- Thread starter cjmac1988
- Start date
If by "carry handle" you mean an A1/A2 upper receiver with an integral carry handle/sight, then the standard FSB is what you need.
If by "carry handle" you mean a detachable carry handle mounted to a flattop upper, then the F-Marked FSB is what you need.
If by "carry handle" you mean a detachable carry handle mounted to a flattop upper, then the F-Marked FSB is what you need.
@#$%!! My reading comp skills need improvement!!
I missed the 20" barrel part of the OPs post somehow.While it is true that the F-marked FSB was originally implemented to allow for proper zeroing on 16" carbines with flattop receivers with a considerably shorter sight radius, the F-marked FSB will work on all flattops regardless of barrel length. Technically, you will be able to achieve a proper zero with either FSB on a 20", RLGS barrel, but you may have to adjust the front sight post down a few clicks for zeroing (again...depending on what range you want to zero). The F-marked with a 20" isn't necessary to obtain a proper zero, but it CAN BE beneficial depending on whether you may be running an Improved Battlesight Zero.
Sorry for the previous confusion if I caused any.
@#$%!! My reading comp skills need improvement!!
While it is true that the F-marked FSB was originally implemented to allow for proper zeroing on 16" carbines with flattop receivers with a considerably shorter sight radius, the F-marked FSB will work on all flattops regardless of barrel length. Technically, you will be able to achieve a proper zero with either FSB on a 20", RLGS barrel, but you may have to adjust the front sight post down a few clicks for zeroing (again...depending on what range you want to zero). The F-marked with a 20" isn't necessary to obtain a proper zero, but it CAN BE beneficial depending on whether you may be running an Improved Battlesight Zero.
Sorry for the previous confusion if I caused any.
The 'F' marked FSB is about forty thousandths higher than an A2 FSB. It is designed for use with the detachable BDC carry handle sight, whether on an M4 or M16A4, which is about forty thousands higher than the A2 fixed carry handle BDC. If you fit an A2 FSB to a rifle or carbine fitted with the detachable carry handle BDC, the gun will be difficult to zero. Usually, with the BDC indexed at its bottom, to properly process a 300 meter BSZ, either rifle or carbine would shoot about 10 minutes high with the front post's flange flush with the FSB deck. Adjusting the front post counter clockwise to lower grouping necessary for a 300 meter BSZ with BDC function intact usually requires two full revs of the front sight, which sets it outside of mil-spec. In other words, the front sight will be loose. On another note, since the 20 inch barreled rifle actually produces a higher velocity than the carbine, its bullet path will intersect line of sight at 32 and 300 meters, unlike the bullet path from the M4, which produces a bullet path intersecting line of sight at 25 and 300 meters. This means to sight in the M16A4 at 25 meters requires the operator to click up the BDC 2 clicks from initial 6/3, to have a good 300 zero when the BDC is clicked back down to the 6/3 setting. Did you get that, yeah I thought so. Also, for what it is worth, the rear elevation wheel on an M16A4 is worth 1/2 minute, the windage is worth 1/2 minute, and a click on the front post is worth 1.2 minutes. On the M4 using the same sight, rear elevation is worth 3/4 minutes, windage is worth 3/4 minutes, and a click on front sight is worth about 1 3/4 minutes. If you think this is complicated add M16A1 and A2 rifles into the training sessions on zeroing for a BSZ.
The only reason I know this stuff is I train folks on this stuff everyday. OH, what I've said is even further complicated by folks coming to the firing line with 16 inch barrels and either .223 or commercial 5.56 ammunition. Another problem is folks who have something other than a mil-spec BDC- who knows what adjustment scale they might use.
Most folks eyes just glaze over when they confront this stuff. They take off the BDC and put on a scope. Thing is, they don't know they've just undermined much of the M4 or M16A4 capability, that's to say, they no longer can address both UKD and KD scenarios with such simplicity as the BDC allows for.
Last edited:
OP,
Colt has many people, including myself, confused with the introduction of their new flat top AR15A4 20" rifle. It has been verified that some of the Colt AR15A4 rifles left the factory with the expected "F" marked FSB (Front Sight Base) while other Colt AR15A4 rifles left the factory with the standard FSB. The jury is still out on how well or even if the standard FSB AR15A4 rifles will zero. All I know is the flat top M16A4 rifles as well as the flat top M4/M4A1 carbines normally have "F" marked FSBs. I am interested in a flat top Colt AR15A4 upper or complete rifle but only if it has the proper "F" marked FSB designed for use with the flat top upper with detachable carry handle/sight. With FN releasing their new 20" flat top FN15 rifle, Colt best get their act together on this.
Colt has many people, including myself, confused with the introduction of their new flat top AR15A4 20" rifle. It has been verified that some of the Colt AR15A4 rifles left the factory with the expected "F" marked FSB (Front Sight Base) while other Colt AR15A4 rifles left the factory with the standard FSB. The jury is still out on how well or even if the standard FSB AR15A4 rifles will zero. All I know is the flat top M16A4 rifles as well as the flat top M4/M4A1 carbines normally have "F" marked FSBs. I am interested in a flat top Colt AR15A4 upper or complete rifle but only if it has the proper "F" marked FSB designed for use with the flat top upper with detachable carry handle/sight. With FN releasing their new 20" flat top FN15 rifle, Colt best get their act together on this.
Last edited:
Some brands, like Windham Weaponry, will use the A2 FSB on the flattop carbine and then take the mil spec BDC and mill it by about forty thousandths. This works, but limits the range for which the BDC can be used; and, if the BDC is replaced with a mil spec folding sight the operator will also need to replace the front sight post with an extended front sight post. Thing is, since most folks don't know how to properly zero for a 300 meter BSZ or even shoot using irons they will not know their BUIS may not zero. They usually just have these sights on for show, using these yet to be zeroed sights only for very close distance shooting where they could probably do as good just shooting instinctively. Interestingly, their scopes appear to be just for show too, since most do not have any interest learning how to use the scope's range finding reticle, or BDC functions that were must haves for these shooters. Bottom line, some folks just want a cool rifle that they can dress up like a Barbie Doll. They have no desire to spend any time learning how to shoot it, much less master it. They will never understand the capability of their purchase even with the stuff they bought to supposedly make their rifle even more capable. It's very queer; but, it is what it is. I'm not complaining, as it seems to be good for the economy.
Here's a sidebar, a few years ago I was assisting a Soldier learning how to shoot his SDMR. On day three, it was discovered his BDC was not mil-spec. The sight was not scaled properly for a BDC function. Apparently, a Soldier turned in this rifle from training on an earlier week with a BDC made in China, which he had purchased at Ranger Joe's. Now, why would that happen? It was easier to buy a new BDC than run back to the dorm where the mil spec BDC was left in trousers worn on an earlier day of training. This incident really messed up the Soldier I was training as there was not much opportunity to flush out a zero while on the 500 yard line with a replacement rifle. The consequence was my student did not produce the results he was clearly capable of producing, a bummer for the student and his coach.
Last edited:
Now I've got another question. Since most of the "lower tier" makers continue to install the "wrong" FSB and according to the above post go to the extra trouble to mill the rear sight to make it halfass work, is the "wrong" non-F-marked sight THAT much cheaper????? Are they really saving enough to justify "cutting corners" and putting sub-standard sights out there?
If they really ARE saving that much, what makes a FSB that's 0.040" shorter so much cheaper and why can't the cheap makers change their tooling enough to build sights a few thousandths taller? Same for milspec/non-milspec buffer tubes. I have yet to find anybody that can explain the savings in a smaller tube.
I'm just bitchin mostly but if anybody can come up with a plausible explanation I'd love to see it. They may be cutting corners but I just can't fathom how these tiny differences in size can save enough money to justify them.
If they really ARE saving that much, what makes a FSB that's 0.040" shorter so much cheaper and why can't the cheap makers change their tooling enough to build sights a few thousandths taller? Same for milspec/non-milspec buffer tubes. I have yet to find anybody that can explain the savings in a smaller tube.
I'm just bitchin mostly but if anybody can come up with a plausible explanation I'd love to see it. They may be cutting corners but I just can't fathom how these tiny differences in size can save enough money to justify them.
I don't know the politics or patents concerning it; but, I believe it was Bushmaster that originally produced such non mil-spec guns. Who knows why, then or today. But, since the fixed carry handle is now days only of benefit to Service Rifle competitors, I too wonder why the higher A-frame is not used on everything being built.
Well! I hadn't considered that little word "patents"---maybe that's the problem. Perhaps anyone who wants to make the PROPER parts has to buy the rights to do so from Colt. Probably at an exhorbitant price, forcing them to charge a lot more for the parts. Thank you.
Here's my final thought on the matter, back when the A4 was authorized for Service Rifle division of NRA HP, Rock River began producing a match conditioned version of such. I had experience with two of them. Both shot about 10 minutes high. I sent both back. I also advised Rock River what I suspected was the problem. Years later, it appears rifles like those I experienced showed up for use at SAFS. The rifles at the two firing points I was coaching on could not be properly zeroed without turning out the front post about two revs. At any rate, it made me think some assemblers of AR's are more about marketing than they are about manufacturing.
Last edited:
SS, another great post, thanks for the information.
In order to address not only the 0.040" height difference between a standard FSB and that of an 'F-marked' FSB, here is an image of a detachable carry handle from an early 90's Colt 'Blue Label' 6700 Competition H-BAR vs that of a current production Colt M4. The Colt 6700 came from the factory with a standard FSB and this carry handle. The M4, of course, has a 'F-marked' FSB.
Last edited:
I have measured numerous BDC's and, like you, have seen, readings all over the place. I had a Stag rifle which had an F marked FSB which was actually an A2 FSB. The gun shot about 10 MOA high. It was returned and sent back with proper sights. Makes me think of Forrest Gump, "Life is like a box of chocolates, you never know what you're going to get".