Problem with buffer tube and rifle buffer length

[jmatt511]

Maybe I'm going crazy or just not seeing it.  I'm using an A1/A2 buffer tube and rifle length buffer in my CMMG lower and DPMS upper with Magpul PRS stock.  The problem is the bolt carrier will not slide back into the buffer tube enough to engage the bolt carrier stop.  Taking out the spring and placing the buffer into the tube, it's still won't go into the buffer tube far enough.  The buffer is a rifle specific 5 15/16 inches long which I believe is the standard length.  A carbine buffer works fine.  What am I missing?

[imschur]

im gonna need to ponder that one.

Is the buffer retainer somehow interfering?

[jmatt511]

No.  It seems to be the length of the buffer somehow.  I'm using a standard DPMS AR-15 rifle length buffer, though I now note the DPMS 308 buffer is .7 inches shorter.  It seems that is still too long.  I'm wondering if I'd end up damaging the lower receiver by using a carbine length DPMS 308 buffer.  The stop milled on the bolt carrier would seem to strike the area of the lower receiver where it mates with the buffer tube.

[imschur]

.7 is a lot. I would start there

[jmatt511]

Problem solved.  I milled .7" from the body of the buffer tube, drilled a new roll pin hole and it will now lock back.  Waiting for my JP handguard and nut to arrive, then it's out to the range for function test , barrel break-in and zero.

[imschur]

jmatt51, Perhaps I'm misunderstanding something. You should not have to mill anything. The buffer tube of the correct length with a proper spring and a buffer of correct length and weight should work without you having to modify anything.

Something is very wrong.

[imschur]

A rifle length DPMS buffer is 5.285 " long Im not sure what it weighs. Slash may have it on his buffer chart

[jmatt511]

I think the problem was I was using an DPMS AR-15 rifle buffer which apparently is longer than the DPMS 308 rifle buffer.

[imschur]

Correct. I think shortening an AR-15 buffer still does not produce one of the correct weight even if the length is machined the same.

[jmatt511]

Kinda figured that.  So I just ordered the correct one from Midway.  Thanks of the help.

[imschur]

Kinda figured that.  So I just ordered the correct one from Midway.  Thanks of the help.

Great!!!

[98Z5V]

One of the very few places you can go radically wrong in a .308 AR build is with the buffers.

What's gonna happen in a .308 build when you use a buffer from an AR-15? 

I usually assume (wrong on many occasions) that most people building a .308 AR have built an AR-15 previously.  Just the sheer size of the bolt carrier on the .308 tells you something - much bigger than the AR-15 little brother, both in diameter and length.  That means something - more to some people than others, I hear.  The receiver extensions (buffer tubes) used on the .308 builds are typical AR-15 stuff, either Rifle (A1 length) or Carbine.  "A2" is not a receiver extension length, either, it's an A1 length extension with a spacer and longer screw, in order to fit an A2 length stock.

OP, this isn't a slam on you, by any means.  It's information for others - many, MANY people are getting into the .308 ARs.  This is just a heads-up to them, should they read this.  Disclaimer finished...

.308 AR buffers are shorter than AR-15 buffers - because .308 AR bolt carriers are longer than AR-15 bolt carriers.  Receiver extensions are the same length.  That extra bolt carrier length has to fit in the same receiver extension when the gun is fired, so the buffers cannot be interchangeable.  The problem you saw was from the use of the wrong buffer, considering the shooting platform.  I'm just glad you didn't shoot it, and saw the problem beforehand.  <thumbsup>

[imschur]

and this is exactly why this site was created.  <thumbsup>

[jmatt511]

98z5v,

No offense taken.  Having been well acquainted with the AR platform since the 80's when our department allowed us to purchase for private ownership and duty carry during the 84 Olympics.  As far as 7.62/.308, I'm a bolt guy.  This is my first forray into this type of rifle and why I chose this site to get info I didn't have.  And thanks to Imschur for the patience and assist.

Jim

[98Z5V]

Right on, Jim - bolt guns rule!  ;D  There's only really a few internal parts on the .308 ARs that are different than the little brothers - bolts and bolt carriers, bolt catch, buffers and buffer springs, ejection port doors and pins, and takedown/pivot pins.  Unless I'm forgetting anything, the other internal stuff is interchangeable.  Since you're used to the bolt guns...  you'll like this thing...  <thumbsup>