View Full Version : Converting a Colt AR15 A2 to M4 flat top
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?
What would you do?
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?
What would you do?
We have Colt M4 Uppers in stock.
C4
You need to find out what size your pivot pin hole is. See if it’s the same size as the rear one or larger.
Are you talking about replacing the complete upper with the barrel, or just the upper receiver and keeping the barrel?
If your pivot pin is the screw type AFAIK you're pretty much hosed. Unless there is legality issue where you are I'd unload it and start over.
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.
How old is it?
Flyingjibus
01-29-10, 13:43
I did exactly what you are trying to do.
You need this
https://www.danieldefense.com/?page=shop/detail&product_id=19
Replaces the stupid screw thing with a solid pin that pushes right out.
I know its not a Colt part but it is top quality.
It has one screw head on the left.
It's a ban model I'm thinking made around the year 2000.
You have the standard pin size. You can put any upper you want on it. Except of course an older Colt with the large pin.
Ready to rock n roll! :cool:
For the stock I know I'll need a new extension tube, latch plate, and lock ring but will my buffer and spring work?
Don't forget that your sight plane will be different and you will not be able to use iron sights.
Don't forget that your sight plane will be different and you will not be able to use iron sights.
"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.
Fortunately even if there is an issue there are taller front sight posts available.
http://www.bushmaster.com/products.asp?cat=21
"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.
Fortunately even if there is an issue there are taller front sight posts available.
http://www.bushmaster.com/products.asp?cat=21
The problem with the easy way out is that it has already been mined!
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.
RogerinTPA
01-30-10, 11:43
For the stock I know I'll need a new extension tube, latch plate, and lock ring but will my buffer and spring work?
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.
RogerinTPA
01-30-10, 11:46
Don't forget that your sight plane will be different and you will not be able to use iron sights.
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.
Don't forget that your sight plane will be different and you will not be able to use iron sights.
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.
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 DD 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.
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.
RogerinTPA
01-30-10, 13:33
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
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.
Only as it applies to Colt, the FSB on a fixed carry handle and flat top receiver are different heights. This is a known fact. If it isn't a Colt, it's a clone!
No, you misunderstood. The stock works and is configured, like a normal M4 type stock. All the parts used are for an M4 stock. It moves throughout it's intended range, in whatever position you choose to select. With the 1st mod I did, you would be correct. Using the A2 receiver extension with a Magpul SPR, there is no movement since it is locked into place by the tension screws on the stock.
I was talking about the first mod when you used the existing a2 tube. I know if you use Carbine parts the stock would work as normal :)
So with the a2 tube would it be locked unextended or extended?
Here's a good article that has detailed info on the FSB.
http://forums.officer.com/showthread.php?t=81462
Since we're on the subject of where the FSB goes, let's dig in. F-marked front sight bases are what the top tier manufacturers use. The F-marked FSB is slightly taller than its commercial brethren, and is what nearly ALL rear sights are designed to work with. With a commercial FSB and an aftermarket rear sight, you will need to adjust the front sight all the way up. Either that, or you could also buy a taller front sight post. With the front sight pin adjusted for use with aftermarket rear sights, the front sight is the same height as the "wings" that were meant to protect it... [shrug]
you will not be able to use iron sights.
Nonsense.
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.
There is no difference in the overall height of an “F” marked front sight base and a standard front sight base. The only difference between the two front sight bases is the height of the sight “shelf” above the top of the barrel. When zeroed, the top of the front sight post will be the same height above the bore line regardless of which front sight base you use. There will be no more “protection” provided by the ears of an “F” marked front sight base than those of a standard front sight base.
http://www.box.net/shared/static/2d520zm04w.jpg
http://www.box.net/shared/static/uei1uar8kw.jpg
Also, you can’t just “swap-out” a standard front sight base for an “F” marked front sight base. The holes for the taper pins in the front sight base are drilled and taper reamed at the factory at the same time that the barrel is. They’re a unique match from the factory.
I have done multiple Colt A2 to flat-top conversions and have never had a problem zeroing the iron sights using a standard front sight base and either the Colt front sight post (#SP62447) or the Bushmaster front sight post.
Colt currently offers four different front sight posts for use on their AR-15/M16 family of weapons. The front sight posts can be categorized according to their basic shape, (square or round) their profile, (straight or tapered) and their height, (short or tall.) The height of the posts are measured from the top of the sight post down to the top of the “flange” of the sight post.
The sight post with the part number SP64507 has a short, square and straight configuration. This post has a nominal height of 0.270”
SP64507
http://www.box.net/shared/static/sfemup3tje.jpg
Part number SP64665 also has a short and square post, but differs from the above sight post in that it has a tapered shape to it; broader at the base and narrowing at the top. The height of this post is also 0.270”.
SP64665
http://www.box.net/shared/static/3ixso44u8q.jpg
Shown below are the square, short/straight and short/tapered sight posts side by side for comparison.
http://www.box.net/shared/static/ym1n1ar5e5.jpg
The sight post associated with the M16/M16A1 series of rifles has a round shape and is also short and tapered. The part number for this post is SP61706. It is slightly shorter than the two posts described above with a height of 0.260”.
SP61706
http://www.box.net/shared/static/9fzjm5e8tq.jpg
The last sight post is Colt’s tall sight post which has a nominal height of 0.300”. It has a square and straight configuration. Its part number is SP62447.
SP62447
http://www.box.net/shared/static/sqgsbhvqc6.jpg
Here is a pic with the round sight post and the tall sight post side by side for comparison.
http://www.box.net/shared/static/fplxodhst6.jpg
The original round sight post has five equally spaced notches around the sight post flange for elevation adjustment. Each notch corresponds to 1 MOA of elevation (when used with a rifle length sight radius). The other three sight posts all have four elevation notches on their flanges corresponding to 1.25 MOA of evlevation.
http://www.box.net/shared/static/d2t6yoj8as.jpg
The two short and square sight posts have a longer threaded shank than the tall sight post and round sight post.
http://www.box.net/shared/static/luagl6ptcj.jpg
Bushmaster makes a front sight post that has a nominal height of 0.310”.
http://www.box.net/shared/static/tsdaq4fnkw.jpg
http://www.box.net/shared/static/dhmek33m8h.jpg
http://www.box.net/shared/static/8bzdzoldfu.jpg
http://www.box.net/shared/static/y5rljba59z.jpg
RogerinTPA
01-30-10, 14:19
I was talking about the first mod when you used the existing a2 tube. I know if you use Carbine parts the stock would work as normal :)
So with the a2 tube would it be locked unextended or extended?
Here's a good article that has detailed info on the FSB.
http://forums.officer.com/showthread.php?t=81462
The misunderstanding is now on me.;) I wouldn't say extended or unextended. The SPR stock is simply fitted into place and secured by the butt plate screw and tension screws on the stock. It is the approximate length as an A1 stock.
RogerinTPA
01-30-10, 14:29
Only as it applies to Colt, the FSB on a fixed carry handle and flat top receiver are different heights. This is a known fact. If it isn't a Colt, it's a clone!
So? It doesn't mean the user will not be capable of zeroing the weapon with an after market upper, unless of course the user doesn't know how to zero the weapon. It all depends on the aftermarket front sight you install, which varies in height, to include different sized FSBs and flip type sights, as posted my Molon.
So if I buy a standard Colt carbine flat top upper and most flip up rear sights, then my front sight post will be the standard height, right? Without it being extended above the protective "ears."
Maybe I didn't make it clear that I didn't want to use a handle but a flip up rear which shouldn't be a problem I wouldn't think since they're made to work with most standard manufacturers.
http://www.gunsnet.net/forums/showthread.php?t=320262
There are 2 front sight heights, and 2 rear sight heights.
F marked FSBs are slightly taller than the unmarked standard A2 FSBs.
The reason they came out with the f marked FSB was so that the front sight post would not stick out over the ears of the sight when you zero the sights.
Some manufactures use standard FSBs on all their barrels (Bushmaster) and instead use a shorter than normal height rear sight to compensate.
For example, Colt carry handles work fine on their M4s with f marked FSBs, but they don't work on bushmasters unless you raise the front sight very high.
RogerinTPA
01-30-10, 15:05
So if I buy a standard Colt carbine flat top upper and most flip up rear sights, then my front sight post will be the standard height, right? Without it being extended above the protective "ears."
Maybe I didn't make it clear that I didn't want to use a handle but a flip up rear which shouldn't be a problem I wouldn't think since they're made to work with most standard manufacturers.
http://www.gunsnet.net/forums/showthread.php?t=320262
Correct, but any front sight that is a standard milspec height, either fixed (F marked) FSB or flip up with the same sight post height, on any after market upper, will do. I use a folding front sight on one of mine, a BCM Folding Battle Sight, which is GTG. http://www.bravocompanyusa.com/AR15-Iron-Sights-s/8.htm
Forgot to ask, do you want to go with a M-4 profile upper or Mid-length?
I don't really care for the 6940. Not enough rail to get my hand out further for a more stable shooting platform. I prefer the rifle length rails on mid-lengths. I don't care for the folding ARMS front sight. If BCM Middies were available back then, I would have went with one of theirs with a 12 inch rail.
So if I buy a standard Colt carbine flat top upper and most flip up rear sights, then my front sight post will be the standard height, right? Without it being extended above the protective "ears."
Maybe I didn't make it clear that I didn't want to use a handle but a flip up rear which shouldn't be a problem I wouldn't think since they're made to work with most standard manufacturers.
http://www.gunsnet.net/forums/showthread.php?t=320262
No. Not if you keep your old barrel and FSB. The standard flip up rear sight will require the taller Colt or Bushmaster front sight post to work well. It will not extend above the ears any more than a regular post on an F-marked FSB. Remember, all the rear sights that mount on flat-top receivers should (as long as they are close to spec) line up with the .040 in higher sight post, either using a taller post or an F-marked FSB. Rifles built that way generally (except Bushy) have the FSB.
If you get a complete upper, including a front sight, then yes, the front sight should be the proper height.
BTW - the poster above somehow got on add-on front sights, which I don't believe has anything to do with your problem.
No. Not if you keep your old barrel and FSB. The standard flip up rear sight will require the taller Colt or Bushmaster front sight post to work well. It will not extend above the ears any more than a regular post on an F-marked FSB. Remember, all the rear sights that mount on flat-top receivers should (as long as they are close to spec) line up with the .040 in higher sight post, either using a taller post or an F-marked FSB. Rifles built that way generally (except Bushy) have the FSB.
If you get a complete upper, including a front sight, then yes, the front sight should be the proper height.
BTW - the poster above somehow got on add-on front sights, which I don't believe has anything to do with your problem.
The poster above you was just being helpful in case I decided to go with a long rail upper and clip on sight. I haven't ruled this out yet.
I've never said I wanted to keep anything off the old upper but thanks to everyone for pointing out the problem if I keep the old FSB :) Barrel shouldn't matter but I did say I wanted a carbine I think.
Correct, but any front sight that is a standard milspec height, either fixed (F marked) FSB or flip up with the same sight post height, on any after market upper, will do. I use a folding front sight on one of mine, a BCM Folding Battle Sight, which is GTG. http://www.bravocompanyusa.com/AR15-Iron-Sights-s/8.htm
Forgot to ask, do you want to go with a M-4 profile upper or Mid-length?
I don't really care for the 6940. Not enough rail to get my hand out further for a more stable shooting platform. I prefer the rifle length rails on mid-lengths. I don't care for the folding ARMS front sight. If BCM Middies were available back then, I would have went with one of theirs with a 12 inch rail.
Glad you asked...
When I was looking at Rock River I was seriously thinking about a mid-length but ruled it out and decided I wanted Colt on Colt. I wish Colt made a mid-length. Not for just the rail length and sight radius but less wear and tear on the parts. Apparently the manufacturers using heavier duty parts remedies the wear issue. If I just wanted the mid-length rail, I could always use a slotted rail to fit over the front post or put on a gas block and cover it to use a flip up like you. Also I want an Aimpoint and really don't care for the post being in the picture. I could live with it for a while.
Wanting to spend the least money as possible up front (keeping in mind I primarily want a Colt) I ruled out the 6940. As sweet as it looks and the price for what you get, it's just so limited on what you can do with it. I don't think it would be easy or even possible to put a longer rail on it if I wanted one. I don't really care for the front flip-up having no detent but I don't have a problem with that style sight in general.
So if I was going to go with Colt, to keep the price down I would get the 6920 and upgrade it later.
I may consider going with a mid-length. What high quality brands do I have to choose from that would keep the cost around the $700 price range of a 6920?
Is the H, H2, or H3 carbine buffer worth the extra money?
RogerinTPA
01-31-10, 11:38
Glad you asked...
When I was looking at Rock River I was seriously thinking about a mid-length but ruled it out and decided I wanted Colt on Colt. I wish Colt made a mid-length. Not for just the rail length and sight radius but less wear and tear on the parts. Apparently the manufacturers using heavier duty parts remedies the wear issue. If I just wanted the mid-length rail, I could always use a slotted rail to fit over the front post or put on a gas block and cover it to use a flip up like you. Also I want an Aimpoint and really don't care for the post being in the picture. I could live with it for a while.
Wanting to spend the least money as possible up front (keeping in mind I primarily want a Colt) I ruled out the 6940. As sweet as it looks and the price for what you get, it's just so limited on what you can do with it. I don't think it would be easy or even possible to put a longer rail on it if I wanted one. I don't really care for the front flip-up having no detent but I don't have a problem with that style sight in general.
So if I was going to go with Colt, to keep the price down I would get the 6920 and upgrade it later.
I may consider going with a mid-length. What high quality brands do I have to choose from that would keep the cost around the $700 price range of a 6920?
I believe the 6940 has a different barrel nut that requires a special proprietary wrench, so you are stuck with that barrel. The rail is monolithic, so you cannot upgrade the rail. If you are willing to live with it, it can be a good carbine. I prefer the 6920 over the 6940 due to it's modularity.
There are 6920 uppers out there available, which are expensive. Adding on the cost of a rail + installation, shipping fees, the cost goes up. I understand folks wanting a "pure bred" weapon, but with BCM having equivalent quality (They follow the TDP), M-4s, Middy barrels, complete with more rail options, it may be the more economical choice.
Not many options available for $700.00, but there are a few good choices. More options if you can wait a while and save an extra 200 bucks. Check out their page and see what options that will suit your needs and budget. http://www.bravocompanyusa.com/AR15-Upper-Receiver-Groups-s/1.htm
As far as Buffers are concerned, H or H2s are best used in M4 carbine barrels. Most recommend an H buffer minimum in a carbine length M4 profile. Is it worth it? To me it is since it does add to the functional reliability of the weapon. For mid-lengths, a carbine buffer will be just fine, but it will run/cycle reliably with an H buffer. I have 2 Middys that have one of each. The 2 Colts have H buffers.
I believe the 6940 has a different barrel nut that requires a special proprietary wrench, so you are stuck with that barrel. The rail is monolithic, so you cannot upgrade the rail. If you are willing to live with it, it can be a good carbine. I prefer the 6920 over the 6940 due to it's modularity.
There are 6920 uppers out there available, which are expensive. Adding on the cost of a rail + installation, shipping fees, the cost goes up. I understand folks wanting a "pure bred" weapon, but with BCM having equivalent quality (They follow the TDP), M-4s, Middy barrels, complete with more rail options, it may be the more economical choice.
Not many options available for $700.00, but there are a few good choices. More options if you can wait a while and save an extra 200 bucks. Check out their page and see what options that will suit your needs and budget. http://www.bravocompanyusa.com/AR15-Upper-Receiver-Groups-s/1.htm
As far as Buffers are concerned, H or H2s are best used in M4 carbine barrels. Most recommend an H buffer minimum in a carbine length M4 profile. Is it worth it? To me it is since it does add to the functional reliability of the weapon. For mid-lengths, a carbine buffer will be just fine, but it will run/cycle reliably with an H buffer. I have 2 Middys that have one of each. The 2 Colts have H buffers.
Great minds think alike I guess! I was just looking at BCM earlier and wanted to know if they're good. They seem to get all 5 star review by customers. If I go with at minimum an aftermarket rear sight an aluminum rail the Colt won't be pure anyway.
http://www.bravocompanyusa.com/ProductDetails.asp?ProductCode=BCM-URG-MID-16+MIT12
Hell, add a BCG and CHG for $145 and you can't beat it.
RogerinTPA
01-31-10, 12:31
Great minds think alike I guess! I was just looking at BCM earlier and wanted to know if they're good. They seem to get all 5 star review by customers. If I go with at minimum an aftermarket rear sight an aluminum rail the Colt won't be pure anyway.
http://www.bravocompanyusa.com/ProductDetails.asp?ProductCode=BCM-URG-MID-16+MIT12
Hell, add a BCG and CHG for $145 and you can't beat it.
That would be my choice except I'd add the Mod4 charging handle. It is one impressive charging handle upgrade. Good luck and post pics when you get it together.
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