So, I've been looking, and I can't find what I am looking for. I have this CMMG rifle, http://www.cmmginc.com/product_p/55ae1bb.htm and I want to find an extended handguard if at all possible that will fit it, but I haven't come across any that are piston compatible. My price range is around $250 at max, if anyone can recommend something I'd appreciate it.

Kind of surprised that there isn't a topic for all the handguards that are piston compatible.

Are you looking for smooth as opposed to railed? Because I've seen folks' posts from other forums looking for smooth hand guards to fit bull barrels (as yours is, yes?) from back in 2011 (that means 2 years of new products, right?). Have you called CMMG? Some were steered to an appropriate product by calling their rifle manufacturer.

If you don't mind switching to quad-rail, try MI and Daniel Defense.

Good luck, let me know what you find out. I'm thinking about a piston upper myself and every bit of info helps.

My rifle isn't a bull barrel, it's a normal military grade barrel. I've looked and Daniel Defense doesn't have an option, nor does MI. All their options are not meant for piston rifles. I want a quad railed handguard, I guess I'll just have to stick to a normal 7" carbine rail, I just wanted the extended handguard for a longer grip.

Edit:

Had some extra time today to do some looking around, a friend of mine recommended ERGO brand, so I looked into them.

http://ergogrips.net/products/rails/overshoot-rail.html

Says it is piston compatible, I have emailed them to see if it is certain ones, or universal. Cheaperthandirt has it for $209, so it falls perfectly in my price range.

Oh? I only assumed it was a bull barrel 'cause the rifle in the link you included, is.  Not the same rifle?
Anyway, glad you found something.  I found this from Samson @ MI: http://www.midwayusa.com/product/2852128132/samson-evolution-series-customizable-free-float-handguard-ar-15-aluminum-matte?cm_vc=subv12696411402
They've got a couple of lengths with the extension / cutout. Not sure if the ID will give enough clearance @ 1.56"
If you don't mind, let me know if you find out the inside diameter of the ergo.

Here's a question: with so many different types, lengths and heights of operating systems and blocks on ARs now, why isn't anyone making a customizable, truly modular rail system?

That's odd, I guess CMMG screwed up on their product description. It is an M4 profile barrel, not a bull barrel, and has the standard plastic handguard which I want to switch out. I was originally thinking about just getting CMMG's handguard, but they are backordered and I would have to take everything off the barrel in order to do it. I came across the ERGO thanks to a friend, and it's a 2 piece free float, so all I have to do is cut the delta off. I'm waiting to hear back from them, so hopefully they can give me a concrete yes or no, because I'd be pissed to spend that kind of money and find it doesn't work with my rifle.

I've been wondering the same thing.  I bought a POF 415 last month and have had a blast shooting it but the quad rail makes it a little front heavy, it loosens often and is really more than I need.  I was looking to replace it with something like the Troy Battle Alpha but have fit concerns.  I've emailed both POF and Troy but haven't heard back.  Anyone have similar issues with the POF?

I've emailed Troy and asked if they had any idea if any of their products will work for a piston rifle. I got a reply saying no, nothing they currently sell is compatible.