6.8 SPC Forums banner
1 - 20 of 25 Posts

·
Registered
Joined
·
154 Posts
Discussion Starter · #1 ·
I have been reloading 6.8 for a while now with pretty good success. I started out by measuring a .004 bump with the empty 9mm cartridge trick. Well today I decided to pick up the Hornady cartridge headspace tool with the "B" insert. I assembled the unit and affixed it to the calipers, inserted a sized SSA cartridge, and zeroed it out. Then I inserted a fired SSA cartridge (one of my reloads) and it measured 0.002. Sounds good, maybe a need to bump it a touch more but good. I checked a few more to make sure things were consistent and even spun the cases to make sure they were not bent, everything checked out. Then I grabbed a fired Hornady factory (VMAX) cartridge and measured it, -0.002. Ummm what? What's going on here? :a32:
 

·
Registered
Joined
·
86 Posts
Brass has some spring to it and 004-.005 is just to stay within a safe margin in semiautos. When reloading sometimes once fired gets mixed with 2 to 7 times fired brass. They all respond differently when sized and sometimes my sized brass has been bumped back .002 or .005, it doesn't really matter unless you are a precision shooter. 002 may not be enough so move back a couple thousands and don't worry about it. Factory brass within its tolerances will vary, and from brand to brand the tolerance gets even greater. Once you set your die and your rifle functions properly........enjoy it!
--tikka
 

·
Premium Member
Joined
·
826 Posts
Uh, I don't know how this might play into the numbers, but the B insert measures the case shoulder at the .350 datum line. A fired 9mm case will measure some where near the .356 line on the shoulder....... As long as you get reliable function from your reloads, it's all good.
 

·
Registered
Joined
·
3,873 Posts
Where you measure the datum line really isn't an issue as long as you do it on the flat area of the shoulder. The Hornady unit is more precise, but as long as you use the same tool to measure fired and sized, you're good to go.

As far as how much to bump, some will tell you .004-.005", but I think that's too much. I've found .003" max to be about right. Enough to ensure good chambering, and almost zero case growth, but not so much you introduce headspace issues.

That's the amount I've used on .277 Wolverine, 6.8, and 7mm Valkyrie, and soon on .358 Yeti, all in ARs. I have never had an issues with chambering or reliability, and case life has been extraordinary due to the fact that brass just doesn't stretch. I haven't trimmed a case in at least three years because they just don't need it. I also don't crimp bottleneck cases so minor differences in length don't matter.

One thing I've learned using the Hornady tool is that you never rely on just one case. I measure a minimum of half a dozen fired cases, and take the average to get the starting number. Same thing on the sized cases after getting the sizing die set. Brass is ductile, and as it work hardens it will vary in the amount of spring back it has. That results in measurements that are similar, but within a range. Measuring several and taking an average cancels out the high and low. If you use the .003" bump there is plenty of leeway in there for everything to work slick as goose grease.
 

·
Super Moderator
Joined
·
12,228 Posts
Where you measure the datum line really isn't an issue as long as you do it on the flat area of the shoulder. The Hornady unit is more precise, but as long as you use the same tool to measure fired and sized, you're good to go.

As far as how much to bump, some will tell you .004-.005", but I think that's too much. I've found .003" max to be about right. Enough to ensure good chambering, and almost zero case growth, but not so much you introduce headspace issues.

That's the amount I've used on .277 Wolverine, 6.8, and 7mm Valkyrie, and soon on .358 Yeti, all in ARs. I have never had an issues with chambering or reliability, and case life has been extraordinary due to the fact that brass just doesn't stretch. I haven't trimmed a case in at least three years because they just don't need it. I also don't crimp bottleneck cases so minor differences in length don't matter.

One thing I've learned using the Hornady tool is that you never rely on just one case. I measure a minimum of half a dozen fired cases, and take the average to get the starting number. Same thing on the sized cases after getting the sizing die set. Brass is ductile, and as it work hardens it will vary in the amount of spring back it has. That results in measurements that are similar, but within a range. Measuring several and taking an average cancels out the high and low. If you use the .003" bump there is plenty of leeway in there for everything to work slick as goose grease.
Agreed. It is for comparative purposes so if you use a 9mm shell case that is fine, just make sure you use the same 9mm shell case each time. I try to bump mine back no more than .002".
 

·
Registered
Joined
·
561 Posts
Agreed. It is for comparative purposes so if you use a 9mm shell case that is fine, just make sure you use the same 9mm shell case each time. I try to bump mine back no more than .002".
I been doing the same try to keep around .002 and I keep my brass in a mtm box from start to scrap.
 

·
Registered
Joined
·
435 Posts
I have found that often the primer will extrude a couple of thousands out of the primer pocket and make exact measurement difficult at best. I have started decapping the brass first with an old Lee Loader decapper. With the primer removed, the measurement of the case is much more exact.

Cazador is correct about brass springback. You can't compare your many times loaded reloads with factory fresh once fired. Even different lots once fired from the same manufacturer will vary in springback.
 

·
Registered
Joined
·
631 Posts
I have found that often the primer will extrude a couple of thousands out of the primer pocket and make exact measurement difficult at best. I have started decapping the brass first with an old Lee Loader decapper. With the primer removed, the measurement of the case is much more exact.
Ditto.

10 characters
 

·
Registered
Joined
·
154 Posts
Discussion Starter · #9 ·
I noticed that and removed it with my lyman decapping die. So it must just be bounce back.
 

·
Super Moderator
Joined
·
12,228 Posts
Cazador is correct about brass springback. You can't compare your many times loaded reloads with factory fresh once fired. Even different lots once fired from the same manufacturer will vary in springback.
But it doesn't matter. Once your die is set up to give you the desired set back for you chamber then you lock it down and leave it. The chamber doesn't change.
 

·
Registered
Joined
·
3,873 Posts
I have found that often the primer will extrude a couple of thousands out of the primer pocket and make exact measurement difficult at best. I have started decapping the brass first with an old Lee Loader decapper. With the primer removed, the measurement of the case is much more exact.

Cazador is correct about brass springback. You can't compare your many times loaded reloads with factory fresh once fired. Even different lots once fired from the same manufacturer will vary in springback.
Just a note. If the primer is backing out like that, it is a guarantee that you have excess headspace on such cases. Use a mike to measure how much of the primer is sticking out and add that to the measurement you get to the datum line. That's what your chamber headspace is. Bump the shoulder back .003" from that for your initial sizer die setting. Refine it as needed on the next firing.
 

·
Registered
Joined
·
2,834 Posts
But it doesn't matter. Once your die is set up to give you the desired set back for you chamber then you lock it down and leave it. The chamber doesn't change.
It does. Work hardened cases will spring back. Bumping a shoulder back .002 is not very much. Annealing should be done at least every two to three cycles. If annealing is not a option the shoulder needs to be bumped more.
 

·
Registered
Joined
·
154 Posts
Discussion Starter · #14 ·
It does. Work hardened cases will spring back. Bumping a shoulder back .002 is not very much. Annealing should be done at least every two to three cycles. If annealing is not a option the shoulder needs to be bumped more.
I don't understand that. Are you saying after a few firings my sized brass might not actually be .002 anymore?
 

·
Super Moderator
Joined
·
12,228 Posts
It does. Work hardened cases will spring back. Bumping a shoulder back .002 is not very much. Annealing should be done at least every two to three cycles. If annealing is not a option the shoulder needs to be bumped more.
You mean they will spring back when sizing? That makes sense. I wasn't following What he was trying to say.
 

·
Banned
Joined
·
2,342 Posts
I don't understand that. Are you saying after a few firings my sized brass might not actually be .002 anymore?
after each new firing the brass will fire form to the chamber, if your die is set to bump back .002 then you will bump it back when you next reload that case and resize it

perhaps someone else can explain it better than i, but each time you fire reloaded brass you resize it during processing, when you load it again (if you haven't changed any of your die set up) it will set the shoulder back
 

·
Registered
Joined
·
3,873 Posts
It does. Work hardened cases will spring back. Bumping a shoulder back .002 is not very much. Annealing should be done at least every two to three cycles. If annealing is not a option the shoulder needs to be bumped more.
We call it spring back, but actually it is resistance to being sized because work hardening makes it less flexible. That's why I use .003". If you're sizing brass that has 4 or 5 loads on it, I would also recommend a second run up into the sizer, just to be sure.

As far as annealing goes, once every 5th load is more than enough, provided you do it right. By that I mean using Tempilaq 750 to adjust the flame and the time in the flame. Don't try to anneal based on how the brass looks. Your eye can't differentiate color changes accurately enough to avoid ruining the brass. Slight differences in alloy and how well the brass was alloyed in the beginning make it nearly impossible to tell with accuracy when you reach the right temperature, without going over.

If you're going to regularly anneal a large amount, then make life easier and get an annealing machine. There are several good commercially made models as well as a couple you can make yourself that work well. I built the one I have, and it does a great job.

One final note: If the load you're using makes the primer pockets expand after the 4th or 5th reload, don't waste time annealing. Annealing won't fix a worn out primer pocket.
 

·
Registered
Joined
·
3,873 Posts
I don't understand that. Are you saying after a few firings my sized brass might not actually be .002 anymore?
The process of firing forces the brass to stretch to match the physical dimensions of the chamber. As pressure drops and temperature falls, the brass retracts (shrinks) a bit to allow it to be extracted. This flexing work hardens the brass, because heat and pressure changes its crystal structure a bit. This builds up eventually to the point that the brass splits or otherwise fails.

Sizing the case after each firing also adds work hardening. After several reloads, its ability to be compressed becomes almost nil, and can make cases hard to chamber. That's why neck sizing only doesn't work after several loads. To overcome this, if the case hasn't failed for some other reason, you need to anneal the case at a specific temperature to realign the crystal structure of the brass and relieve the stress due to work hardening, to make the brass flexible again in the shoulder/neck area.

As we've said earlier, each case is slightly different and will size to the dimension you want to a greater or lesser degree. That's why you take an average to cancel out those differences. Even though you may set the sizer to a nominal .003" bump back, the actual measurements will have a range of values between .002" and .004", centered around .003". Keep that in mind when you're trying to set your sizer. Trying to get each case to size to the exact same measurement is impossible and will make you crazy in short order.
 

·
Registered
Joined
·
631 Posts
Just a note. If the primer is backing out like that, it is a guarantee that you have excess headspace on such cases. Use a mike to measure how much of the primer is sticking out and add that to the measurement you get to the datum line. That's what your chamber headspace is. Bump the shoulder back .003" from that for your initial sizer die setting. Refine it as needed on the next firing.
Trying to get this straight in my head. Not trying to be argumentative.
-----
Assume excess headspace of .010.

From your comment then:
As the round is chambered the ejector holds the round against the front of the chamber.

The firing pin strikes the primer and momentarily increase the force pushing the round forward.

As the powder ignites and pressure builds, the case expands into the chamber and is held fast. The initial pressure also pushes the primer from the pocket and against the bolt face resulting in a .010 primer protrusion from the pocket.

As the pressure increases the case is stretched until the case head makes contact with the bolt face, thereby forming the case to the chamber dimensions.
------
In the above, it seems to me that the force of pushing the case head against the bolt face is sufficient to either push the primer back into the pocket, or more likely, to flatten the primer and give the appearance of excess pressure.

As stated above, not trying to be a nay sayer, just trying to get the correct amount of shoulder bump.

Is it possible that as chamber pressure drops, case springback starts while sufficient chamber pressure remains to unseat the primer as the springback pulls the case head away from the bolt face? The ejector would add some slight force to keep the case forward as the case becomes shorter from springback.

When shooting S&B 110 PTS rounds that have by my measurement about .011 to .014 headspace clearance, the primers protrude about .001 to .002 after being fired. My resized to what I think is about .003 to .004 headspace clearance also have primer protrusion of about .001 to .002 after firing. The S&B primers are flattened as though they have higher pressure than my reloads.

Thinking the process through a bit farther makes me wonder if as the brass cools once ejected from the chamber, does it shrink from cooling enough that it actually requires no shoulder bump. The brass will shrink approximately .002 to .004 from ejected temperature to room temperature.
 

·
Registered
Joined
·
3,873 Posts
Trying to get this straight in my head. Not trying to be argumentative.
-----
Assume excess headspace of .010.

From your comment then:
As the round is chambered the ejector holds the round against the front of the chamber.

The firing pin strikes the primer and momentarily increase the force pushing the round forward.

As the powder ignites and pressure builds, the case expands into the chamber and is held fast. The initial pressure also pushes the primer from the pocket and against the bolt face resulting in a .010 primer protrusion from the pocket.

As the pressure increases the case is stretched until the case head makes contact with the bolt face, thereby forming the case to the chamber dimensions.
------
In the above, it seems to me that the force of pushing the case head against the bolt face is sufficient to either push the primer back into the pocket, or more likely, to flatten the primer and give the appearance of excess pressure.

As stated above, not trying to be a nay sayer, just trying to get the correct amount of shoulder bump.

Is it possible that as chamber pressure drops, case springback starts while sufficient chamber pressure remains to unseat the primer as the springback pulls the case head away from the bolt face? The ejector would add some slight force to keep the case forward as the case becomes shorter from springback.

When shooting S&B 110 PTS rounds that have by my measurement about .011 to .014 headspace clearance, the primers protrude about .001 to .002 after being fired. My resized to what I think is about .003 to .004 headspace clearance also have primer protrusion of about .001 to .002 after firing. The S&B primers are flattened as though they have higher pressure than my reloads.

Thinking the process through a bit farther makes me wonder if as the brass cools once ejected from the chamber, does it shrink from cooling enough that it actually requires no shoulder bump. The brass will shrink approximately .002 to .004 from ejected temperature to room temperature.
You're overthinking it. If the primer is backing out and stays that way, then either the pressure isn't rising to a sufficient level to force the case back and reseat it, or you have too much clearance between the case shoulder and chamber shoulder. If the primer backs out and gets flattened as the case tries to reseat, then you have enough pressure and the shoulder will form to the chamber but it may also stretch at the head to do that, and that isn't good.

The problem with factory ammo is that nearly all of it is excessively sized to ensure that it fits in every rifle of that caliber in the wild. It is also why I loaded my own from day one. Any new brass got loaded with a lighter bullet and a moderate load as a fire forming load. That was enough to fully fire form the case, but didn't overly stress it, so head stretch was nil. After that, cases got sized based on the fired headspace for a bump of no more than .003".

There is usually enough slop in the system for the firing pin to push the case up against the shoulder in the chamber when it hits the primer. The force of the primer going off will pin it in place long enough for the main charge pressure to lock the neck and shoulder tight then force the case walls to stretch to seat the case head against the bolt face. So even with the extractor hooked over the rim, that's what happens. If the case is already .008-.014" short, that will start to cause an incipient head separation. One way to eliminate this is to pull the bullets, and set the powder and bullet aside. Then run the neck over a 7mm expander ball to enlarge it slightly. Then resize the neck just enough to leave a small secondary shoulder at the base of the neck. This will cause the case to be a slight force fit in the chamber and hold the case tight against the bolt face. Reassemble the powder and seat the bullet. Now when you fire, you will get a zero headspace condition and a perfect fireformed case. Set you sizer accordingly for a .003" bump.

Another way to do it is to pull the bullet just enough that it is just barely shy of contacting the rifling. The end result is the same as above, but you also run pressures up some. I've done it both ways, but the doughnut on the neck base is better.

The bottom line is that ALL factory ammo starts out with excess headspace. Depending on your chamber dimensions and how they loaded the ammo, your end result will vary. If you aren't going to reload, don't worry about it as long as you get the accuracy you need. But if you do want to reload, then you have to take the factory issues into account, and do what is necessary to cause as little stress to the brass as you feel comfortable doing. When I got into the 6.8, I already knew I was going to have to deal with the headspace issue, so I planned accordingly. I only fired 6 rounds of factory SSA to get a few reference cases, and reloaded everything myself from that point on. I used new Remington cases, and ran them over the expander for my .280 Rem to true up the mouth and expand it to .284. Then I ran the new cases into the sizer (had a shim under it to allow it to create the slight doughnut on the base of the neck. That gave me the zero headspace I needed so I got a perfectly fireformed case every time. If the case head is tight against the bolt face, you'll get zero primer backout, and no false pressure signs. Just use a powder charge that is about mid range and a medium weight bullet. Use those rounds for fouling shots, and to develop trigger control, etc.
 
1 - 20 of 25 Posts
Top