Warning

 

Close

Confirm Action

Are you sure you wish to do this?

Confirm Cancel
BCM
Member Login

Site Notices
4/18/2021 9:59:29 PM
Posted: 9/16/2004 9:21:18 AM EDT
I read somewhere a while back that Colt / LMT M4 uppers have a different height FSB than other brands.
Are there also detachable carry handles of different heights?  Or are all detachable carry handles the same?
Link Posted: 9/16/2004 10:01:15 PM EDT
Anyone?
Link Posted: 9/17/2004 6:25:34 AM EDT
Yes, there is something to what you say.  I asked this same question a while ago and never got a straight answer.  

Here is what I experienced - I bought an A3 kit from J&T with detachable carry handle.  They sent it with a taller front sight (not the base, just the post itself).  I also have an A2... when both are properly sighted-in the A3 has the sight post significantly higher than the A2.  I replaced my J&T barrel with a Bushmaster barrel... and I had to put that taller front sight post in to get it to sight in correctly.  

I've heard Colt uses taller front sight bases because the detachable carry handles do sit up a little higher, but I have no idea if that is true.  Sorry I can't be more help.

Scot
Link Posted: 9/17/2004 6:57:43 AM EDT
My RRA upper needed the taller front sight for the detachable carry handle.  IIRC you need the taller front sight with all ARs that use the standard FSB and detachable carry handle.  

Through research on this sight i have also heard that you need the taller front sight for the Arms40 series flip up sights.  Hope this helps.

Later,

AR

Link Posted: 9/17/2004 10:43:32 AM EDT
When I put one of my M4 uppers together, using a Bushmaster 14.5" 1/7, M4 barrel, I purchased the taller front post for use with an ARMS#40, but I ended up not needing it,..
Link Posted: 9/17/2004 1:14:20 PM EDT
There was a long thread on this a while back.  Paraphrased:

On Colts the flat-top sights are higher than A2 sights (by .040" IIRC).  The front sight bases are marked with an "F" to indicate they're they're higher.  The difference is that the flat part between the ears is higher.  Everyone else just uses the same FSB for all rifles.  So all non-Colt detachable handles are .040" lower than Colt ones, to match up with the A2 FSB height.  BUISs like the ARMS#40 that are designed for military rifles are made to the taller Colt spec height to go with the taller "F" FSBs.  FSBs like the PRI that are intended for military use are "F" height.

Depending on the exact tolerences of your rifle, you may or may not need the extra .040" of front sight height to make everything zero OK.  Many have used ARMS#40s on non-Colt rifles with no trouble.  Others have run out of elevation and needed the longer post.  An "F" height FSB with a non-Colt handle would have the opposite problem.
Link Posted: 9/17/2004 3:32:47 PM EDT

Quoted:

BUISs like the ARMS#40 that are designed for military rifles are made to the taller Colt spec height to go with the taller "F" FSBs.  FSBs like the PRI that are intended for military use are "F" height.



Mike,

Are you saying that some BUIS (non-military contract) work with lower non "F" front sight bases thus eliminating the taller sight post?

Thanks,

Scot
Link Posted: 9/17/2004 6:03:11 PM EDT

Quoted:

Quoted:

BUISs like the ARMS#40 that are designed for military rifles are made to the taller Colt spec height to go with the taller "F" FSBs.  FSBs like the PRI that are intended for military use are "F" height.



Mike,

Are you saying that some BUIS (non-military contract) work with lower non "F" front sight bases thus eliminating the taller sight post?

Thanks,

Scot



As I understand it, all aftermarket rear sights are Colt height.  As Mike posted, some have problems and some don't.  The disadvantage to using the taller front sight pin is that your post will be higher than your protective ears.
Link Posted: 9/28/2004 12:01:42 PM EDT
Sorry for the late response--been out of town for 10 days.

I would guess that some BUISs like the DPMS or Bushmaster/YHM ones would be made to the same heights as their carry handles.  But that's just a guess.  I'm not even sure how you'd measure to find out.


The disadvantage to using the taller front sight pin is that your post will be higher than your protective ears.

That came up in the old long thread too.  At least one person on the other thread said the top of the post should be in the same place relative to the ears with the taller post as with the "F" base and regular post.  I agree, but that assumes that the forgings start out the same and the barrel hole is located in the same place relative to the top of the ears.  I don't remember ever getting an answer on the other thread as to whether that was truely the case.  
Link Posted: 9/28/2004 12:37:09 PM EDT

Quoted:
Sorry for the late response--been out of town for 10 days.

I would guess that some BUISs like the DPMS or Bushmaster/YHM ones would be made to the same heights as their carry handles.  But that's just a guess.  I'm not even sure how you'd measure to find out.


The disadvantage to using the taller front sight pin is that your post will be higher than your protective ears.

That came up in the old long thread too.  At least one person on the other thread said the top of the post should be in the same place relative to the ears with the taller post as with the "F" base and regular post.  I agree, but that assumes that the forgings start out the same and the barrel hole is located in the same place relative to the top of the ears.  I don't remember ever getting an answer on the other thread as to whether that was truely the case.  



I would like to know this also.

I have an armalite/eagle arms flattop upper with carry handle, and it does require the taller front sight post to properly zero with a bushmaster barrel.   I was getting worried that maybe my flattop was out of spec causing this problem, since I thought only colt used the taller carry handles, but it sounds like all the carry handle manufacturers are following the colt spec.

Even with the taller front sight post, the top of the post is still below (although just barely) the level of the protective ears.  It's not enough lower than the ears to be able to tell by just looking at it (it looks level), but if you lay a straight edge across the ears, you can see it is slightly lower.  Hopefully, someone with a colt can let us know whether thiers is similar.  Thanks.
Link Posted: 9/28/2004 12:44:06 PM EDT

Quoted:
some have problems and some don't



The USMC have a different front sight hold than the Army.  Think this contributes to the some do/some don't thing??  Just trying to figure out why.
Link Posted: 9/28/2004 12:50:17 PM EDT

Quoted:
I read somewhere a while back that Colt / LMT M4 uppers have a different height FSB than other brands.
Are there also detachable carry handles of different heights?  Or are all detachable carry handles the same?



I think this will help u out:

www.ar15.com/forums/topic.html?b=3&f=66&t=187102

What the heck happened to the thread?
Link Posted: 9/28/2004 1:43:45 PM EDT

I think this will help u out:

www.ar15.com/forums/topic.html?b=3&f=66&t=187102

What the heck happened to the thread?


Good question.  I think the one titled "Hmm...Didn't know this about front sight bases" linked under the Excessive Elevation section of the one you linked to was the original long thread we're referring to, but the link seems to be broken.
Link Posted: 9/28/2004 2:07:21 PM EDT
I have the same problem with two different uppers, DPMS and M&A Parts (which I'm pretty sure is really a RRA). RB Precision can provide you with offset rear sight leaves, which have the apertures .040 lower than standard, which doesn't sound like much but do the math and it works out to a bunch @ 100 yards. Unfortunately, the leaf wouldn't go on the screw in the cheap SARCO carry handle on one of my guns (different threads--I'm sure the Chinese screw is wrong), and I haven't had a chance to put it on the other one (a genuine Colt) yet.

But it SHOULD work.
Link Posted: 9/28/2004 4:03:48 PM EDT
Just got a response from TWEAK.  Seems that the links have been archived.  TWEAK is gonna see what can be done to get them back up.
Link Posted: 9/30/2004 9:34:05 PM EDT
so what if you are using a rail mounted forward sight like YHM, would an ARMS 40 be off then because of height issues?

Would a taller post for the front sight correct it?

And if I bought a carry handle from Fulton, any idea if its taller milspec or not.  It does have the little anvil mark on it....
Link Posted: 10/1/2004 6:03:36 AM EDT
I bought a detachable carry handle from Fulton a few years ago, and it was a Colt.  I'd assume they're still selling Colt carry handles today, but I'm not positive.
Link Posted: 10/1/2004 6:50:29 PM EDT
Mike is on the right track.  Ok, let me see if I can simplify this, here goes.

Front sight bases (FSB) fit into two catagories, standard A2 (low FSB) and the Colt "F" marked FSB (high FSB).  Colt puts the high FSB on all barrels that they install on flattop uppers, all barrels installed on fixed carry handle uppers (A1/A2) use the low FSB.  I believe LMT also uses high FSB's on their barrels, but then all their barrels are install on flattop uppers (they do not make A1/A2 upper receivers).  All other manufacturers use the low FSB on their barrels that are installed on either flattop uppers or A1/A2 uppers.

What makes the low and high FSB's different?

Both low and high FSB are machined from the same dimensioned forgings.  The only difference between them is that on the high FSB, the area between the ears is not machined as far down as on the low FSB.  This difference in height is 0.040 inches.  When using standard height FSB posts in both low and high FSB's, this would but the post approx. 0.040 inches higher above the barrel in a high FSB.  Because the area between the ears on a high FSB is not machined as far down, the top of the standard post will be closer to the top of the ears than a low FSB.  However, the top of the post should still be slightly below the top of the ears on the high FSB.  If a "tall" post is used in a low FSB, this should put the top of the post at the same level, in relation to the ears, as a standard post in a high FSB, meaning that the top of the tall post in a low FSB should be slightly below the top of the ears.

Ok, pretty simple right?  Well due to the fact that FSB are forgings, their dimensions can vary a little bit from forging to forging, since the forging process is nowhere near as accurate a way to make a part as is casting or CNC machining it from billet or bar stock.  As a result, the location of the hole machined into the bottom of the FSB for the barrel can vary a little from FSB to FSB.  This means that the distance from the top of the ears to the center of the hole for the barrel can vary also.  When all this is taken into consideration, I believe this explains why the standard post works some times and some times the tall post is needed in low FSB's.

Ok lets talk about the rear sights now, in particular removable carry handles (RCH) versus fixed A1/A2 carry handles (FCH).  The FCH is made to use the low FSB with standard height post, if a high FSB is used with a standard post, the post will most likely have to be turned down into the FSB a bit (approx. 0.040 inches) to get things zeroed.  Colt RCH's are made to be used with the high FSB with standard height post.  Where things start to get real confusing is that other manufacturers may make their RCH to work with a high FSB or they may make them to work with a low FSB.  All manufacturers including Colt use the same RCH forgings and it is my understanding that all manufacturers including Colt machine the forgings to the same final dimensions.  Where things differ is between the rear sight bases.

There are at least two different types of rear sight bases and possibly as many as three.  There is the standard FCH sight base that is the same for all manufacturers including Colt.  Then there is the Colt RCH rear sight base which differs from the FCH sight base by having both a base and screw that is shorter.  Even though the RCH sight base is shorter than the FCH sight base, the RCH rear sight still sits up higher (approx. 0.040 inches) above the bore axis than on a FCH due to the sight base shelf on the RCH being higher than that on a FCH.  

Sufficiently confused yet?  Well there is more.  It is my understanding (and I could be wrong here) that Bushmaster, and possibly other manufacturers, use a third type of rear sight base that is even shorter (by 0.040 inches) than that of a Colt RCH sight base.  This allows for the use of a low FSB combined with a standard RCH forging machined to mil spec.  If I am wrong about the different sight base, then for Bushmaster to get their RCH to work with a low FSB would mean that they would have to machine the sight base shelf lower on the RCH forging and possibly also cut the sight base screw shorter as well so that it doesn't come in contact with the flattop.

Ok, I just went downstairs to do some measuring.  The FCH sight base thickness is 0.250 inches and a Colt RCH sight base thickness is 0.200 inches.  I don't have a Bushmaster RCH so I can't give any numbers on what thickness sight base they use.

As for the question on PRI, LMT, KAC, ARMS, etc. iron sight heights, I believe all them are set up to be used with high FSB's and Colt RCH height rear sights.

Well I am not sure I simplified the issue any but I hope this helped.  As for the reason Colt decided to raise the sights 0.040 inches on their flattop models, that anybody's guess.  Possibly it was to allow for enough elevation adjustment in the RCH while not making the rear sight base too thin, that's my guess anyway.
Link Posted: 10/1/2004 6:56:22 PM EDT
Jason, thanks so much for taking the time to post that very full and informative report on the problem. That ought to be made a sticky!
Link Posted: 10/5/2004 8:52:25 AM EDT
I tried to measure the rear sight base on my BM detachable handle last night.  Couldn't get a really good measurement without disassembling it but I came up with ~.203" so it appears they use the same base as Colt (which I would have expected).  The shelf for it was ~.791" (?-memory failing) from the base surface that would contact the top of the rail.
Link Posted: 10/5/2004 9:45:38 AM EDT
OH here is the answer I am looking for.
Link Posted: 10/5/2004 7:50:45 PM EDT
Eureka!  Thanks for the measurements Mike, I figured out where the difference is between BM and Colt removable carry handles.  I just measured my Colt RCH and found that the shelf height from top of flattop is 0.833", which gives us a difference between makes of approx. 0.040".  So it's not the sight base that is different, but the machining of the RCH forging.

With the sight base turned all the way down against the shelf, I measured the distance from the bottom of the screw to the bottom of the carry handle and got a reading of 0.055".  I have a feeling if Mike were to measure this same distance, it would be approx. 0.015" on his BM carry handle.  I guess this raises the question again on why Colt decided to raise the sights 0.040" on flattop models, since it doesn't look like they needed to, to get the full 600m of elevation adjustment.
Link Posted: 10/5/2004 7:56:00 PM EDT
Link Posted: 10/5/2004 8:33:02 PM EDT
Link Posted: 10/5/2004 8:56:31 PM EDT
huh, my RRA upper with stainless 20" barrel has an F marked FSB...
Link Posted: 10/5/2004 9:29:05 PM EDT
Link Posted: 10/5/2004 9:29:17 PM EDT
Okay, so should this theoretically work:

I have a PRI FRONT FLIP UP SIGHT (older 2 screw model) being used in conjunction with the stock BUSHMASTER RCH.  Should this combo work as the PRI mimics the taller COLT "F" FSB?  So I have a taller FSB being used in conjunction with a "shorter" RCH.  So does this mean that this combo is easier to deal with because in order to zero I had to turn DOWN the FRONT SIGHT POST to zero?  Man, I shoulda just bought a damn COLT!

Oh wait a minute, if I was having problems, IN MY CASE, a COLT RCH would have solved the problem because the COLT RCH, being taller, would have lined up with my PRI front sight, huh?  
Link Posted: 10/5/2004 9:47:53 PM EDT
Link Posted: 10/5/2004 11:39:11 PM EDT
Ain't that a load of bullcrap!  I tell ya, I should have just bought a freaking COLT!  But, seriously, my rifle is a Bushy Dissy.  

So, you mean to tell me that the upper itself might not be to spec??? WTF?
Link Posted: 10/6/2004 12:00:41 AM EDT
Link Posted: 10/6/2004 12:04:06 AM EDT
Read the FAQ?  Hell, one of my questions MADE the FAQ!!!   But, no, I didn't know that BFI wasn't using correct height uppers.  Now I definitely know I shoulda bought a COLT!  

By the way, when are the links gonna be fixed???
Link Posted: 10/6/2004 12:12:51 AM EDT
Link Posted: 10/6/2004 12:15:59 AM EDT
Hopefully the one I bought in '00/01 is cool.
Link Posted: 10/6/2004 12:35:11 AM EDT
Link Posted: 10/6/2004 10:14:38 AM EDT
What's a decent set of calipers that I can buy?  And is digital okay, or am I just lazy?  
Link Posted: 10/6/2004 9:04:31 PM EDT
Who sells the longer front sight posts? I need to get one. Thanks.
Link Posted: 10/6/2004 9:51:56 PM EDT
Link Posted: 10/6/2004 9:53:27 PM EDT
Link Posted: 10/7/2004 12:24:12 AM EDT
Link Posted: 10/8/2004 11:46:35 AM EDT
Measured some more stuff last night.  On my BM handle with the sight turned all the way down the screw measured approximately .019" which isn't exactly what jason_h predicted, but close enough to confirm the theory.

I also measured the hight of the upper as described by Tweak on a late-90s BM and a 2003 RRA.  The RRA was .722" to the bottom of the slot and .835" to the top of the rail.  The BM was .721" to the bottom of the slot and .839" to the top of the rail.
Link Posted: 10/10/2004 12:09:38 AM EDT
Link Posted: 10/11/2004 8:16:15 AM EDT
jason_h said:

With the sight base turned all the way down against the shelf, I measured the distance from the bottom of the screw to the bottom of the carry handle and got a reading of 0.055". I have a feeling if Mike were to measure this same distance, it would be approx. 0.015" on his BM carry handle.


That's what I was measuring to be .019" which seems to confirm the idea that the BM and Colt sight bases are the same part.
Link Posted: 10/11/2004 8:16:47 PM EDT
Link Posted: 10/11/2004 10:47:20 PM EDT
I just ran into this issue when using a LMT BUIS, CMT Upper and BFI Brl/FST.  
Link Posted: 10/25/2004 11:41:29 AM EDT
detachable carry handles and FSB height differences.

I have just ran in to this with a "Model 1 Sales" CAR 16 with A3 upper and carry handle.

I called them and the guy I talked to said " they have not had any problems with this."

I told him about the .040" hight differance and how Colt marks there front sight bases with an "F" on the left side. Also told the guy that our range has a retired UMC ammor that showed me the same proble a guy out on the range Sunday was having with another CAR from a diffrent manufacture.

I called BushMaster and got the .040" taller sight.

If there is any other people out there that is having the same problem with a Model 1 Sales CAR kit with flat top + carry handle PLEASE POST HERE OR Email me.

TWEAK what parts of Washington are you from? I am in SE Washington.
Link Posted: 10/25/2004 6:35:53 PM EDT
How very strange. I measured my Oly A3 upper and it came out 1.721/1.844. Hard to imagine Oly having much variation when they have to match the upper to the FIRSH.

Since the whole point of the taller front sight base is to restore the trigonometry involved in calibrating the elevation wheel to zero the rifle, any method of raising the height of the front site post should work, whether it's a taller FSB with a standard front sight post, or just a taller front sight post.
Link Posted: 10/25/2004 7:48:43 PM EDT
Link Posted: 10/29/2004 6:16:36 PM EDT
Link Posted: 10/29/2004 6:43:44 PM EDT
So, here's my question:

Are most all rear BUISs (LMT, ARM #40 etc..) made based on (to work with) the F/higher Colt FSB?

If this is true, then isn't everyone without the higher FSB having issues using these BUISs?

So everyone without a higher FSB using these BUISs has to buy a higher sight pin?

Is this right? Oh...I'm so confused.


Link Posted: 10/29/2004 7:47:48 PM EDT
Link Posted: 10/30/2004 12:29:09 AM EDT
Top Top