Jump to content
308AR.com Community
  • Visit Aero Precision
  • Visit Brownells
  • Visit EuroOptic
  • Visit Site
  • Visit Beachin Tactical
  • Visit Rainier Arms
  • Visit Ballistic Advantage
  • Visit Palmetto State Armory
  • Visit Cabelas
  • Visit Sportsmans Guide

Zero headspace is perfect


Alan Waters
 Share

Recommended Posts

2 minutes ago, 308kiwi said:

Alan suggested removing material from the rear of the barrel extension shoulder or the front of the receiver face, either would shift the entire assembly rearward.

But yes absolutely, Fit the barrel extension, cut the chamber to head space THEN drill the gas port?

Flange is what threw me at first too, I believe he's referring to the face on the muzzle end of the barrel extension.

Link to comment
Share on other sites

  • Replies 65
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Posted Images

3 minutes ago, Alan Waters said:

I'm not measuring the headspace between the the barrel extension and the chamber. That is not headspace. If you set the barrel/extension back it locates the chamber closer to the bolt face.

Sorry dude but head space in an AR system is measured between the front face of the barrel extension lugs and the datum point of the shoulder area of the chamber, end of story.

You can shift the barrel/barrel extension assembly wherever you want but it will not change this relationship.

Link to comment
Share on other sites

6 minutes ago, 308kiwi said:

Sorry dude but head space in an AR system is measured between the front face of the barrel extension lugs and the datum point of the shoulder area of the chamber, end of story.

You can shift the barrel/barrel extension assembly wherever you want but it will not change this relationship.

Well, the whole conversion is not about how headspace is measured. It's about how to reduce the distance between the bolt face and the rear of the cartridge which is headspace.

Link to comment
Share on other sites

5 minutes ago, Alan Waters said:

 

No doubt about that. Its a steep learning curve for me.

Then quit trying the stubborn-machinist argument on it, sating your idea will work, then defending it...   and listen to what's being given to you.  You have any idea how many precision AR barrels Kiwi has made from scratch and bar stock, in New Zealand, for customers?  You should re-read, and listen to what he's trying to tell you.

Shiit, I can't make an AR barrel for anything - but I know the steps it takes to make one.  Even I can see that what you're describing will not work. You can't afford to set a BCG body back int he upper at all.  Nothing.  Nada.  You fuk with that, and you just fucked up the whole operation of the gun.  Then you're compensating for the next fuk-up, machining something else...

Listen, man, listen. 

Link to comment
Share on other sites

46 minutes ago, Alan Waters said:

Well, the whole conversion is not about how headspace is measured. It's about how to reduce the distance between the bolt face and the rear of the cartridge which is headspace.

 

56 minutes ago, 308kiwi said:

Sorry dude but head space in an AR system is measured between the front face of the barrel extension lugs and the datum point of the shoulder area of the chamber, end of story.

You can shift the barrel/barrel extension assembly wherever you want but it will not change this relationship.

This needs to be reviewed - Kiwi is giving the proper information on headspace.

Edited by 98Z5V
Link to comment
Share on other sites

46 minutes ago, 98Z5V said:

 

This needs to be reviewed - Kiwi is giving the proper information on headspace.

To clarify what I said "head space in an AR system is measured between the front face of the barrel extension lugs and the datum point of the shoulder area of the chamber"

This obviously includes the bolt, which is a fixed component that you cannot, (should not) alter.

 

Link to comment
Share on other sites

 Just saying HS is measured from the Bolt Face to the  SAAMI Shoulder datum point is enough . The fitment of the Barrels Extension is Critical for proper Function & HS .Sketch is a little crude , but shows it's just not as easy to adj. HS by trimming the Barrels Extension . The relationship of all the Action components are critical , as has been said , it's a domino effect , if you start changing the dimensions/spec's of one of them.

Headspace_zps7iieakyk.thumb.png.fc8f6ba5a24b8edfeea89f874fec6597.png

Link to comment
Share on other sites

If my AR fails headspace, I just email the manufacturer of the quality bolt or quality barrel I purchased and most of the time one of those good folks will help sort it out and replace the offending item. This sure doesnt look like a DIY fix for 90% of shooters. 

Edited by blue109
Link to comment
Share on other sites

3 hours ago, blue109 said:

If my AR fails headspace, I just email the manufacturer of the quality bolt or quality barrel I purchased and most of the time one of those good folks will help sort it out and replace the offending item. This sure doesnt look like a DIY fix for 90% of shooters. 

Think this is the best advise on the thread so far for the average builder.

Still glad this topic came up and was discussed. @Alan Waters you may have been off on some of your statements but that's ok as long as you learned some things. I for one learned a lot through the discussion. Thanks for bringing the subject up. 

Link to comment
Share on other sites

  • 3 weeks later...
On 11/13/2018 at 11:17 PM, Alan Waters said:

Agreeing that the barrel and extension become one after proper assembly, if material is removed from the rear of the barrel extension flange, the chamber will move rearward an equal amount. You could get the same effect by setting back the receiver face.

Well, I am embarrassed to say it has taken 21 days for the light to come on at 2 am this morning. The statement above is true. However, what I did not see was if the chamber is moved by the method described above the barrel extension lugs also move an equal amount so the headspace would remain the same. Obviously, I am not a smart as I thought. My apologies to all those who's blood pressure spiked during the exchange.  Regards to all, Alan 

Link to comment
Share on other sites

5 hours ago, ARTrooper said:

Takes a good man to admit when he was wrong. we all make mistakes

Absolutely.

9 hours ago, Alan Waters said:

Well, I am embarrassed to say it has taken 21 days for the light to come on at 2 am this morning. The statement above is true. However, what I did not see was if the chamber is moved by the method described above the barrel extension lugs also move an equal amount so the headspace would remain the same. Obviously, I am not a smart as I thought. My apologies to all those who's blood pressure spiked during the exchange.  Regards to all, Alan 

Blood pressure remained stable throughout the conversation?

This is the best forum to learn about these particular firearms, so much information available from a lot of guys that have done the hard yards.

 

Link to comment
Share on other sites

headspace is an interesting topic. During the firing process the pressure in the primer pocket created by the primer firing makes the primer act like a DI gas pistol, the primer backs out of the pocket, taking up any headspace present. Then if pressure in the case rises (it does not if you are say just popping primers in an empty case, or firing wax bullets or the Speer plastic bullets) the case gets forced back over the primer. if you measure most new 45 acp brass you well see that it is WAY shorter than the saami chamber. This process I laid out with the primer is exactly why some fired 45 acp primers look like the gun had 65,000 PSI going on, in a ctg where the maximum pressure is a sedate 21,000 PSi. The primer cup IMHO sort of "rivets" or bulges before the case gets shoved back onto it, creating that flattened primer look you would get with a REALLY hot rifle round and maybe even 0.000 headspace. As far as the primers backing out, you can see that with almost any revolver by just firing primed cases only, sometimes they will even tie the gun up, folks who shoot wax bullets drill out the flash holes to prevent this from happening.

So now back to high pressure rifles. The danger there is that the round will be driven forward by the firing pin, the primer will back out, the pressure will rise, the case will GRIP the chamber up where it is thinner on the front end, then the back end of the case will get stretched back to the breechface, and forced back over the primer. The reason the field gauge is .008 is because in the real world that will still work fine, it is not where things SHOULD be...and perhaps if it STARTED out right how the heck did it GET to .008..what moved ??

The excessive headspace deal really crops up if you take the same case and fire it, then resize it back to a saami headspace length of the shoulder datum, then stretch it again, then resize it, and stretch it again...to that a few times and you will see an incipient head separation, a shiny spot where the case head will break right off if you keep going.

When chambering a barrel on a bolt action I typically aim for a "feel" on a go gauge..this means we are at zero. Then ideally I bump the shoulder for .002 headspace when sizing brass. .001 or zero might be fine too, -.001 or more means the rifle has to cam the bolt shut with some force, which creates unnecessary wear over time. Folks that just neck size brass can see that over time, I did it to one 22-250 rifle myself thinking neck sizing was the way into some secret accuracy society :-).

Now say you have some oddball rifle that has excessive headspace...not a common caliber or one you shoot a LOT, so what do you do ?? I have a Savage model 24V 222/20 gauge that is exactly that way. Not practical to re barrel, it is a cool gun for what my dad bought it for. So what he did, and I do, is just make brass to fit that chamber length. If you want to do it "perfect" you neck the 222 brass up to say 25 caliber, or 6mm, then back out the 222 full length sizer die, and gently bump the false shoulder back a little at a time until the gun just closes on the brass, then fire form them, and just bump the shoulder enough to allow the gun to close on the rounds. So you have created a special case just to fit that gun.

The same can be done with belted magnums, technically they headspace on the belt, but you CAN adjust your dies to get that .002 shoulder bump, then you are not blowing the shoulder out each time, then shoving it back again when you resize. Obviously one might make different decisions if the rifle is a 375 H&H used for dangerous game, or a 7mm or 300 win mag used for beasts that eat grass :-).

Just some stuff to think about on the subject and maybe flesh out the "why" of this stuff...I would not lose a wink of sleep about a rifle that was .001 past no go, unless it started out "snug on a go" and somehow ended up .001 over no go, I would not let that rest until I knew exactly why things moved.

 

Also back to the 45 acp thing from above, the headspace gauge only considers the chamber, the brass or the ammo is the other half. Guy I know was a Thompson Center sponsored shooter, mostly black powder stuff. But TC took a group of them up to Canada on a hunting trip, TC supplied guns and ammo....I seem to recall it being 6.5x55. Well they stalked game, Deer I think, took aim, and when they fired "click"...cock the gun again..."click", cock it again...."BOOM".

Root cause ?? Saami chambers...right on the money, and ammo that was way short on the datum dimension to the shoulder.

So if you totally wig out about the headspace of your chamber.......and lose sleep over that extra .001" ?? have you ever checked your ammunition ?? :-).

 

Bill

 

Bill

 

Edited by willbird
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...