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Is a barrel extension always compatible? .308 platforms


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#1 fngmike

 
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Posted 01 October 2010 - 02:01 PM

Hi,

Just as the subject states.  I know there's a difference in thread for barrel nuts between dpms and armalite, but is the extensiion the same?  It would seem that it should be but I would like to confirm this once and for all.  I've changed the barrel on the 15, but only assembled lowers on the 10s.  Also, would anyone happen to know how much length the extension adds to the barrel?

Thanks for any help,

Mike

#2 imschur

 
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Posted 02 October 2010 - 02:02 AM

They are different. You must use the correct bolt to match the extension

#3 98Z5V

 
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Posted 02 October 2010 - 02:07 AM

  Also, would anyone happen to know how much length the extension adds to the barrel?

Thanks for any help,

Mike


It doesn't add any length to the barrel, if you're wondering about barrel length determination.  Barrel length would be from the face of a closed bolt (seated inside the extension), to the end of your barrel.  It gets particular if you're running a barrel under the 16" legal minimum, for ATF Tax Stamp (SBR) purposes.

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#4 fngmike

 
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Posted 02 October 2010 - 02:31 AM

I understand what you're saying about measuring from the bolt face for the actual length.  The reason I ask the question is that I've been looking at Lilja's replacement AR10 barrels.  That's how he depicts them on his website at least.  However, he has a shorter barrel that he's listing as a 20" including the length of the extension.  So to me that means it's slightly shorter since the bolt face is inserted an inch or so through the extension.

I've only seen Rem 700s rebarreled at a friend's machine shop.  That's kind of fairly intuitive when you watch how it's done.  I understand that this would be similar but again it's a different perspective.  I would assume once the blank is profiled, you would need to know a finite distance of thread to cut for the extension.  Where this gets tricky in my mind is installing the indexing pin and drilling the gas port between lands. 

I would then guess once the extension is in the proper place you cut the chamber using a new bolt and go gauge as you cut.

I found a link for this site over on SnipersHide.  There's a lot more info here on gas guns and I'm really not interested in 15s any longer.  SH has a group buy going on over there for 6.5 Creedmoor barrels from Lilja.  But I can't get much info as they're already saying for dpms.  If the extensions are different because of different bolts then I guess I won't be in on that.

#5 imschur

 
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Posted 02 October 2010 - 02:34 AM

Keep in mind its just the bolt you need and not a complete bolt carrier group

#6 98Z5V

 
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Posted 02 October 2010 - 03:37 AM

  I would assume once the blank is profiled, you would need to know a finite distance of thread to cut for the extension.   Where this gets tricky in my mind is installing the indexing pin and drilling the gas port between lands. 


Exactly.  That is the very thing that determines headspace dimensions.  Mill the barrel extension wrong, and the headspace is off.  That has to happen first - barrel extension milling.  After that, once the extension is fixed in place, you have barrel centerline.  With that, you can mill for the pin and gas port.

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#7 fngmike

 
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Posted 02 October 2010 - 04:28 PM

Keep in mind its just the bolt you need and not a complete bolt carrier group


I'm not sure what you mean.  Yes, you'd only need the bolt to headspace.  I've been trying to find this info, could you use a dpms bolt in an armalite carrier?  From what I've found so far the answer looks like no to me.  It looks as though the dpms uses a different cam pin.  If I have time I might run over to a dpms owner's house and find out for sure.  This is the most frustrating thing to me about the .308s, nothing is standard between the manufacturers hardly.  I was hoping with the adoption of the M110 we might see some standardization but I doubt it.

#8 imschur

 
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Posted 02 October 2010 - 04:44 PM

No first hand experience with the Armalite carrier but by the data I have researched you can use either carrier on both platforms as long as you use the proper bolt to match the extension type.

I look forward to hearing back on what you observe.

BTW I would love to have some pictures of the two BCG's side by side for comparison


#9 VictorCharlie

 
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Posted 09 November 2010 - 02:35 AM

fngmike,

The timing issues are set by the gas port. First the port is drilled in the barrel and then everything [thread form and shoulder] is set off the port location.

The same held true for FAL variants if port location was desired to be in the groove as opposed to land/groove overlap or on the land. Personally, I've never experienced anything detrimental [accuracy] or seen an actual benefit from having the port in a given place. What was paramount was the absence of a burr at the break-out point, where the tool came through to the interior of the barrel.

As to the thread pitch commonality between manufacturers: Good luck on getting two manufacturers to actually talk to each other and  set the major, minor, etc. tolerances to a common tolerance and stick to the agreement.

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#10 fngmike

 
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Posted 09 January 2011 - 07:01 PM

I thought I had posted back to the forum but ran into yet another question.  Even though I found DPMS instructions online, I couldn't remove the bolt from the carrier.  And since I only bought an upper and stripped lower, I had no manual for my Eagle Arms.  So I guess my next question is who has removed their bolt for cleaning?  Because on both these rifles you'd have to remove the gas key to remove the bolt.  I'm considering filing or dremeling one side of my cam pin so I could actually rotate and remove it.

But I did learn a couple of things.  First, my EA upper from Armalite doesn't have a floating firing pin.  There is a spring on the firing pin to prevent the primer from being struck when the action closes.

Second, since I couldn't remove the bolts, I swapped the entire BCAs between rifles.  And guess what?  The actions would cycle.  Now I didn't headspace these and I am NOT saying I recommend this in any way shape or form.  If you try this and fire a round, you're on your own and consider yourself warned.  I wasn't able to take any photos and didn't bother with any measurements.  Visually the bolt faces appear to be about the same.  And they definitely fit into the other's barrel extension.

So I am back to my original question.

#11 fngmike

 
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Posted 09 January 2011 - 09:51 PM

Ok, hold the phone.  I finally pried, beat the cam pin out of my carrier.  I even took some pics of it to show how tight that cam pin is.  I can't believe they aren't made like the 15s.  The panther's owner couldn't get his bolt out either so I certainly wasn't going to mess with his rifle.  I'm going to borrow the rifle again when I relocate my headspace gauges.  I want to see what I get if I swap the bolts. 




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