Someone commented on another thread that the Bushmaster Carbon 15 uses a non-standard barrel nut.  Can anyone tell me what is non-standard about the nut, particularly dimensions?

Just asking again.

I don't know the dimensions.  I would not remove the barrel nut.  The last one I took off took some of the threads with it and now the upper is no good.

That's nasty.  Makes it kind of hard to put a carbon tube on one.  What kind of receiver vice block were you using?

The threaded portion of the carbon 15 receiver has a larger diameter than an aluminum receiver.  The nut is "non-standard" because it's inner diameter is larger.

I think I used a ratworx block.  The standard clam shell block doesn't fit the carbon15 upper.  There is a carbon forearm available that bolts to the standard barrel nut.  I don't remember if you have to remove the barrel nut for installation or not.  I'll see if I can find a link.

I found the carbon hand guard that attaches to the stock barrel nut.  It looks like you will have to take off the nut and re-use it for proper installation.



http://www.apcustomusa.com/ar15.html



Here is a thread about rails that use the stock barrel nuts.



http://www.ar15.com/forums/t_3_12/588729_FF_Quad_Rails_that_use_the_factory_Nut.html

Thanks,  The reason for my question was that I was considering putting an AP Customs hand guard on one.  I have an AP on my Colt and really like it.  It doesn't have to be glued on or need a special barrel nut; as long as the outside diameter and shape of the barrel nut on the Carbon 15 is standard the AP should fit.  I'm just thinking about playing around and building the lightest AR I can come up with.  I have a Carbon 15 .22 upper (I got one of the few that actually works and got it for $200) on a Cav Arms lower that is just over 4 pounds.  It is a fun plinker.  I am thinking about a Carbon 15 upper with the pencil barrel and an AP Customs carbine tube with MBS sights on either a Cav lower or on the Carbon 14 lower with an ACE stock.  I built a rifle for my finance with a Carbon 15 upper on a chartreuse Cav lower and a chartreuse hand guard.  It weighs an even 5 pounds.  She loves it.  I think I can knock another half pound off that with the AP tube.

Sounds like a fun setup.  Post some pictures when you get it finished.

The old saying, "When you have weapons for defense and hunting, everything else is just a toy."  I've got a Colt 6920 set up the way I like it, a Sig M400 with a 20" heavy barrel for varmints that shoots 1/2 inch and better groups, a 450 Bushmaster for hogs, a CMMG MK 3 308 for deer and a Carbon 15 .22 for plinking.  If I build this one it will be just for fun and to watch people comment on how light it is.