Small gap.

[randyrucker]

I shot my new AAC 9 inch upper last weekend. I noticed when I looked at it close there is a small gap between the URX rail the the upper receiver. On my other upper this fits flush together. Does this gap look normal ? Thanks for the help.

[BlogSarge]

Sadly, yes.

Even on the Knight website there is a gap there. You'd think they could mill the uppers and rails so they fit together with an uninterrupted rail section.

[randyrucker]

Is there any way to fix this gap to make it flush ?

[jryock]

The gap on my AAC upper isn't nearly that wide, but I also have the KAC URX II Mid-Length on it. It must have something to do with the nut.

IMHO... I wouldn't have any issue if it is firmly attached (doesn't move) and the grooves of the rail sections are evenly spaced. If the rail is not continuous (in the groove sense), I'd be kindly asking AAC to fix the issue.

[bstark]

I don't think it's installed correctly and I agree with jryock that it involves the barrel nut. I have the same AAC/KAC setup (but a 16" barrel) and no gap. I bet if you took it apart and reassembled it, you could eliminate the gap through adjustment of the nut in the rail. Keep in mind though if you want to take it apart, you need a proprietary barrel nut wrench. They run about $115.

[randyrucker]

Well AAC must be on vacation. I sent a PM message to Paul VanWagner about 4 days ago and got no response. I called his office today and left a message. So far nothing from AAC. I'm not going to go out and buy a $115 dollar barrel nut wrench and try to take this apart and re-assemble it. This thing is brand new and it looks like a quality control issue to me. It's like buying a fine china dish and after taking it home you notice a hairline crack in it. I don't think it affects functionality, it just shouldn't be there.

[tallburnedmidget]

FYI, I have removed a KAC URX II from a 16" AAC upper and man are you in for some surprises if you think it will be a simple ordeal to do yourself.

Here is what I had to do:
- Get a KAC compatible barrel nut wrench (like $200+ for KAC, or on ebay you can find after market machined ones for $80-$100)
- Removing the flash hider is a pain as it is rockset. After trying all kinds of things (except buying the $30 flash hider remover tool) I was pissed off and layed the upper on my bed, got on it with my knees so it wouldn't move, and took a long file wrapped in cloth (to not scratch the flash hider) and then placed the whole thing inbetween the prongs of the flash hider, and finally yanked with all my frustration and strength and POP it came loose.

I didn't even try to apply any steady easy force to pop the rockset as I had already begun stripping the flats on the flash hider using various wrenches and other methods. I was going to succeed in one go, and did hahahaha.

-Next, you must work through the hand guard to remove gas block and gas tube and know there is a retaining pin on the gas block, then tap the gas block off the barrel while applying force to the hand guard to ensure the gas block doesn't catch within the grooves of the handguard
-Gas block removed, so you can take apart the lower half of the 2 part hand rail which then gives you access to the barrel nut for the wrench
-At this point you learn that the freakin 2nd half of the handguard is ROCKSET to the barrel nut!!!!!! Use your barrel nut wrench to remove the barrel nut and remaining half of the handguard as one piece.

I could see that in your case you may only need to rotate the handguard around the barrel nut to get flush/reduce the gap BUT the rockset will rear it's head and I don't even know how one would separate the barrel nut and handguard from one another w/out paying hundreds of $$$ for the hand rail tool that would allow you to vice the handguard and crank on the barrel nut until the rockset broke.

I give it to AAC they make something that will not come apart so if you have an AAC upper in a war you can feel confident it wont come lose or break apart hahahah.

I hope my trials and tribulations of fighting with an AAC KAC handguard give you some insight on what you may be dealing with

[randyrucker]

Thanks for the replies guys. Paul from AAC called me back pretty quick. He told me the KAC rails fit like that and it in no way affects accuracy or functionality. Having said that he recommended returing it to AAC to see if the barrel nut can be tightened. I'm probably being somewhat anal about the gap, but I just think it looks like a more professonal fit when the URX rail is flush with the upper receiver. I would like to thank Paul for calling me back so quickly and taking the time to explain to me how a KAC rail is made. Good luck Paul on your move to greener pastures. I hope you still visit the forums sometimes.

[TKanaley]

The KAC URX 3 is installed by turning the threaded handguard onto the threaded upper receiver until it can go no further. Depending on the receiver itself, the rails on the receiver and handguard may line up perfectly as the handguard is fully tightened, or you may need to back off the handguard a bit till it does. That can be anywhere up to a 3/4 turn which will determine the size of your gap. I have an SR15 where the URX 3 handguard fits flush against the receiver and I have an old Armalite that I'm installing one on, that which will need to be backed off about half a turn to line up the rails. I will end up with a small gap, but that's just the nature of the beast. Tightening the BBl nut won't accomplish a thing as far as reducing the gap goes. You would have to machine the base of the handguard where it contacts the receiver in order to get rid of the gap. I personally wouldn't worry about it.

[TCustoms]

FWIW, Rockset is easy to break free. A few drops of water and it will soften after an hr or so.