[68inSS]

Last week I posted my Range Report for my Bison Armory 20" SS upper.
I just finaly got around to measuring my chambers headspace and am wondering if I'm out of spec.
Using the Hornady "B" bushing, I got 1.355" from head to datum line. I know the min headspace is 1.346. What is the max?
I averaged my measurement from 10 fired cases. To be fair to Bison, my bolt was purchased from Stag. I hope max is 1.356, I'd then be JUST in but is it recommended that I have some work done to tighten up my headspace?

 

[M14 Kid]

Measuring FIRED cases out of your semi-auto AR chamber will NOT give you accurate headspace information. The headspace dimension of the fired cartridge cases that come out of your rifle will most likely be larger than the headspace dimension of the chamber.

 

[M14 Kid]

I just finaly got around to measuring my chambers headspace and am wondering if I'm out of spec.

OK... continuing onward ---- did you ACTUALLY measure the chamber headspace on the rifle? Or, are you measuring the headspace on fired cartridge cases from the rifle's chamber and ASSUMING that THAT dimension is the same as your rifle chamber?

Using the Hornady "B" bushing, I got 1.355" from head to datum line. I know the min headspace is 1.346. What is the max?

For what it's worth, the difference between min and max SAAMI headspace dimension for most of the bottleneck rifle cartridges that I've dealt with over the years is 0.010".

I averaged my measurement from 10 fired cases. To be fair to Bison, my bolt was purchased from Stag. I hope max is 1.356, I'd then be JUST in but is it recommended that I have some work done to tighten up my headspace?

Again (if I'm reading you right) .... measuring your FIRED cartridge cases isn't telling you what you need to know.

 

[68inSS]

Again (if I'm reading you right) .... measuring your FIRED cartridge cases isn't telling you what you need to know.

Huh? How the heck do I set my reloading die if I don't measure fired cases??? Go/No-go gauges only tell you if your within spec but give you no measurements. Measuring fired cases is what MUST be done to determine your headspace measurements. I've done this countless times!! This method is a tried and true way of determining a chambers headspace dimension.

I think your mistaken, my friend.

 

[Dr Dues]

Measuring FIRED cases out of your semi-auto AR chamber will NOT give you accurate headspace information. The headspace dimension of the fired cartridge cases that come out of your rifle will most likely be larger than the headspace dimension of the chamber.

Ok, then please enlighten us! HOW do YOU measure YOUR barrels ACTUAL headspace??????

 

[owenslee]

Chamber headspace is measured with precision Go/No Go gauges as M14 Kid points out. It is the primary method to determine actual chamber headspace. The only more accurate alternate method to determine chamber headspace / dimensions is to make a cerro cast of the chamber. Using Go / No Go gages is the preferred method used by every professional gunsmith to insure chamber headspace is within specification.

Measuring a fired case may be appropriate to determine the settings for a resize die...but it is NOT the acceptable method for measuring chamber headspace as it introduces additional variables (temporary expansion / case stretch and case contraction...bushing clearances...tolerance stack ups...etc.) that added together can adversely affect the resultant measurement.

Any quality gage is made from a hardened steel or nodular iron material having low rates of expansion to avoid wear and avoid temperature influences. By using a fired case...you are attempting to use a "gage" made from a soft easily distorted brass material that is highly susceptible to expansion or contraction.

The Hornady "headspace" gage should be relegated to resizing brass only. It is not an acceptable method to measure the chamber headspace.


Kerry

 

[owenslee]

... is it recommended that I have some work done to tighten up my headspace?

Decreasing the headspace on an AR barrel is limited to selectively fitting a bolt with locking lugs to the long end of the acceptable tolerance (.277")...and / or selectively fitting a bolt with a case head recess to the shallow end of the allowable tolerance.

Removing the barrel extension and shortening the barrel to reduce headspace requires removing one full thread pitch in order to index the gas port correctly. This shortens the entire barrel by the same distance...including the gas tube. The barrel extension is then reinstalled and the barrel is rechambered to proper specs. A PITA by the time it's all done. And if your original barrel happened to be just barely 16" in length...after shortening the barrel by one thread pitch...it could now be under 16" and technically illegal in the eyes of ATF.

Kerry

 

[Dr Dues]

Chamber headspace is measured with precision Go/No Go gauges as M14 Kid points out. It is the primary method to determine actual chamber headspace. The only more accurate alternate method to determine chamber headspace / dimensions is to make a cerro cast of the chamber. Using Go / No Go gages is the preferred method used by every professional gunsmith to insure chamber headspace is within specification.

Measuring a fired case may be appropriate to determine the settings for a resize die...but it is NOT the acceptable method for measuring chamber headspace as it introduces additional variables (temporary expansion / case stretch and case contraction...bushing clearances...tolerance stack ups...etc.) that added together can adversely affect the resultant measurement.

Any quality gage is made from a hardened steel or nodular iron material having low rates of expansion to avoid wear and avoid temperature influences. By using a fired case...you are attempting to use a "gage" made from a soft easily distorted brass material that is highly susceptible to expansion or contraction.

The Hornady "headspace" gage should be relegated to resizing brass only. It is not an acceptable method to measure the chamber headspace.

Kerry

Thats fine, but Go/No Go gauges don't tell you the exact headspace of the barrel your testing. They ONLY tell you if your barrel falls within those "specs". Yes, brass has its issues WRT measurements, but gives you additional information than only running two gauges .
Yes, casting a chamber is the most accurate way to check those dimensions but measuring your brass can give you good and valuable info.

 

[paulosantos]

Last week I posted my Range Report for my Bison Armory 20" SS upper.
I just finaly got around to measuring my chambers headspace and am wondering if I'm out of spec.
Using the Hornady "B" bushing, I got 1.355" from head to datum line. I know the min headspace is 1.346. What is the max?
I averaged my measurement from 10 fired cases. To be fair to Bison, my bolt was purchased from Stag. I hope max is 1.356, I'd then be JUST in but is it recommended that I have some work done to tighten up my headspace?

When I measure my fired brass, I usually get 1.360". YOu are fine. When you re-size your brass, just bump the shoulder back to approximately 1.350-1.351" and you will be fine.

I think some of the confusion is that some of the guys are using the wrong terminology. The Hornady Headspace Gauge is use to measure the growth of the brass. I don't know why they call it a headspace gauge. It is not to be confused with the go/no-go gauges.

 

[68inSS]

and I'm ordering the go/no-go gauges today!
and thank you all for your input

 

[paulosantos]

and I'm ordering the go/no-go gauges today!
and thank you all for your input

Did you order the Monson ones from Brownells?

 

[borderpatrol]

Forster makes ten or eleven gage sets for .223 and .308 in .001" increments. You can obtain an exact reading in those specific calibers. I would think a custom 6.8 set with 10/11 gauges would be expensive. Another way you can get very very close is to fire the same case at least three times in a row with your regular loads. In order for this to work the gas system must be made inoperable. Bolt guns - no problem. Neck size only after each firing. The case, after being fired in the same chamber without resizing, will be within .001" or so. Don't be surprised if you have some extraction problems.

 

[68inSS]

Did you order the Monson ones from Brownells?

I was going to go with the PTG but I'll take your suggestion.

 

[paulosantos]

I was going to go with the PTG but I'll take your suggestion.

The monsoon are nice because you don't have to disassemble your bolt completely.

 

[68inSS]

As a follow up to this discussion, I just noticed the infamous case head rings on some of my brass (not all). I'm short on time but I will follow up in the next few days with pics. I will also cut one of the cases and get pic, too.

Using my RCBS Case Master I don't see any thinning of the brass though. The gauge should show continually thicker brass as you head toward the case head (and it did) but I did find a few that leveled out above the rings and then continued getting thicker. Never did the gauge go backwards (indicating a crack).

The interesting thing is that this brass was virgin. We just had a discussion about someone knocking the shoulders too soon but this is not the case. For reference here it is: http://68forums.com/forums/showthread.php?t=6272

 

[M14 Kid]

Chamber headspace is measured with precision Go/No Go gauges as M14 Kid points out. It is the primary method to determine actual chamber headspace. The only more accurate alternate method to determine chamber headspace / dimensions is to make a cerro cast of the chamber. Using Go / No Go gages is the preferred method used by every professional gunsmith to insure chamber headspace is within specification.

Measuring a fired case may be appropriate to determine the settings for a resize die...but it is NOT the acceptable method for measuring chamber headspace as it introduces additional variables (temporary expansion / case stretch and case contraction...bushing clearances...tolerance stack ups...etc.) that added together can adversely affect the resultant measurement.

Any quality gage is made from a hardened steel or nodular iron material having low rates of expansion to avoid wear and avoid temperature influences. By using a fired case...you are attempting to use a "gage" made from a soft easily distorted brass material that is highly susceptible to expansion or contraction.

The Hornady "headspace" gage should be relegated to resizing brass only. It is not an acceptable method to measure the chamber headspace.

Kerry

Saved me a LOT of typing! Thanks!

We're both in Missouri and have a lot in common ---- would love the opportunity to meet/chat/shoot together! Got a good place for me to shoot my new 6.8 build??

 

[owenslee]

Saved me a LOT of typing! Thanks!

We're both in Missouri and have a lot in common ---- would love the opportunity to meet/chat/shoot together! Got a good place for me to shoot my new 6.8 build??

Where in Missouri?

I shoot at 67 Gun Club out to 300 yds because it's close to Farmington and a nice facility. For longer distances I shoot at our farm in Mark Twain Nat'l Forest out to 600 yds. Plenty of places to shoot.

There are enough Missouri members that perhaps we could arrange a local shoot if the gun club agrees.

Kerry

 

[M14 Kid]

I didn't realize where Farmington is --- had to look it up on a map. I'm across the state in the KC area, just a few miles from Kansas. Pretty long drive to shoot some guns!

 

[68inSS]

Pictures and measurements

Ok. Finally got around to getting pictures taken of one of my fired cartridges. Remember, not all of my fired cartridges look like this but I'd say 30% are as clearly visible as this one.

Here's the pic that is typical with the measurement from the case head to the "ring":

Remember this was virgin brass, no sizing was done to it. Here's a pic to showing the case head to show that there weren't obvious signs of over pressure. I've got some of my brass that had 30.5 grains of Re10x that show flow signs but there are no case rings:

I cut two cases open that had obvious rings. I only show one half because this is typical of what I found - nothing, really:

IMPORTANT FINDING: the ring on the outside exactly corrisponds to the top of the web-ring on the inside of the case where the webbing blends into the wall.

Using my RCBS case master to measure brass thickness, the brass grew in thickness from mouth to head but ,curiously, stayed constant for a short span ABOVE the outside ring then began to rise again after the ring and the rest of the way to the head.

On the outside of the cases (I measured 4 fired cases), the ring consistantly measured .4195 inches and tapered down from there to the mouth. The rim (I know it's rimmless) of the case measured between .416-.418 inches.

For comparison, several virgin brass cases measured .4165 at the "ring" location and .416 at the rim.

In my amatuer opinion, I lean toward excessive headspace:
1) My fired brass is measuring just short of the max allowed headspace - my fired brass measured 1.355 inches. SAAMI spec is 1.346. +.01
2) Since brass typically "snaps" back after firing, my actual headspace is probably larger then my measured brass
3) No obvious signs on the case head of overpressure

I did order the Manson Go/No-Go gauges and should have them mid-week next week.

Even if I am within specs, would it not be in my best interest to have the barrel reworked so my brass lasts longer? Or after the one big stretch of the virgin brass would I then be OK?

I look forward to all your expert opinions and appreciate your input!
Thanks!

 

[paulosantos]

I honestly wouldn't be too worried since you are reloading and you can re-size the brass to your barrel. As long as your accuracy is good, I wouldn't worry too much about it. I can tell you that the brass in all of my barrels grew to 1.360" and on 3 out of the 6 of my barrels that I checked, the bolt would close on the no gauge with just a little bit of resistance.