I had this complete M15T Armalite 20 inch complete factory upper that ran very well on a full length buffer tube and fixed stock.  I changed lowers to a cheaper one to sell the upper and it now has an M4 stock, buffer, and spring.  With this new buffer setup, it runs like it is very undergassed.  It ejects but will not strip another round or lock back on an empty mag. 

The last setup with the Armalite factory lower,  it also ran very well with the BCM A5 buffer tube kit.  

So, this being a gun I am trying to sell, what is the cheapest way to make it run?  Should I just get another A5 or is there a better inexpensive buffer and spring that would work better in the M4 tube?  If I have to spend $100 to make it run, I will probably keep it.  I wish I had a fixed stock setup to install.  

Thanks.   Not to big a deal.  My Larue build ran like a raped ape today!!!!

I will check those things tonight.  It is a standard carbine buffer with no marks.  The bolt will lock back manually.  I didn't notice anything odd about the spring tension but will check again.  

I was running it pretty wet so it shouldn't have been a lube issue.

Nothing special about the spring, carbine spring.

They were reloads, definitely not max, but data from the 5.56 loads, not the .223 loads.  Ramshot TAC was the powder with 62 gr bullets.  I need to take a look at my data and try some factory stuff.

I dusted off my chronograph, so I will see where my loads really are.

i have no answer here but i experienced the same issue with a 20" and a carbine buffer set up long ago, when using less than full power 556 ammo. it sure doesnt make any sense.

i just went with the A5 and never had anymore issues.

Buffer weighs 2.9 oz, standard CAR buffer.  
Last two sets of reloads were 25.7 and 25.8 gr of TAC with 62gr RMR bullets (M855 style)

Data range for the load shows 23.6 to 26.2, so I will chrono them and inspect primers at some point to see where I am at.  

My plan is to take some factory M855 and see if it runs.  Then I will try my reloads again with a Spikes ST-T2 that is on my shelf.  These buffers seem to be real problem solvers.  I can't make my brain think this is bolt bounce, but stranger things have happened.  I could swap my Vltor buffer next time too.    I am now seeing the light on this.   VLTOR A5 system addresses this specific issue of the collapsible stock on a 20" upper, it is patented, and the USMC uses them.  Apparently the Army went with some A6 buffer to use a standard length buffer in a standard carbine tube.  The A5 buffer will only work in the extended VLTOR tube.  It looks like the answer is among these things I read tonight.  

I am gonna still try the Spikes ST-T2 buffer, but I bet it is too light.