Got my barrel in for a build, I noticed that it had two holes in the barrel estension. A quick search of the net shows that it must have been drilled for a extractor support pin. Wonder why they just didn't finish installing the pin?. your thoughts?

Never heard of an extractor support pin.

You are correct it was drilled for an extractor support pin. I don't know why it was not included. I've only seen pictures and know very little about them, but my understanding is that the pin just slips in there and is held in place by by the "wall" of the upper receiver (where the barrel threads are). They are apparently some fail safe in the event of an obstructed bore. I may be wrong and am sure someone will correct me if I am.

Having said that I wouldn't be happy if my barrel was like that. I don't know if it would cause problems, I don't think it would but I still wouldn't be happy. I would contact the seller.

Sig Sauer uses this setup in a couple of models my Sig M400 has it

No, it's a kludge fix for the extractor losing grip on fired cases due to rapid unlocking, which happens generally in piston operated or overgassed/short timed DI configurations.

The standard AR15 extractor is unbalanced... it has more mass in front of the pivot pin than behind, so the higher the rotational velocity of the bolt, the less force is exerted on the extractor to grip the rim of the fired case.  The pin provides a mechanical block to prevent the extractor from loosening while the bolt is rotating to unlock.

The more elegant fixes for the problem are to redesign the bolt and extractor so it's balanced (the LWRC ACB and LMT advanced bolt designs), or just use a weapon configuration with a more reasonable unlocking speed.

Ok, gotcha. Thanks for the clarification. Don't know if we've answered the OP's original question though lol.

In theory it should not hurt the firing of the weapon. That would be behind the front of the bolt face.
My personal theory is that if you did not order it with that hole return it and get one with out. Either way carbon and dirt will build up in it.

thanks guys, I have ask the dealer for a return. I have read over the ad for the barrel and the only thing noted about the extension is, "This barrel is setup with the full auto barrel extension for increased reliability".

If returning the barrel for another turns out to be difficult, I'd say just go with what you have.  I can't imagine how those little holes will affect form/fit/function. - CW