win GO 1.630"

The Hornday bushing is the reason you are getting a small reading and the below diagrams
that I made show you why. The Caliper is being zeroed to the "FACE" of the insert, however there
is a chamfer on the inside of the lip that changes the actual datum point inward from the face of the tool.
Thus you are not getting a good reading based upon the 1.630" headspace that everyone references to.
What you can do is insert a "GO GAGE" into your setup and see what the difference is from the 1.630".
That will be you setback dimension for the chamfer on the inside lip of your tool. Back off that dimension
from the face of your tool and re-zero your tool and I bet the number will be more acceptable for what you
expect to see.

View attachment 6519
View attachment 6520

View attachment 6521

Terry

 

[JRM83]

The RCBS precision mic or Redding instant indicator both work very well at measuring headspace. As bonus they will both also measure to the ogive for seating depth, and the Redding can do case length as well. Downside is that they are caliber specific which is expensive if you reload more than a few different calibers.

 

[tactserv]

I'm not sure why this is so complicated.. Shoot 5 factory rounds througn the gun. Use a bump gauge and measure the empties (average). Now size the brass to bump the shoulder back to between .001" to .002" for bolt action rifles and between .003" and .005" for AR15 type rifles. Easy as pie.

This minimizes case stretch and mazimizes case life.

OR, resize to factory specs and toss after 1 reload.

Mike

 

[Dacam]

You guys have some great info on this matter, thanks a bunch.
I will get a "GO" gauge and reference that number with my Hornady bushing.
Thanks again,