Double feeds are over-cycling related.
Extraction problems are typical shorty issues too.
I'd estimate that you have over a thousand rounds thru the gun and are experiencing the effects of gas port erosion, which apparently some on this board don't seem to think ever happens.
Action speeds too high.
Try a heavy buffer.
I'd of course recommend the MGI RRB.
The D-Fender is the D-ring made to improve extraction.
These items are made to address the problems you mention.

If you want to go the cheap route, a 9mm buffer and an o-ring  or extra power extractor spring will help.

Thank you DevL.
You hit the nail on the head, as usual.

And yes, Simon, they are addressing the symptoms.
The real problem is gas port erosion combined with the short gas system that has perpetually been the cause of these problems for years. The LMT systems do very well with the shorties, but they are still subject to the same issues.

Sullivan and Gwinn designed the D-Fender, RRB buffer, and adjustable gas tube as a system to overcome the carbine gas system's shortcomings, and it worked(works) very well as a package. The RRB and D-Fender are the most popular parts of the package, ironically, even though they are the 2 parts that don't deal with the gas metering. People seem to be a bit touchy about fooling around with the gas metering, so not as many people order the adjustable gas tubes.

If the barrel makers would chrome the gas ports internally, that would go a long way to resisting erosion, and staving it off to much later in the barrel's life. But they don't often(hardly ever) do that, so the 26000psi hot gasses act like a plasma cutter on the gas port and it opens right up in just a thousand or two thousand rounds to accellerate the action speeds. Then these kinds of reliability problems begin to rear their ugly heads. Funny how those shorties start out when they are new, running in full auto around 850rpm, and in not too long they start getting up to 1000rpm or 1100rpm, and people seem to think that nothing needs to be addressed!? It's only when the gun quits working right that it dawns on them that they should be doing something to remedy these increasing action speeds.

If the upper is brand new, then your gas port is probably drilled on the large end of the spec range.
You'll be able to get it running fine, with some of the usual methods.
You're on the right track.

Quoted: I'm Using M193 ammo the 10.5" MRP is having failure to extract/double feed problems, swapping it with a 14.5" barrel its aok.  What seems to be the problem? I'm using a brand new LMT standard FA bolt, no O ring.

Sounds like your having extractor spring issues. I would suggest using a Wolf heavy duty
extractor spring first. They are about double the spring strength of the standard extractor spring.
This should resolve your issue. Here's Wollf gunsprings link: www.gunsprings.com/RifleShotgun/Colt_Rs.html#M4Ext
and pricing:
Stock No. 30090 - each.....$ 4.49  
Stock No. 30091 - pak of 3.......$ 10.99  
Stock No. 30092 - pak of 10.....$ 22.99  

If this doesn't resolve the issue then purchase and heavy buffer also.
Hope this information helps.
Best regards,

Frank
Patriot Ordnace Factory (POF-USA)
www.POF-USA.com

I am not a MGI dealer and nor do I know TWL, but that darn D-fender ring is nice.  I have install them on several hard used CQB guns that is running flewlessly right now.  

A local LE dept cut down a bunch Colt 16inch guns to 11 inch and the thing just palin won't run reliable.  I helped them check the gas port size and installed D-fender ring, upgraded spring and a heavy buffer. now the gun is running and have not had a hiccup since.

With your LMT MRP, the 10.5 inch barrel you might ran into some functional problem, but once you do what the guys tell you as far as H-buffer or d-ring, your CQB will run great and yoiur 14.5 will run even better.

By the way, if you got the cash, spring for one of those MGI buffer, it is so choice..

SOLUTIONS, IN ASCENDING ORDER OF COST:

CRANE O-RING
WOLFF EXT SPRING
SAW EXTRACTOR SPRING


WES GRANT
M.S.T.N.