I was trying to upgrade my Bushmaster Carbon 15 with a new handguard from the original standard issue handguard. I bought a Yankee Hill Machine quad rail that had excellent reviews. After getting to the point of threading on the new handguard, I found out the threads don't match. Called YHM, and they informed me that the handguard only fits mil-spec uppers.

Just curious if anyone knows of a solution to make this one work, or a recommendation for a different handguard that is similar to a quad rail?

I have a Bushmaster Carbon 15 pistol that I took the stock handgaurd off put a YHM free float handgaurd on it with no problem. Did you buy a new barrel nut from YHM?

Yes, it is a free float handguard, and came with the barrel nut. I took off the existing delta ring, and tried to screw the YHM barrel nut on, but no luck. The tech on the phone said they get that question all the time, and that their barrel nuts don't fit the BM C-15? Sounds like you got yours to work. The YHM barrel nut I received seems just a slight bit smaller I.D.?

This is the barrel nut that I used.

http://yhm.net/index.php?main_page=product_info&cPath=2_4_51&products_id=32

My pistol is about 5 years old so maybe they made the uppers milspec back then and don't now. I didn't have a problem at all. I probably even had a few beers while doing it.

Edit: I am rookie and have run out of posts for the day so I will just edit this one.

Per the ar15.com link "The Carbon 15 pistols with serial number D12144 and higher have upper receivers with a larger barrel nut thread diameter than an AR-15 handguard barrel nut so the AR-15 handguards that replace the barrel nut will not fit."

Mine is D11XXX so that would explain it. You could always just order a milspec upper. These are supposedly in stock for $89.

http://aeroprecisionusa.com/shop/ar15-stripped-upper-receiver


[This message has been edited by CrossBowAg99 (edited 1/16/2013 10:10p).]

Yep, that looks like the same nut to me. It works great with the handguard, but just won'c thread onto my upper. Frustrating to backorder and wait for it, only to find out it doesn't match up. There are a few types of the C-15, and I got mine last year. Maybe that's the difference.

CrossBow,

I found this, which helps explain my predicament. Guess you got lucky to avoid this mess.

http://www.ar15.com/archive/topic.html?b=3&f=4&t=547430

And, you guessed it....the replacement nut is out of stock.