The title pretty much explains it. I’m having a hard time finding a Colt upper other than the LE6940CK monolithic upper. I’ve tried Fidelis but they don’t have anything in their auctions and haven’t answered my email from a week ago.
From what I’ve read, the front hinge pin may be an issue. Mine has the screw but I think an adapter can be found for about any one.
I’m not totally opposed to another brand but I’d rather match with a Colt.
If I could find the parts, I could use my existing bolt/charging handle but that would require head spacing every time I switched uppers, right?
IIRC the Colts with the double screw arrangement (you needed two flat tip screw drivers to disassemble it) had the large hole. The newer ones with the standard hole size only had one screw on the left side.
The OP says that he has the screw type but I’m not sure which one he is talking about.
“Will” or “might”? I have seen several flattop conversions where the tolerance stacking worked in the shooter’s favor and they were able to zero with the original FSB without issue.
Any standard pin upper will fit it as long as the lower pin holes are standard, screw or not and there are several adapters on the market in case your holes dont match. As far as head spacing thats a different issue. You dont have to headspace when you change receiver, thats between the barrel and the bolt. I’ve been building these rifles , pistols & carbines since 1989 , ran into just about every issue in the last 21 yrs. As far as finding a COLT m4 upper receiver send member scottryan an email he deals in alot of colt parts. I’ve bought from him in the past from __15.com . He always has top shelf Parts
I have done numerous Colt AR15A2’s (front screw pivot) to flat top / push pin conversions and the Colt A3 carry handle zero’s nicely. Most are right on the money to a standard A3 factory zero.
I’ve converted two of these rifles, and they both zeroed out with front sight posts that were excessively high, in terms of where the base of the front sight hit the front sight base. One was actually entirely above the FSB. I was told that Colt puts a different FSB on the A3 and A4, in order to have one unitary detachable carry handle.
The Bushmaster extended front sight fixed the issue affordably, but there’s still the fact that the sight itself is awfully high, and prone to getting damaged. Probably not a big deal, but the ideal solution would be to procure the correct FSB and install it.
My first mod I used a Magpul SPR stock, with the original receiver extension, buffer spring and A2 buffer. It worked very well. No problems with cycling at all.
A few months ago, I wanted a complete carbine configuration so I replaced mine with a MilSpec Daniel Defense carbine receiver extension, DD QD end plate, DD castle nut, carbine spring, buffer and ACR stock. The upper is a Sabre Defense Mid-length (I believe this was prior to any other manufacturer, producing Mid-lengths, pre-BCM/Larue). You can always sell off the upper and dremel the sear block, if you feel an AR15 BCG isn’t enough for you. It works very well as my recce-ish carbine.
Why not? The upper is a solid mate to the lower, with no day light or rattling. My Sabre Defense middy upper has a standard F FSB, and it took the same time it did to zero the irons as it did for any of my other AR weapons, including my AK variant.
I don’t understand how the sight plane could be different if the upper is a tight fit with the lower. Then the sights shouldn’t be an issue.
Wouldn’t that mean the stock would be in the compact position and couldn’t lock in the extended position? Unless I’m misunderstanding how it would work, I think I’d rather just go ahead and buy the new parts than having to shoot with a stock too short.
No, you misunderstood. The stock works and is configured, like a normal M4 type stock. All the parts used, as previously mentioned, are for a MilSpec M4 stock. It moves throughout it’s intended range, in whatever position you choose to select. I omitted the fact that it was a “carbine” receiver extension, since I thought it was what I implied, since it wouldn’t be logical to replace an A2 receiver extension, with another DD A2 receiver extension. I edited my original post to include the word “carbine” with “receiver extension” for more clarity. All the carbine parts, are Daniel Defense (DD), purchased from DSG Arms: http://dsgarms.com/index.cfm/category/13/lower-receiver-parts.cfm
