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Page AR-15 » AR-15 / M-16 Retro Forum
AR Sponsor: bravocompany
Posted: 11/17/2014 11:14:31 AM EDT
So I just got my tax stamp back on my Colt M4 lower, which I already have a 10.5" Mk18 Mod1 upper for, but next on the chopping block (pun ) is my XM build. It currently sits with an NDS XM16 lower, M16A1 upper, and a 16" light barrel. I have it set up right now to be an XM177E1 when it's chopped, but I may change it to a 607, not sure yet. Anyway, either way I'll need the barrel cut to 10". I have a buddy whose dad owns a gun shop and I would trust them to cut the barrel, but would I need to have the gas port opened up at all with the muzzle being so close to the FSB? Also, does anyone possibly have a picture or diagram of the exact dimensions needed for the 10" XM barrel at the muzzle? Thanks!!
Link Posted: 11/17/2014 11:47:34 AM EDT
[#1]
Don't have specs, but personally I'd size the gas port same as a Mk18 - we know that works...
Link Posted: 11/17/2014 1:38:46 PM EDT
[#2]
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Quoted:
Don't have specs, but personally I'd size the gas port same as a Mk18 - we know that works...
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Yeah, a lot has changed since the original xm's were built, as far as getting one to run.  The Mk18's with their 10.3 inch barrels run fine with .070-.071 ports, ASSUMING you are running mil-spec 5.56 ammo. Daniel Defense sells their commercial 10.3 inch barrels with an .080 port, as they assume you might run Wolf, Tula, PMC or other lower pressure .223 ammo.  When you have the barrel cut the frt sight will be off.............a good time to size the hole.
Link Posted: 11/17/2014 2:08:15 PM EDT
[#3]
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Quoted:

Yeah, a lot has changed since the original xm's were built, as far as getting one to run.  The Mk18's with their 10.3 inch barrels run fine with .070-.071 ports, ASSUMING you are running mil-spec 5.56 ammo. Daniel Defense sells their commercial 10.3 inch barrels with an .080 port, as they assume you might run Wolf, Tula, PMC or other lower pressure .223 ammo.  When you have the barrel cut the frt sight will be off.............a good time to size the hole.
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Quoted:
Quoted:
Don't have specs, but personally I'd size the gas port same as a Mk18 - we know that works...

Yeah, a lot has changed since the original xm's were built, as far as getting one to run.  The Mk18's with their 10.3 inch barrels run fine with .070-.071 ports, ASSUMING you are running mil-spec 5.56 ammo. Daniel Defense sells their commercial 10.3 inch barrels with an .080 port, as they assume you might run Wolf, Tula, PMC or other lower pressure .223 ammo.  When you have the barrel cut the frt sight will be off.............a good time to size the hole.

I think .080 sounds like a good spot, then, seeing as how I will not be running exclusively mil-spec 5.56. Thanks!!
Link Posted: 11/17/2014 6:23:02 PM EDT
[#4]
I highly recommend starting with one of the smaller hole sizes. You can always open it up more...You can't make a hole smaller.
Link Posted: 11/18/2014 1:15:00 AM EDT
[#5]
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I highly recommend starting with one of the smaller hole sizes. You can always open it up more...You can't make a hole smaller.
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This. The 10" is highly overgassed to begin with...I would start with stock Colt specs.
Link Posted: 11/18/2014 2:24:32 AM EDT
[#6]
Not retro - but solves oversized port issues, perhaps:

https://www.rubbercityarmory.com/scripts/prodView.asp?idproduct=33
Link Posted: 11/18/2014 10:57:37 AM EDT
[#7]
There are reams of information about port sizes in a thread in the AR Discussion section. The thread is http://www.ar15.com/forums/t_3_118/641240_Official_Mk_18_and_CQBR_Photo_and_Discussion_Thread.html. In a nutshell........Colt uses .070 in the Mk18, knowing that only 5.56 mil-spec ammo is going to be used, primarily suppressed. Yes, the Mk18 is overgassed when run suppressed.    Daniel Defense uses an .080 size port because they know you may run .223 ammo. It also is overgasses when using 5.56, but not when running .223 ammo.
      For what it's worth, and this is just with my experience, my DD Mk18 upper is very tame when running .223 bulk Federal ammo.  When running 5.56 there is a very noticable difference......it is much harsher as far as recoil...........and obviously more muzzle flash.   I am also running an H3 weight buffer with an unknown spring.
Disclaimer........my DD MK18 is actually a CQBR Block II.   Why they call it a MK18, I do not know.  Same thing as far as operation though...10.3 inch barrel.
Link Posted: 11/18/2014 11:03:41 AM EDT
[#8]
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Quoted:
Not retro - but solves oversized port issues, perhaps:

https://www.rubbercityarmory.com/scripts/prodView.asp?idproduct=33
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I have to ask how the adj. gas keys work.  It would seem to me that it would have to work by adjusting how much gas is vented through a bypass, thus lessening the amount of gas that is actually impinging on the carrier. I'm sure that works but at the expense of more gas being dumped into the action, and thus your face/eyes.  Anyone ever use one of these to report on how they work?    I've seen other adjustable gas systems that could be used on a retro (where everything would be under the handguards) and they would have the benefit of reducing gas flow TO the key/carrier, thus reducing gas flow into the action.
Link Posted: 11/21/2014 6:04:09 PM EDT
[#9]
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Quoted:

I have to ask how the adj. gas keys work.  It would seem to me that it would have to work by adjusting how much gas is vented through a bypass, thus lessening the amount of gas that is actually impinging on the carrier. I'm sure that works but at the expense of more gas being dumped into the action, and thus your face/eyes.  Anyone ever use one of these to report on how they work?    I've seen other adjustable gas systems that could be used on a retro (where everything would be under the handguards) and they would have the benefit of reducing gas flow TO the key/carrier, thus reducing gas flow into the action.
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Quoted:
Quoted:
Not retro - but solves oversized port issues, perhaps:

https://www.rubbercityarmory.com/scripts/prodView.asp?idproduct=33

I have to ask how the adj. gas keys work.  It would seem to me that it would have to work by adjusting how much gas is vented through a bypass, thus lessening the amount of gas that is actually impinging on the carrier. I'm sure that works but at the expense of more gas being dumped into the action, and thus your face/eyes.  Anyone ever use one of these to report on how they work?    I've seen other adjustable gas systems that could be used on a retro (where everything would be under the handguards) and they would have the benefit of reducing gas flow TO the key/carrier, thus reducing gas flow into the action.


Seems to me the best way is to adjust it at the gas port. Seeing as a number of Colt FSBs (including many XM177s) were drilled, then plugged with an allen head set screw, it seems to me that that would be a very logical way to make an adjustable gas system. Perhaps a needle valve with spring from an old carb as an example? I think the right way would be to turn down a long allen head cap screw to match, but you see where I'm going. It would look retro too!
Link Posted: 11/21/2014 6:25:51 PM EDT
[#10]
Discussion ForumsJump to Quoted PostQuote History
Quoted:


Seems to me the best way is to adjust it at the gas port. Seeing as a number of Colt FSBs (including many XM177s) were drilled, then plugged with an allen head set screw, it seems to me that that would be a very logical way to make an adjustable gas system. Perhaps a needle valve with spring from an old carb as an example? I think the right way would be to turn down a long allen head cap screw to match, but you see where I'm going. It would look retro too!
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Discussion ForumsJump to Quoted PostQuote History
Quoted:
Quoted:
Quoted:
Not retro - but solves oversized port issues, perhaps:

https://www.rubbercityarmory.com/scripts/prodView.asp?idproduct=33

I have to ask how the adj. gas keys work.  It would seem to me that it would have to work by adjusting how much gas is vented through a bypass, thus lessening the amount of gas that is actually impinging on the carrier. I'm sure that works but at the expense of more gas being dumped into the action, and thus your face/eyes.  Anyone ever use one of these to report on how they work?    I've seen other adjustable gas systems that could be used on a retro (where everything would be under the handguards) and they would have the benefit of reducing gas flow TO the key/carrier, thus reducing gas flow into the action.


Seems to me the best way is to adjust it at the gas port. Seeing as a number of Colt FSBs (including many XM177s) were drilled, then plugged with an allen head set screw, it seems to me that that would be a very logical way to make an adjustable gas system. Perhaps a needle valve with spring from an old carb as an example? I think the right way would be to turn down a long allen head cap screw to match, but you see where I'm going. It would look retro too!

Morg, I've thought about it many times. Lately, I've been looking at it from the standpoint of how to do one on my CQBR Block II.  MOdern junk with the gasblock under a rail. Anyway, back to the A1 frt sight base being modified to be adjustable.  I have/had a few frt bases that have the set screw in the front. The problem is twofold.........1) the gas tube is solid in the front  2) there is a rollpin crossways though the tube that prevents an adjusting needle being used in the location of where the setscrews are. (drilled/tapped in the front of the sight base)
  I've machined a few gasblocks for SR25's that used an adjustment needle, but the needle is placed perpendicular to the gas tube. Made the needle out of stainless, and they still erode relatively quickly is used to only diminish the gas flow. They last much longer if used in the fully closed or fully open position. I used them fully closed to make the SR a single shot, for 1000 yd competition.  With the CQBR, which is waiting on a suppressor to get out of jail, I'll need something that is just a restrictor, to adj. for suppressed and non-suppressed fire.
Link Posted: 11/21/2014 6:27:42 PM EDT
[#11]
3 ways to currently adjust gas on an AR - adjustable gas block, adjustable gas tube, or adjustable gas key.
Link Posted: 11/21/2014 6:28:12 PM EDT
[#12]
Doesn't JR make an adjustable gas tube? I always wondered how they work...
Link Posted: 11/21/2014 6:57:31 PM EDT
[#13]
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Quoted:
Doesn't JR make an adjustable gas tube? I always wondered how they work...
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The tube that I am somewhat familier with has a small block pressed over it.......maybe an inch or two behind the frt sight base. The block has been drilled/tapped to use an adjusting screw that protrudes into the gas tube, thus restricting the flow.
Gas blocks, adjustable, come in all kinds of configurations. I really like the Govnah.
Gas keys. Those are the ones that I can't figure out. I can imagine how I would do one, but I'm not sure how they are actually done. Seems like the key type would just be vented, to reduce bcg velocity, which would work I suppose, but would still dump a full charge of gas into the action with each shot.
Link Posted: 11/21/2014 7:44:43 PM EDT
[#14]
I would have ADCO chop it and open the gas port. They're pretty much the go-to for these kinds of things
Link Posted: 11/21/2014 7:59:17 PM EDT
[#15]
Blanco............hey, just dawned on my that some of your original question wasn't answered. My apologies for steering your post off on a gas block tangent.  Let me ask..........when you say the exact muzzle dimensions, just what exactly are you askin for?.......do you want from the shoulder (shoulder that sight base abutts) forward?      shoulder to very end of barrel.   I can measure my 607 and draw/post up a sketch/picture.
Link Posted: 11/22/2014 8:29:55 PM EDT
[#16]
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Quoted:
Blanco............hey, just dawned on my that some of your original question wasn't answered. My apologies for steering your post off on a gas block tangent.  Let me ask..........when you say the exact muzzle dimensions, just what exactly are you askin for?.......do you want from the shoulder (shoulder that sight base abutts) forward?      shoulder to very end of barrel.   I can measure my 607 and draw/post up a sketch/picture.
View Quote

Haha no problem! What I was looking for was dimensions as far as in front of the FSB? As in, how far between FSB and shoulder, and then shoulder and very end of muzzle (threaded portion)?
Link Posted: 11/24/2014 1:23:54 PM EDT
[#17]
Blanco, I just measured the 10" barrel on my 607.  Here goes......length from shoulder that frt sight abuts, to the very end (muzzle) is 2.556.    The 1/2x28 threaded section is  .642 long.
Said another way......625 diameter forward of shoulder that sight abuts for (2.556 minus .642= 1.914 inches), then steps down to .495 diameter for .642 length........for a total of 2.556 inches
I turn the 1/2" diameter to .495 for cutting the 1/2X28 threads.  Threads are cut for a 2A class fit.
If this is clear as mud I'll do a drawing in AutoCan and try to post.
Link Posted: 11/24/2014 1:46:09 PM EDT
[#18]
You won't know until you assemble them.

Link Posted: 11/24/2014 2:16:18 PM EDT
[#19]
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Quoted:
You won't know until you assemble them.

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He won't know what until he assembles what?
Page AR-15 » AR-15 / M-16 Retro Forum
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