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Posted: 4/22/2015 1:02:17 PM EDT
I just built a 16" mid length carbine.  It is a 16" BCM upper with a Aero Precision lower.  It has a Rainier Arms bolt carrier, BCM gunfighter charging handle, 37 coil carbine buffer spring, Spikes Tactical T2 carbine buffer.  The lower was built with a standard CMMG lower parts kit.

My problem is the bolt does not want to stay open after last shot or on an empty magazine.  When comparing to other AR's I have built using the same components the bolt is not going back far enough for the bolt catch to contact the front of the bolt and stop it.  It won't work with a empty mag or when pulling back the charging handle and engaging the bolt catch.  The bolt catch moves freely and it is working but the bolt just doesn't go back far enough to catch the front of the bolt face.

By the way the buffer tube is a mil-spec tube.

Any Ideas?  I think I have tried everything.
Link Posted: 4/22/2015 1:59:16 PM EDT
How far off from catching are you?  Try backing off the buffer tube one turn at a time until you can get it to lock back.
Link Posted: 4/22/2015 4:24:05 PM EDT
Originally Posted By bclemans:
I just built a 16" mid length carbine.  It is a 16" BCM upper with a Aero Precision lower.  It has a Rainier Arms bolt carrier, BCM gunfighter charging handle, 37 coil carbine buffer spring, Spikes Tactical T2 carbine buffer.  The lower was built with a standard CMMG lower parts kit.

My problem is the bolt does not want to stay open after last shot or on an empty magazine.  When comparing to other AR's I have built using the same components the bolt is not going back far enough for the bolt catch to contact the front of the bolt and stop it.  It won't work with a empty mag or when pulling back the charging handle and engaging the bolt catch.  The bolt catch moves freely and it is working but the bolt just doesn't go back far enough to catch the front of the bolt face.

By the way the buffer tube is a mil-spec tube.

Any Ideas?  I think I have tried everything.
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Was the spring installed on the bolt catch?

Was the gas block properly aligned?


Link Posted: 4/22/2015 6:19:32 PM EDT
make sure that something didn't get into the receiver extension and is preventing full travel of the buffer. it may be pounded into the back of the extension by now, so take a hard look inside with a flashlight.
Link Posted: 4/22/2015 6:49:12 PM EDT
Please measure the action spring length.  It should be:
RIFLE: 11 3/4 Inches (29 85 cm) minimum to 13 1./2 inches (34 29 cm) maximum.
CARBINE: 10 1/16 inches (25.56 cm) minimum to 11 1/4 inches (28.58 cm) maximum.
Link Posted: 4/22/2015 6:49:44 PM EDT
Link Posted: 4/23/2015 6:46:32 PM EDT
To answer a few of the above questions.  The buffer spring is correct.  It measured 10.5" and had 37 coils.

The bolt face stops about 3/8" in front of the catch.  That is as far back as I can get the bolt and carrier.  Basically the carrier will pull back far enough for the catch to contact on the front of the carrier but not the bolt face.

When looking down the receiver extension it looks to me like there is a ridge about 1/2" from the bottom inside the tube.  It looks like the spring could possibly bind up on this but I am not sure.  

Also when comparing this mil spec buffer tube to a commercial tube it is a good 3/4" or more shorter.  I realize these two types of tubes are different in size but are they supposed to be that much different in length?

My next possible fix is that I have ordered a new buffer tube to try.  It is a Vltor 6 position mil spec made from 7075 and is 8.1" overall length.  This was the longest 6 position I found and I'll see if that fixes the problem.

Everything else you guys mentioned above I checked out and I am hoping this new buffer tube is the fix.

Thanks for all the advice and I'll let you know how it all worked out.
Link Posted: 4/23/2015 7:53:21 PM EDT
By the way I measured the inside of the buffer tube from the lip to bottom of the tube and it is less than 7".  It is about 6 5/8" to 6 3/4".

I am beginning to think this buffer tube is the problem.

I should know tomorrow as my new buffer tube should be here in the mail.
Link Posted: 4/23/2015 11:35:17 PM EDT
Link Posted: 4/24/2015 3:59:34 PM EDT
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Originally Posted By Dano523:



What you got was a receiver extension tube for the bb airsoft rifles, and why the depth of the void is not deep enough for an actual AR-15 rifle instead
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Originally Posted By Dano523:
Originally Posted By bclemans:
By the way I measured the inside of the buffer tube from the lip to bottom of the tube and it is less than 7".  It is about 6 5/8" to 6 3/4".

I am beginning to think this buffer tube is the problem.

I should know tomorrow as my new buffer tube should be here in the mail.



What you got was a receiver extension tube for the bb airsoft rifles, and why the depth of the void is not deep enough for an actual AR-15 rifle instead



Well I got the new buffer tube on and that was the problem.  This new Vltor A5 tube is longer and the gun functions with the bolt stop correctly.  In the Vltor instructions it says to use only the Vltor buffer and spring.  I am using a carbine spring with a Spikes T2 buffer.  I haven't shot it yet but I would assume the setup I have will work.  The previous setup worked perfectly except for the bolt holding open.  I fired 70 rounds through the old setup with zero malfunctions and very good accuracy.  I'll take this new setup out and try it later.
Thanks for all the help, it was much appreciated.
Link Posted: 4/24/2015 8:03:23 PM EDT
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Originally Posted By bclemans:
By the way the buffer tube is a mil-spec tube.
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Originally Posted By bclemans:
By the way the buffer tube is a mil-spec tube.

Originally Posted By Dano523:
Originally Posted By bclemans:
By the way I measured the inside of the buffer tube from the lip to bottom of the tube and it is less than 7".  It is about 6 5/8" to 6 3/4".

I am beginning to think this buffer tube is the problem.

I should know tomorrow as my new buffer tube should be here in the mail.



What you got was a receiver extension tube for the bb airsoft rifles, and why the depth of the void is not deep enough for an actual AR-15 rifle instead

Where did you get the buffer tube from, OP, and what led you to believe it was mil-spec?  I'm not criticizing. I just want to know what to be wary of when I shop for stuff.
Link Posted: 4/24/2015 8:31:24 PM EDT
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Originally Posted By mimunroe:
How far off from catching are you?  Try backing off the buffer tube one turn at a time until you can get it to lock back.
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huh?
Link Posted: 4/24/2015 11:54:06 PM EDT
[Last Edit: 4/24/2015 11:55:07 PM EDT by bclemans]
I can't remember for sure where I got the original buffer tube from.  One of the on line AR stores.  It was advertised as Mil Spec and I believe the company was Slide Fire that made the buffer tube..
Link Posted: 4/25/2015 7:17:21 AM EDT
If you bought the Vltor A5 receiver ext, you cannot use a standard length buffer or standard length carbine spring as the A5 uses it's own proprietary buffer and rifle length spring.  You will end up damaging your lower receiver.
If you bought this
Link Posted: 4/25/2015 8:18:44 PM EDT
I found that out after I bought the buffer tube.  I have ordered the proper spring and buffer to put in it before I fire it.

If anyone knows however could this buffer tube be used with the rifle length spring and a Spikes T2  buffer?

Why would the lower be damaged using a different buffer?  The only difference between the Vltor buffer and the Spikes would be the weight and length.  Vltor makes lighter buffers to help tune the gun should it need it.
Link Posted: 4/26/2015 9:16:42 AM EDT
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Originally Posted By bclemans:
I found that out after I bought the buffer tube.  I have ordered the proper spring and buffer to put in it before I fire it.

If anyone knows however could this buffer tube be used with the rifle length spring and a Spikes T2  buffer?Nope

Why would the lower be damaged using a different buffer?  The only difference between the Vltor buffer and the Spikes would be the weight and length.  Vltor makes lighter buffers to help tune the gun should it need it.
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It's the length difference of the buffers as the buffer acts a stop for the bcg, too short and the bcg collides with the rec. ext. ring and will likely cause a crack.
Link Posted: 4/30/2015 2:07:29 PM EDT
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Originally Posted By bclemans:



Well I got the new buffer tube on and that was the problem.  This new Vltor A5 tube is longer and the gun functions with the bolt stop correctly.  In the Vltor instructions it says to use only the Vltor buffer and spring.  I am using a carbine spring with a Spikes T2 buffer.  I haven't shot it yet but I would assume the setup I have will work.  The previous setup worked perfectly except for the bolt holding open.  I fired 70 rounds through the old setup with zero malfunctions and very good accuracy.  I'll take this new setup out and try it later.
Thanks for all the help, it was much appreciated.
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Originally Posted By bclemans:
Originally Posted By Dano523:
Originally Posted By bclemans:
By the way I measured the inside of the buffer tube from the lip to bottom of the tube and it is less than 7".  It is about 6 5/8" to 6 3/4".

I am beginning to think this buffer tube is the problem.

I should know tomorrow as my new buffer tube should be here in the mail.



What you got was a receiver extension tube for the bb airsoft rifles, and why the depth of the void is not deep enough for an actual AR-15 rifle instead



Well I got the new buffer tube on and that was the problem.  This new Vltor A5 tube is longer and the gun functions with the bolt stop correctly.  In the Vltor instructions it says to use only the Vltor buffer and spring.  I am using a carbine spring with a Spikes T2 buffer.  I haven't shot it yet but I would assume the setup I have will work.  The previous setup worked perfectly except for the bolt holding open.  I fired 70 rounds through the old setup with zero malfunctions and very good accuracy.  I'll take this new setup out and try it later.
Thanks for all the help, it was much appreciated.


As stated above, DO NOT shoot a rifle with the A5 buffer tube and Spikes buffer. You WILL damage your lower, on the first shot. The buffer is what is supposed to stop rearward travel as it bottoms out against the back of the tube. If you use a buffer that is too short, it will no longer be able to stop rearward travel and instead the bolt carrier key will run into your lower receiver. This will crack the lower, damage the key, break the key off, or all of the above.
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