308B headspacing

[tomgonz]

I just finished building a DPMS styled AR 308, and used a 18 inch BHW barrel.  I think I understand the 308B chamber is only very slightly different in the throat for improved accuracy.  My question is, should the standard SAAMI spec .308 Win Go/NoGo gauges work as expected?  I would think so, if you did not change the neck area.

I ask because, the bolt will not close on my Clymer 1.630 Go gauge.

So, now I have a lot of questions.

Thanks,
Tom

[GLSHOOTER]

Sept 27, 2013 12:14:42 GMT -8 tomgonz said:

I just finished building a DPMS styled AR 308, and used a 18 inch BHW barrel.  I think I understand the 308B chamber is only very slightly different in the throat for improved accuracy.  My question is, should the standard SAAMI spec .308 Win Go/NoGo gauges work as expected?  I would think so, if you did not change the neck area.

I ask because, the bolt will not close on my Clymer 1.630 Go gauge.

So, now I have a lot of questions.

Thanks,
Tom

Did you pull the extarctor and ejector out of the bolt?

Greg

[tomgonz]

Yep, both were removed, and the bolt would not rotate into position.

[tomgonz]

What do you guys use when making the barrels to set the headspace?  An off the self DPMS bolt, or something you made to the spec needed?  And what is that spec, the back of the lug to bolt face?  I'd like to measure my bolt and verify it is in spec.

Thanks,
Tom

[Deleted]

Sept 27, 2013 12:54:04 GMT -8 tomgonz said:

What do you guys use when making the barrels to set the headspace?  An off the self DPMS bolt, or something you made to the spec needed?  And what is that spec, the back of the lug to bolt face?  I'd like to measure my bolt and verify it is in spec.

Thanks,
Tom

Unless things have changed they use a DPMS bolt. At least that is what mine are done with.

[tomgonz]

Since no one wants to tell what the specification of a DPMS 308 bolt is, bolt face to rear lug distance, I have no way to verify the bolt I do have is in spec or not.  I've done lots of internet searching and made several phone calls, and no luck.  If anyone has the blueprint spec for the DPMS 308 bolt, I would like to see it.

So, I have placed an order for a DPMS 308 bolt from Brownell's.  Of course it is back ordered, like everywhere else on the internet.  So, I don't know how long it will take to get it.  I will use this new bolt to check the headspace.  If it also fails to close on the GO gauge, then the barrel will need to be fixed or replaced.  If it does work, then the previous bolt was likely out of spec.

BHW, please check with your barrel designers and/or makers, they must know what the bolt face to rear lug distance it.  I'd like to know the official spec.  Thanks.

Also, what is the warranty for getting a barrel fixed or replaced for incorrect headspacing?  I bought it a couple of weeks ago, and I don't know when the new bolt will be in.  I will not shoot it until I see the headspace pass.

Thanks again.
Tom

[GLSHOOTER]

Sept 28, 2013 12:00:55 GMT -8 tomgonz said:

Since no one wants to tell what the specification of a DPMS 308 bolt is, bolt face to rear lug distance, I have no way to verify the bolt I do have is in spec or not.  I've done lots of internet searching and made several phone calls, and no luck.  If anyone has the blueprint spec for the DPMS 308 bolt, I would like to see it.

So, I have placed an order for a DPMS 308 bolt from Brownell's.  Of course it is back ordered, like everywhere else on the internet.  So, I don't know how long it will take to get it.  I will use this new bolt to check the headspace.  If it also fails to close on the GO gauge, then the barrel will need to be fixed or replaced.  If it does work, then the previous bolt was likely out of spec.

BHW, please check with your barrel designers and/or makers, they must know what the bolt face to rear lug distance it.  I'd like to know the official spec.  Thanks.

Also, what is the warranty for getting a barrel fixed or replaced for incorrect headspacing?  I bought it a couple of weeks ago, and I don't know when the new bolt will be in.  I will not shoot it until I see the headspace pass.

Thanks again.
Tom

Replacing an out of spec barrel is not a problem. Perhaps Ritch can measure his bolt and see what he comes up with. Alternatively a visit to AR15Armory.com should generate some specs over in their Alternative area. I'm sure we can get some numbers up after Monday. I'll spend some time sleuthing for you.

I have seen various 223/556 HS gauges that while "in spec" one would allow a bolt to seat while the other did not. Have you tried to put a new unfired case or loaded round in it by any chance? If it fits you are good to go. Does the bolt without a HS gauge in place turn and lock up? Go Gauges would have to be specific to the reamer. This is a PTG reamer set up and BHW gauges are specific to those reamers. A Clymer HS gauge will not work on the BHW chamber.

Greg

I got the measurement on the DPMS for bottom to top of lugs is 0.320. The bolt depth is 0.130.

PS: What brand of bolt do you have now?

[GLSHOOTER]

PM Sent.

Greg

[tomgonz]

UPDATE.  After speaking with Ritch, we determined most of the signs are more likely over gassing.  I've ordered a BTE micro adjustable gas block.  That should take care of it.  I also noticed a burr on the bolt face.  After taking that down, and polishing the whole bolt face down a bit, the go gauge will now go. Just barely, but it does.  He said the BHW chambers were on the tight side, that's fine.  Based on a couple of other conversations I've had, I'm thinking now that the manufacture did not take into account the nickel boron plating thickness on the bolt face.  When it's tight, every little bit matters.  I guess that is something to keep in mind.

Can't wait to get it on the range again.  Waiting for the gas block.

Tom

[tomgonz]

Okay, mostly just wanted to know if these measurements make sense.



I have the Hornady case gauge (HCG), using the .400 bushing for .308.  I measured the Go gauge that was
marked 1.630 and the HCG gave a length of 1.622 in.  Am I missing
something here?  Shouldn't it say 1.630 in?



Here are some measurements with the HCG.



New Federal round = 1.620

Fired Federal case = 1.629

Sized Federal case = 1.625

Clymer Go gauge (1.630) = 1.622



I understand it's use is more to check the difference of the fired to sized cases.

But, I guess I thought the HCG would give me a 1.630 on the 1.630 Go gauge.



Thoughts?

Tom

[GLSHOOTER]

Oct 4, 2013 14:54:02 GMT -8 tomgonz said:

Okay, mostly just wanted to know if these measurements make sense.



I have the Hornady case gauge (HCG), using the .400 bushing for .308.  I measured the Go gauge that was
marked 1.630 and the HCG gave a length of 1.622 in.  Am I missing
something here?  Shouldn't it say 1.630 in?



Here are some measurements with the HCG.

New Federal round = 1.620

Fired Federal case = 1.629

Sized Federal case = 1.625

Clymer Go gauge (1.630) = 1.622

I understand it's use is more to check the difference of the fired to sized cases.

But, I guess I thought the HCG would give me a 1.630 on the 1.630 Go gauge.

Thoughts?

Tom

Looks like a call to CLYMER is in order on that one.

Greg

[tomgonz]

At first I was thinking there is something up with the Hornady Case Gauge.
As a sanity check, I also picked up a Foster .308 Go gauge, which is also marked as 1.630 in.  It measures 1.6215 with the HCG.

I just look at the HCG bushings with a 10x loop.  They all have a very slight bevel right where the case shoulder would touch the bushing.  That would easily cause measurements to loose about 0.008-0.009 inch.  I guess that is a mystery solved.

I guess the HCG is not meant to measure true datum line to case head, but rather a means to measure the difference from a fired case to a sized case.  The bevel would not affect that.

Has anyone else notice this bevel on the HCG bushings, or measured a Go Gauge with it?  Just curious.

All is fine here. Thanks.

Tom

[Deleted]

Oct 5, 2013 11:07:13 GMT -8 tomgonz said:

At first I was thinking there is something up with the Hornady Case Gauge.
As a sanity check, I also picked up a Foster .308 Go gauge, which is also marked as 1.630 in.  It measures 1.6215 with the HCG.

I just look at the HCG bushings with a 10x loop.  They all have a very slight bevel right where the case shoulder would touch the bushing.  That would easily cause measurements to loose about 0.008-0.009 inch.  I guess that is a mystery solved.

I guess the HCG is not meant to measure true datum line to case head, but rather a means to measure the difference from a fired case to a sized case.  The bevel would not affect that.

Has anyone else notice this bevel on the HCG bushings, or measured a Go Gauge with it?  Just curious.

All is fine here. Thanks.

Tom

I just measured my Forester go gauge. I used the same Hornady tool you did. Mine measures 1.618. Go figure.

[tomgonz]

Hi all,

I just wanted to wrap up this posting with my last range visit.  Short version, everything seems to be working well.

I did get and install the BTE adjustable gas block.  I'm completely sure I needed it, but I feel better that it's there and adjusted properly.  If go in half turn on the adj screw, then it fails to lock back on last round.  I back the screw out a half turn, and all my Match ammo locks back on last round.

I ran several brands of Match ammo through the gun, and all function well without any signs of over gassing from the extractor on the rim.  A few still showed some shinny ejector marks on the head, but nothing else.  Everything cycled, everything locked the bolt back, and no brass got stuck in the chamber.

Federal 168 gr Match
Winchester 168 gr Match
Black Hills 168 gr Match
PPU 168 gr Match

I forgot to bring my normal bench rest pads, so I'm sure I'll do better on the next visit.  I was able to keep 5 shot groups within 1.5 inch at 100 yards.  I had a couple of groups under 1 inch.  Very happy with that.  Hope to reproduce that with some reloads.

The cheap PPU 145gr ball ammo still ran a bit hot.  I guess that is not a big surprise to most.  I pulled a few of them and dropped the powder charge from 46 gr to 42 gr.  Those all run just fine.  I'll probably do that to rest of the 3 boxes I have.

That's it.  Now to paint some parts of the gun, and start some reloading.

Tom

[GLSHOOTER]

Congratulations in getting it running. Always nice to hear of a success. The adjustable block is teh only way to fly IMHO on many of these rifles and with the disaprity of loadings out there it makes good sense to cover all your bases.

Greg