I have a colt preban with M4 14.5 upper.  It jams alot.  The problem is not the mag has I tried many mags.  The shell casing gets jamed in upper receiver.  There are severe scratch marks on the casing.  I suspect my gun need more lubrication.  It works fine than jams.  Jams alot.  thanks.

There are many variables here but let's start with the basics:

1. What kind of ammo are you using?
2. Are the empty casings not ejecting or are the new rounds not making it into the chamber?
3. Are the scratches from the bolt riding over top the new round?

If "the shell casing gets jammed in the upper" means failure to extract a spent case from the chamber, and you have a 14.5", my guess is you need either a wolff extrctor spring (best solution) or a D-fender O ring (second choice).  

Glockshooter

This wolf extractor spring. where to purchase, how much, and is it easy to install.  Should I replace springs on all my bolts or only those used in 16 barrels or shorter?  Thanks

Go to http://www.tacticalforums.com/ - there is a long thread under "customs guns". Basically they recommend a reliability package by http://www.specializedarmament.com

same thing going on with mine....new round partially in the chamber, with the spent casing jammed between the new round, and the top inside of the reciever/charging handle area, tried a different setup, whole new carrier/bolt assy, and it works fine.....

IIRC I bought a 3 pack from Brownells [url]www.brownells.com[/url].  you could go to the source [url]www.gunsprings.com[/url]  I needed one when I went from 16" to 14.5 w perm f/s.

I suggest you buy a 3 pack and replace any that need it.  No problem with the gun, don't replace.  If you get a fte in another gun, swap it out and see if that helps.  

Easy to install.  I did it, and I'm "not real handy".  Just do it over a whilte towell in an area with good light in case anything falls.  

BTW, before you ask, insert color does not matter. [url]http://www.ar15.com/forums/topic.html?b=3&f=118&t=156824&page=2[/url]

I had prolems with 16" PWA that I used at tactical carbine class a while back.  Cleaned it, lube it, using Hornady TAP still had problems. After the school, I replaced the gas tube and have not had a problem since, about 1500 rds.  Before you spend lots of money in getting a "tune up" on the rifle.. check out some of the other possiblities...

Good shooting, after you get it fixed...
T2

GlockShooter is CORRECT  ! ! !

I too had a 14 1/2 inch carbine (Bushmaster) giving me fits  !   Replaced the extractor spring with one from Wolff.  No more problems  ! !  

I would suggest you order a "10 pak" of springs and replace them in ALL your your AR rifles and carbines.  Keep a couple in you range bag/shooter's box, and make new friends at the local range.  

I have found that if you MAINTAIN  your rifle, any rifle, and lube it sparingly, it will run just fine.  No need to over due on the lube  ! !