Greetings from the North Country...
I need some help. I have an Oly-arms 7-1/2 carbine. Picked it up with problems - fail to extract, double feed ...ammo sensitive.
Rebuilt it with flat top upper, JP ent. adju.gas block, pigtail tube. also changed extractor and spring, ejector and spring, buffer and spring. have put it all together over 6 times all with the same result. fail to extract, double feed, ammo sensitive.
I have 11 other ar's in the stable and this is the only one with problems. I really like the super short barrel and would appreciate any help anyone can give me.
Thanks in advance...
Eagleone is clear.

 Seems like you did all you are supposed to do.

 Just thinking out loud here:  
 Gas leaking somewhere.
 Parts not milspec.

 Last resort--send to best AR15/M16 smith.

My take on your problem would be that with the short barrel, the rifle has insufficient gas pressure to cycle the action.  The solution would be to open up the barrel's gas port. Since you already have an adjustable gas block, you can tailor the pressure to exactly the load you're shooting.  

Do you have a D-Fender in your bolt? It may help with the Extraction problems...$12 fix.



"Simply install this small hi-tech composite "D" ring and increase the force of the extractor spring by Four Times!  Yes, it's a simple device, but if extraction is your problem, this is the simplest, and cheapest, solution you will find in the marketplace today.  Extractor and Spring are not included."
http://www.buffertech.com/dfender.htm

There was a police officer complaining a few months back about 11" barreled AR's being ammo sensitive.  Seems to me the more you shorten them, the more problems start to arise.

I agree it's probably a gas tube problem, the trick is to figure out how to modify it to run smoother with such a short barrel.

A 7.5" barreled AR must be fun just from a coolness point of view, however.

I don't know if this works but there's a company that sells product to fix all the problems you mentioned.  I have no
experience with this product.

ar15pigtail.com/