Whenever I read about H&K registered sears, it's always expressly noted how they're registered for multiple calibers: 9mm, .223 and .308 (or 5.56mm and 7.62mm), but the potential host rifles are not necessarily mentioned, as there's a whole host of H&K delayed-roller locked options.    Are registered FNC sears expressly assigned in the BATFE registration paperwork exclusively to FNC rifles, or are they registered as machine guns for a certain caliber as the H&K sears?

My FNC sear shows the following registration information.

4a: (Manufacturer)  S&H Arms
4b: (Type of Firearm) Machinegun Sear
4c: (Caliber) "N/A"
4d: (Model) "FNC"
4e: (Length Barrel) "N/A"
4f: (Length overall) "N/A"

So to answer your question it is not tied to a particular caliber and the only reference to FNC is in box 4d.  The same goes for all three of the HK Sears I own,  pretty much everything is marked on the F4 is the same as the FNC sear except the Model box 4d says "HK" and the caliber is noted as "9MM, 223, 308"

However, the ATF has taken the position via numerous tech branch letters denying any "host creativity" that a FNC Sear will only be installed into an FNC.   They have further taken the position via rule 2008-1 that only Belgian made FN FNC's can be called "FNC"s and that any future Domestic Clones must be called something else.

I would be willing to bet that if you wanted to go the distance in court to install an FNC sear into a non FNC Host you could probably given a long enough time horizon and deep enough pockets.

However as it stand for the average Joe, FNC Sears are legally only going into 5.56 FN FNC Hosts.  Although you could probably drop a homemade 22LR conversion kit into the upper it without too much concern.

Agreed.

ATF's Rule 2008-1 is certainly a rule, but likely would not stand up to any legal challenge.

The foundation of this is sound since a registered sear for one platform would necessarily need to either be modified to work in another platform or fire control parts would need to be made or modified for it to work.  Either way, a new MG is being created.  Makes sense.  But requiring an FNC sear be used only in a factory FN FNC rifle?  I don't think so. Unfortunately, there really isn't another option for these sears anyway, which, I believe, is the point of the Rule.  There are more FNC sears registered than there are imported semi-auto FNCs in the country.  In "assigning" these sears to only factory FNC rifles, they are ensuring that a new FNC-type rifle is not created expressly to take advantage of this situation.

Here is the parts diagram for the FN FNC lower:

http://i215.photobucket.com/albums/cc305/c33m0n3y/fncparts3.jpg

Here is the same diagram but for the FN SCAR L:

http://i215.photobucket.com/albums/cc305/c33m0n3y/FNSCARFCG.jpg

The FNC sear/trip lever (Part 521) looks remarkably similar to the SCAR's (part 581), and the FCG are also very similar.   Since both lowers are not serialized they are not the firearm, so tweaks/modifications to a lower would not "make" a second machine gun should one have a registered sear.  

HK sears are installed in all kinds of clones, legally, so I'm investigating whether approaching a legal conversion of a SCAR to full auto with a registered FNC sear is a fool's errand or if it's something worth pursuing.

I've seen this come up before, the FN FNC is pretty much the original fire control of the new SCAR.  One would look at it just like an HK sear but it's the ATF so maybe they had a bad day few years back and made a ruling on it.  I've seen them for under 3g and thought about doing the same thing..... Well maybe I should pick one up.

It can be done mechanically. The ATF says no but that is just their opinion and no one has challenged it yet. I think ATF would lose but I don't have the dollars to fight them and I sold my sear and FNFNC so ..................

Actually for around $3000 for the sear and FNC rifle going for between $2500 - $3000 on Gunbroker' that is not a bad price for a 5.56mm full auto .  If I was back home in the U.S. I would certainly think about it.

But for $6500 you could have a RLL, and that would be much easier to play with, get parts for, host rifles, etc.

Yea, but it's a lot easier to explain to momma that a rifle could be worth 6500 bucks rather then a little piece of sheet metal with a hole in it. plus a 6500 dollar RLL still needs another grand worth of rifle to go bang.

So,  Is there any thing to look out for when trying to buy an FNC to convert?

Make sure the host gun is in top condition as if much of anything major breaks you can find yourself SOL pretty quick.  Pay extremely good attention to the bolt and bolt carrier as they are pretty much irreplaceble (unless you want to buy another FNC or troll the auction sites and boards looking for that one or two per year that pop up)  The one spare complete BCG group that I have managed to snag about 5 years ago cost me what a whole brand new AR would cost at Academy.  Thankfully barrels are now available via green mountain but it's not real straight forward to change them out so get one with a good barrel and dont abuse it by dumping betas through it.

Other than that go try and get all the spare extractors, springs, hammers, etc. you will need for your lifetime as there is only one source (Chris @ Vulcan imports) and he isnt really the most responsive so if you manage to get in touch with him or his "proxy contact" over on the Uzitalk FNC board, just buy one of everything to start.  If he ever goes away pretty much all new small spare part for the FNC go away with him.

As much as I like the FNC if forced to pick between my FNC and my RLL I would keep the link over the FNC for the host platform flexibility and spare parts alone.  However, honestly the best deal is really to spend $2500 more and buy a M16 RR for $9,000ish. It will be the best $2500 upgrade you ever spent.

+1

To emphasize the general FNC support issue...

I watched a factory FNC Maintenance/Repair manual sell on eBay a few weeks back for......$880. Yes, it was a manual.