I have a MT6400c from colt. I'm looking to use the magpul fixed carbine stock. The problem I'm having is that the buffer tube is to long. I've removed the buffer spring to measure it and depth inside the buffer tube. It's all cabin Length, but the out side length of the buffer tube is to long about 3 inches and will not work with the magpul stock. Is this because its a nj Compliant rifle? Do I have to replace the buffer tube with the real carbine length? Or can I cut the tube down to carbine length to work with the fixed magpul stock?

Don't cut your existing tube. Just remove it and out it to the side until you decide to switch things up or sell the gun. Buy yourself a new mil spec carbine buffer tube.

 

http://www.bravocompanyusa.com/Milspec-Receiver-Extension-Buffer-Tube-p/bcm%20milspec%20receiver%20extension.htm

http://www.spikestactical.com/new/z/milspec-buffer-tube-p-1049.html

 

You can find cheaper ones out there but these are what I would get.

 

Not sure about being able to use the same spring though. Carbine lenght springs are about 11". You said the tube is longer by 3" so measure the spring and see how long it is. Do you know what type of buffer you're using?

Yes, Colt uses a Carbine buffer and carbine spring in a modified rifle length tube. You will need to get a carbine extension, about $25.00, and reuse the carbine buffer and buffer spring. 

 

Make sure you get the right extension for stock.  Mil-spec or commercial.  Size matters.

 

You will not be using the position notches on the tube so 4 or six will work.

 

http://palmettostatearmory.com/index.php/ar-15-05/lower-parts/individual-parts/palmetto-state-armory-6-position-mil-spec-buffer-tube.html

 

Amazon sells this part as well.

 

http://www.amazon.com/Receiver-Extension-Carbines-Tactical-Intent/dp/B004MAFZPO/ref=sr_1_14?ie=UTF8&qid=1376245039&sr=8-14&keywords=ar15+buffer+tube

I also have the colt mt6400 and recently installed the magpul fixed stock. All I did was change out the buffer tube to a new vltor one and reused my old buffer and spring.

 

This is besides the point but - how did you get the factory stock off of the colt tube? Besides being pinned mine seems to be permanently fused on, possibly epoxied I can't tell

I also have the colt mt6400 and recently installed the magpul fixed stock. All I did was change out the buffer tube to a new vltor one and reused my old buffer and spring.

 

This is besides the point but - how did you get the factory stock off of the colt tube? Besides being pinned mine seems to be permanently fused on, possibly epoxied I can't tell

I carefully drilled out the roll pins. Also, the bolt in the position selector is threaded into the buffer tube.

Thanks for the help everyone.