I purchased a never fired M4 upper from a member here at the EE.
I purchased a new RRA LPK, RRA M4 stock and Oly lower.

When shooting, the combo short stroked badly. It only cycled normally about 1%-5% of the time. I put 500 rounds through it , cycling the vast majority by hand. The last 100 rounds or so showed some improvement as things started to wear in but still only about 25% successful cycling.

I switched out parts from another rifle and determined that the unshrouded BC on my M4 might be the cause as the rifle(s) would work fine in any other configuration that did not use the unshrouded BC.
My firing pin is badly peened and damaged from these short stroke incidents (see pic in my other post below)

I sent the upper back to the factory and they say it is 100% OK. They suggest the stair step notch on the back of my RRA hammer is causing the problems.
I tested my complete upper on a friends complete Oly lower which had the smooth, un-notched hammer and it still short stroked.

Should I ...

1 - Buy an un-notched hammer as the factory suggested   hat

3 - Send the upper back to the guy I bought it from and try and get a refund   hy

http://tinypic.com/bi5d6q.jpg

I looked at your other thread to see the FP damage.

Get a shrouded BC.

This will address/fix the FP damage issue.

The short stroking could be caused by something else.

Fix one problem at a time.