6mm BR headspace discrepancy

[Old Navy]

I have one of Brad's pre-fits on a Razor action. I never thought to check it before setting the headspace. To cold to play outside so I took some measurements on two Savage barrels and one Shilen and one Critorian. I measured the recess in two bolts out of Savage 12 target actions. The recess difference between the two is .006". I tried several combinations of barrels and and bolts. I got from .002" to .020" nose to barrel clearance. These measurements assume the go gages are correct.

 

[Webster]

I’m confused. First I am a Pharmacist - not a machinist or gunsmith.....

I have a 6mm BR Remage barrel setup. Using the .350 bushing on my Hornady Lock N load tool, my Forster go gauge reads 1.158. I screwed the barrel on and set the headspace using the go gauge with the ejector removed. A fired Norma case ends up with a datum measurement 1.189 with some signs of case stretch. I reheadspaced the barrel and got approximately the same results. A different action also got about the same case stretch.

I found some new Lapua brass that measures 1.156. Obviously my action swallows the new Laupa brass. I reset the headspace using the Lapua brass. I haven't fired a round with this setup yet.

Now the datum COL measurement for loaded rounds at the lands using the Hornady tool have gotten longer even as the length of the “long” Norma brass is the same as new “normal” Lapua brass at 1.555. I would think the datum OAL of a loaded round would be shorter since I screwed the barrel in slightly while setting the headspace with Lapua brass.

1. Why was the brass from using the go gauge to set headspace so long after firing?

2. Why are the loaded round Datum COL’s with the new headspace longer and not shorter?

3. Are my measuring tools and or methods suspect?

4. My reamer print reads using the .375 bushing my headspace datum should be between 1.1456-1.1495. New Lapua brass is 1.135 which is shorter than what the reamer print says. I assume this is “room to grow” when the round is fired?

Many thanks for any replies.

I reloaded for many years without measuring anything except OAL. Never had a no/go guage. It really isn't that complecated. Maybe the problem is you think everything has to fit some measuement you have.

 

[Just Say Moe]

The answer to your original question “are your measuring methods suspect” is yes.

Assuming you are using the Hornady tool with the dummy case, most of your confusion is probably there.

Have you measured case base to shoulder datum of the Hornady case? Most likely it will be different than the same measurement on your brass. Best is to make a custom case based on a fired case, sized as you will actually shoot it.

You have two things going on. Headspace, bolt face to shoulder contact point in the chamber, and bolt face to bullet contact at the lands. You said that you are seating the bullet to touch the lands. Technically you are headspacing off the bullet, not the shoulder of the brass. In a perfect world, or brass sized the same, both points would be touching. That’s why the Hornady tool case needs to be the same size as what you shoot to have any real meaning.

The cartridge overall length Should remain constant with the bullet touching the lands. That’s base to bullet ogive. But headspace could vary, base to shoulder. That’s why it is possible to to have different headspace measurements and the same overall length. When you are loading to a touch or jam, the headspace is determined more or less by neck length, the distance from the shoulder to bullet ogive. More neck, less body, is more headspace, more gap between case shoulder and chamber. The bullet is touching.

More body, less headspace will give You the same overall length, with the same bullet touch.

This is easier seen with a jump. The jump distance will change with a headspace change when moving the shoulder. As base to shoulder distance changes, you push or pull the neck back and forth.

The Hornady gauge gets even less precise if the shoulder angle is different than the brass you are shooting.

There is a learning curve to learning to use the tools and being able to trust the measurements, that curve is reduced by using a case sized as it will be shot. I would bet that measuring the Hornady case might clear up a lot of your confusion.

They are pretty darn close

Forster Go gauge​

1.160​

Hornady gauge​	1.158​
New Lapua brass​	1.159​

 

[Just Say Moe]

If you have .031 of stretch the first thing I think of is that you didn't get the go gauge snapped into the extractor before you tightened the nut. Your go gauge was stopping on the extractor, not the bolt face.
Edit to add, you can check this by putting an witness mark from the barrel onto the nut with a Sharpie marker. When you tighten it up make sure the go gauge is against the bolt face then tighten. If the witness mark is in a different place, you found the problem. Good luck.
If you have pressure problems with it when you get it squared away, try a Beta Blocker.

I made sure the gauge was under the extractor in the bolt and not stuck in the chamber.

I think your chamber is too deep.
That screwing the barrel in more is merely causing you to run out of breech clearance (that Jackie mentioned).
Possibly making you 'think' that you're at 0 HS on the go gauge, while you're still too long to chamber shoulder..

Could be. Same result on 2 different actions - an R700 and a Tenacity. I used new Lapua brass with 2 pieces of tape to reset the headspace and fired cases were 1.168 - for a 0.009 stretch. I'll take that. Bolt will not close on the go gauge now - it is a Criterion barrel from Northland. I have the same one in 20VT on another R700 action headspaced using a go gauge with no issues.

 

[Texas Solo]

I used a Forster go gauge, then one piece of tape. I have .002 headspace. My brass is sized and bumped accordingly. Smooth as silk.
Absolutely won't chamber any factory ammo.

 

[mikecr]

I reloaded for many years without measuring anything except OAL. Never had a no/go guage. It really isn't that complecated. Maybe the problem is you think everything has to fit some measuement you have.

So you just screw non-shouldered barrels in without establishing head space?

 

[BoydAllen]

I used a Forster go gauge, then one piece of tape. I have .002 headspace. My brass is sized and bumped accordingly. Smooth as silk.
Absolutely won't chamber any factory ammo.

How is your barrel throated? How does the GO gauge compare to factory ammo when measured with a headspace comparitor?

 

[mikecr]

I didn't search this here but I seem to remember some or one of the Remage barrel makers were sending the barrel out too long (cut too deep) and you needed to cut .010" off to properly headspace it?

Better long than short, but then you'd need a local barrel finisher to finish it. Otherwise, you'd have a mess.

I have one of Brad's pre-fits on a Razor action. I never thought to check it before setting the headspace. To cold to play outside so I took some measurements on two Savage barrels and one Shilen and one Critorian. I measured the recess in two bolts out of Savage 12 target actions. The recess difference between the two is .006". I tried several combinations of barrels and and bolts. I got from .002" to .020" nose to barrel clearance. These measurements assume the go gages are correct.

This is some pretty appalling standards..

 

[Old Navy]

Better long than short, but then you'd need a local barrel finisher to finish it. Otherwise, you'd have a mess.

This is some pretty appalling standards..

I agree. The .020 was the 6.5X284 that had two different neck outside diameters.

 

[searcher]

You planning on using "factory loaded ammo"??
If SO, headspace gauge.
If NOT, set it where YOU want it.
I prefer "snug" headspace. That way, less brass to move when bumping the shoulder.
None of mine could chamber factory loaded ammo on a bet.

This is good advice. I very seldom use factory ammo - but needed to fire factory ammo when I first got a .17 Hornet due to unavailability of brass at the time. As I was putting a new tube on the Hornet, I gave my gunsmith a few rounds of my factory Hornady ammo and he couldn't have set the headspace better - slight resistance when the bolt is almost bottomed when locking. No case stretch problems and incredible accuracy with the factory ammo. Since you only plan to reload, I'd fiddle with it till you get less headspace clearance. I keep a notebook with all the specs on my rifles in case (when) I croak - I outline which rifles can be shot with factory ammo - and which ones not. I'd not want them in the hands of folks thinking OK to run factory ammo in them. Same with unmarked barrels as to what the caliber is.

 

[Webster]

So you just screw non-shouldered barrels in without establishing head space?

Always paid a gunsmith and paid a lot of money. Don't know anything about a prefit barrel. A guess I assumed they were machined to just screw in and they were up to spec. I think I am hung up on a lot of guys getting OCD about cartridges and no conversation about how to improve personal skills.

 

[mikecr]

It really isn't that complecated. Maybe the problem is you think everything has to fit some measuement you have.

Says the man who has never done it. I've never done it either.

Always paid a gunsmith and paid a lot of money. Don't know anything about a prefit barrel. A guess I assumed they were machined to just screw in and they were up to spec.

And this is an opportunity for both of us to learn something about it.

 

[doninreno]

I'm kinda working on this same problem on my 6mm BR Norma since I had some fairly "stretched" looking (Lapua) cases (none failed YET, though)...using a .350 diameter measuring tool and measuring some of the cases that have been fired in that chamber and they are 1.155 or 6....., plus or minus maybe a thou. Lapua virgin cases measure out to be1.152 or 3...... and Petersen cases (also virgins) measure out at 1.154 or so. With the ejector removed and continuing to measure with the same tools, I find that at 1.1525 to 1.153 is where the bolt drops with pretty much NO effort applied and then when I stick in a .001 longer measured case it takes some effort, but not much to make the bolt handle to move down...add another .001 and the bolt can be closed but with a fairly hard push downward. Any more length and it becomes only possible to lower the bolt if you happen to be King Kong. So...it looks to me that my zero headspace number is 1.1525........ and if I want to use about .002 headspace, the number I need to shoot for when sizing the cases for my desired headspace is 1.1505 or so, but that sounds kinda short to me!

Comments/suggestions/critics/ideas welcome, but it seems to me that I have the headspace thing figured out, but if what I seem to have found doesn't sound to any of you that I'm not going at this right, I'd sure like to get some input/comments from you all.

As it stands now, (last year's results), my groups at 300 yards (F-Open shot off of a SEB NEO and a Edgewood rear rest) are running around .35 to .4 MOA or so and I am doing OK in the matches (At least when the wind isn't blowing since I am a wind doping DUMMY), but I'd sure like to feel like I have everything done as well as I possibly can and see if the groups shrink at all or get the SD down into somewhat lower digits. I still have a few leftover loads from last year and will look at my notes from then and also measure to see what the headspace was set at (as we all know, measurements do tend to wander a bit even when made with great care due to things like temperature differences, etc., etc.)

Tuesday morning EDIT/ADDED---OK..I did just now measured the "leftovers" from last season (1.153) as loaded but not fired and also measured the fired cases (1.156)....hmmm...that looks to me to be about .003 headspace. I think I'll see if I can cut that maybe in half for my first batch for 2024.

PS..the bolt in that rifle is a one piece PT&G and the barrel is a Bartlein 8 twist that was chambered and installed by a fellow match shooter/gunsmith who is a GREAT shot and quite respected around here as a 'smith.

Thanks!

 

[doninreno]

I don't really get how this got stretched this far with the headspace set at .003", but my best guess is that somehow a "short case" snuck into the loading block and this happened....see the pics! It didn't come apart during extraction, but I don't think it was far from doing just that! I cut this (6mm BR) open so I could look at it and not just have to go by what I could see on the exterior or feel with the dental pick I use for checking such stuff (or maybe my borescope).

 

[Rocketvapor]

I measured the recess in two bolts out of Savage 12 target actions. The recess difference between the two is .006".

Did you also measure the lug length? Or just the recess?

 

[Old Navy]

Did you also measure the lug length? Or just the recess?

No, just the recess but since I first posted I had my son son check them with a tool that has a dial indicator. The difference is .0035". Both bolts went to Carlsbad to be bushed. He machined the recess of one just a little to clean it up. My son just turned a used 6.5X47L barrel into a 6.5 CM for me. I took the barrel and the bolt with deepest recess down to him. He gave it back with .005" clearance.