Head Space Fired cases ?

[Chubbs]

Hi Guys
On a bolt action rifle what is the biggest expansion of the case head space you could expect ?
Example. If bumping a fired case using a F/L die so that it just chambered freely at a head space 1.617" and tight at 1.618".
How much bigger could the head space on an ejected case be. Are we talking 1 or 2 thou or could it be as large as 5 thou?

 

[Richard Coody]

Hi Guys
On a bolt action rifle what is the biggest expansion of the case head space you could expect ?
Example. If bumping a fired case using a F/L die so that it just chambered freely at a head space 1.617" and tight at 1.618".
How much bigger could the head space on an ejected case be. Are we talking 1 or 2 thou or could it be as large as 5 thou?

I would say that either you brass was not fully fireformed before you set up your shoulder bump or your technique for calculating your shoulder bump was flawed.

If a fired.case chambers easily then you are not ready to set up your shoulder bump. When a fired case chambers.tight then set up a 1 or 2 thousandth bump. That falling free just might not work for your rifle.

 

[rjtfroggy]

I have one that a fired case measures .006 bigger than sized. Measured for free fall of bolt Wheeler way. Now I will not bump that far (only go .002) but now after firing a couple of times ( 3-4) they have stabilized.

 

[Chubbs]

I have just pulled out 90 old cases which have been fired at least 8 - 12 times. Never been annealed. They are measuring at
4 x 1.6120"
55 x 1.619"
23 x 1.618"
8 x 1.617"

Brand new Lapua cases are measuring at around 1.614" / 1.615" with some down as far 1.613"
Measuring some once fired 308 Federal cases which have been sized down to Rem 260 and fired again I have some which measure 1.622"
Which is why I wondered how big the case can stretch as I am almost positive the chamber will be around 1.618" / 1.619"

 

[JFrank]

Hard for a case to expand further than the chamber. 3x fired has been plenty for my purposes.

 

[LCazador]

Like SPJ says hard for a case to go past the chamber limits but a hot load could push it past. On a new case yes you could go past a couple of thou even using a moderate load to fireform. Your 308 cases could give 4-5 thou difference till fireformed.

 

[Chubbs]

Like SPJ says hard for a case to go past the chamber limits but a hot load could push it past. On a new case yes you could go past a couple of thou even using a moderate load to fireform. Your 308 cases could give 4-5 thou difference till fireformed.

That was more my thought thinking the shoulder is not fully formed from the 308 to the 260 which has a sharper angle.

 

[K22]

It takes some practice to develop a consistent technique to measure case head space. The amount of pressure applied and the alignment of the case needs to be consistent.

One of the reasons I like the Whidden Gun Work single piece bump gauge is that there are not interfaces between the gauge and holder because there is no holder. By marking the holder and insert bump gauges designed as such to create indexing marks, you can obtain more consistent insert and holder assembly if you are disassembling the holder and insert after each reloading session.

Also if you're using the Hornady anvil to center and position the case in the caliper, check to make sure it mates squarely with the jaw of the caliber. A friend had consistency problems and we traced it to the anvil.

I've encountered some similar variation in fire case head space in some rifle with some brands of cases. However when sized the cases are within + or - .001" after bumping. Some things that can affect bump consistency: type of lube, consistency of lube application, and sizing technique i.e. speed of running the case through the sizing die.

My rifle smith also told me that excessive lug wear especially uneven wear can also affect fired case head space consistency.

 

[Brians356]

Hard for a case to expand further than the chamber.

It can happen. Bolt compression.
-

 

[Texas10]

If you are measuring the base to shoulder of a fired case WITH the primer still installed, chances are that you are not getting a true measurement. You must remove the spent primer to get a reliable measurement.

As SPJ wrote, once the case is fully fireformed, it typically will only grow enough that chambering it again right after firing may produce a stiff bolt close. If the bolt will not close on a freshly fired case, you're probably dealing with a very hot load, so beware.

A case that's had a number of cycles on it will work harden and not spring back after firing, making extraction difficult. These are known as "bolt clickers" for the sound of the bolt as it's forced open. That when you know it's time to anneal.

 

[Ron Blain]

Hi Guys
On a bolt action rifle what is the biggest expansion of the case head space you could expect ?
Example. If bumping a fired case using a F/L die so that it just chambered freely at a head space 1.617" and tight at 1.618".
How much bigger could the head space on an ejected case be. Are we talking 1 or 2 thou or could it be as large as 5 thou?

I am not sure which cartridge you are speaking about but I can give you an example using a .308 Winchester case. A new factory round should measure case head to datum a bout 1.630" and the datum line is where the case neck crossed the 0.400' inch line of the case neck. When a case id fired under normal conditions it expands to fill the entire chamber and then shrinks bout 0.001" It should measure chamber headspace less 0.001". Using the .308 Winchester if my loaded case was 1.630" (nominal) and comes out measuring 1.633' it leads me to believe my chamber headspace is about 1.602"

Before getting any deeper into this rabbit hole It would be good to knoe the cartridge in question ans which gauge(s) you are using to get your numbers?

Ron.

 

[mr45man]

It can happen. Bolt compression.
-

Maybe, but you need a hammer to open it

 

[Chubbs]

All measurements with primers removed.
Lapua Rem 260 cases.
Hornady H/S comparator and CBTO comparator used together to give flat base. Found anvil not level. Seems Hornady forgot it's quality control on these. Lots of people saying not level. Probably the coating throwing them off.

 

[rjtfroggy]

Get in touch with member Delv7915 and never worry about your caliper base again.
Best $21 I've spent for reloading in a long time.
He is also in the classifieds.

 

[Ron Blain]

All measurements with primers removed.
Lapua Rem 260 cases.
Hornady H/S comparator and CBTO comparator used together to give flat base. Found anvil not level. Seems Hornady forgot it's quality control on these. Lots of people saying not level. Probably the coating throwing them off.

OK, the problem with The Hornady Lock-N-Load® Headspace Comparator & Anvil Base Kit is that it really is only good as a comparator. The numbers you are seeing are not at all accurate. However if you measure a case after firing and adjust your sizing die to set the shoulder back 0.002" (or any other number) the sized case should easily chamber in your rifle. The 260 Remington has the same headspace dimensions I mentioned earlier. Here is where the glitch is. Below I zero a 0.400 bushing and measure a known accurate 1.630" chamber headspace gauge.

With known 1.630 chamber headspace gauge:

The gauge is reading 0.006" low. Now I insert a known 1.634" chamber headspace gauge actually a No/Go gauge.

The gauge does accurately measure the difference going from 1.624" to 1.628" but the number is still way off the true dimension.

"The gauge measures variations in brass before and after firing or re-sizing. It allows for headspace comparison between fire-formed brass and re-sized brass".

That's all it is, a comparator arrangement for before and after. Also, as mentioned the cases need to be inserted straight and the same for each measurement. The inaccuracy rest with the bushings and if you look closely the bushings do not have a sharp corner break but rather a slight radius.

There are better systems made and better bushings made as pointed out.

Ron

 

[Chubbs]

Thank you Ron for your replies. Very informative. I was aware that the Hornady gauges where not giving actual chamber dimensions. I was referring to them as a datum point to explain the difference in datum readings which you have explained far better than I.

---

 

[Doom]

Without going into a detailed metallurgical explanation, suffice it to say that on firing both the case and chamber expand radially and axially. The chamber expands then returns to it original dimensions. The case expands but does not return to its original dimensions but does return to an interim dimension that hopefully is less than the chamber dimension. Otherwise the case is hard to extract. There is a point at which where with enough pressure the case will expand and will conform to the chamber dimensions but this pressure is beyond normal chamber pressures. It is likely that even after multiple firings that the case does not conform exactly to the chamber dimensions.

 

[fatelvis]

It takes some practice to develop a consistent technique to measure case head space.

I agree with this, but I found using Redding's "Instant Indicator die w/ gauge" eliminates ALL variance in measuring technique, and is rock solid repeatable without a "feel" or practice with the tool. For me, it is worth the money to know for sure. To make it REALLY user friendly, couple the indicator with a digital gauge instead of a dial gauge.

 

[K22]

I agree with this, but I found using Redding's "Instant Indicator die w/ gauge" eliminates ALL variance in measuring technique, and is rock solid repeatable without a "feel" or practice with the tool. For me, it is worth the money to know for sure. To make it REALLY user friendly, couple the indicator with a digital gauge instead of a dial gauge.

Not familiar with it but it sound like an improvement over the bump gauge / caliper system. I know it took me some time to get my technique down to I could obtain repeatable measurement.

Not cheap either but I wonder if the RCBS Precision Mic might be easier to use also and less apt to drift since there is no dial gauge involved.

 

[Lefty Trigger]

If you have fairly new brass, sometimes the base is not exactly flat and the will cause weird readings on a few cases.