[Konstantin835]

If I have a flat-top receiver AR-15 with an A2 front sight base will I be able to zero the rifle? As I understand it the A2 FSB is .040 inches shorter than an F marker FSB which might cause problems zeroing. Anybody have any experience with this?

[john5036]

Uh, what potential problems are you referring to?

You shoot the rifle, adjust accordingly, zeroed. The .04" is not an insane difference to where you shouldn't be able to zero.

[Konstantin835]

I was wondering if the front sight is adjustable enough to compensate for the difference.

[john5036]

Depends on the gun and then maybe your rear sights as well. The carry handle probably will give you issues. To make it easy, I'd just pick up an F marked FS and be done with it.

[ugaarguy]

Have you measured it to see if it's F height, but just not F marked? LMT used to do that on some of their carbines. You could also get a taller front sight pin - that was Bushmaster's cheap "solution" to using A2 FSBs when they should have been using F height FSBs.

[Konstantin835]

I don't have the gun yet so I can't measure it, I'm ordering a parts kit tomorrow from Del-ton. Because I'm in NY I can't have a bayonet lug so I have to choose between an A2 FSB with no lug (legal as is but maybe a sight problem) or an F marked FSB with a bayonet lug which would require me to grind off the lug before assembling the rifle. I would rather not grind anything so thats why I'm hoping the A2 FSB is good to go.

[ants]

K, when you get it you may find the range of adjustment of the front sight post to be adequate. It goes up and down about a tenth of an inch. The old one I had adjusted with no problem. Yours may be OK.

If anyone discovers a problem with a short front sight base, they can buy an extended sight post from DPMS for about 15 bucks. It just screws into place.




EDIT: Oh, I should have paid closer attention to ugaar guy, he already mentioned the extended front sight post. Sorry I came late.

[Konstantin835]

Thanks, I guess I'll just hope the range of adjustment is big enough.

[Zach S]

The F FSB is the same forging as the A2, the shelf is just machined .040" higher.

The .04" is not an insane difference to where you shouldn't be able to zero.The issue isn't being able to zero, its the base of the front sight post being higher than the shelf machined for it in the FSB.

http://i699.photobucket.com/albums/vv360/zephyr351c/StandardFSB4.jpg

If you get your rifle zeroed, and the base of the front sight post is higher than the machined shelf of the FSB, you can probably buy a taller front sight post. I know they used to be available though Bushmaster.

Some manufactures use F FSBs that don't have the F mark. I know I have one AR15 that falls into this group. If you have a caliper handy, you can measure from the the shelf to the bbl. F is 1.98" where the standard A2 is 1.94".

To be honest, I don't think its as big a deal as some make it out to be. I have owned 5 ARs (still own four), all flat tops, with various BUIS and carry handles installed and I've never had an issue. The only front sight post I changed was on my Olympic .45 upper - and that want zero related. It just drove me nuts having one AR with an A1 front sight post.

I've edited this post half a dozen times fixing typos and such. I'm going to get my morning dose of caffeine and re-read it to make sure it makes sense...

[john5036]

The issue isn't being able to zero, its the base of the front sight post being higher than the shelf machined for it in the FSB.

The OP's concern was regarding zero...

If I have a flat-top receiver AR-15 with an A2 front sight base will I be able to zero the rifle? As I understand it the A2 FSB is .040 inches shorter than an F marker FSB which might cause problems zeroing. Anybody have any experience with this?

[Zach S]

The OP's concern was regarding zero...
And as I said in my previous post, the issue isn't being able to zero, its the base of the front sight post being higher than the shelf machined for it in the FSB.

In the future, if you feel the need to call me on something, please provide some useful information when doing so. Thanks.

[madcratebuilder]

Zach S covered it well, the difference is the height of the shelf.

You well be able to "zero" the rifle just fine. The round adjustment portion of the sight post may be above the shelf of the gas block. If this bothers you replace the post with the +.040 post. This is only a issue with the A1 rear sight as the A2 has elevation adjustment. Aftermarket rear sights may be different. My Ma-Tech buis works without the need for a +.040 post on a Bushmaster flat top.

[Onmilo]

I have some tall front sights from Bushmaster.
I only had to use one to get a rifle to zero and that gun was built using a Colt flat top upper with a 20" standard base rifle barrel.
That upper was a secondary for a guys Colt M4 Carbine.

[john5036]

And as I said in my previous post, the issue isn't being able to zero, its the base of the front sight post being higher than the shelf machined for it in the FSB.

In the future, if you feel the need to call me on something, please provide some useful information when doing so. Thanks.

I'm not trying to be confrontational, so give me the benefit of a doubt here, but what you are saying is the issue does not directly address the OPs really clear question.

Did you read the OPs actual question? I cited it for you. He was concerned about zeroing, enough he mentioned it twice. I understand that your information is useful and explains the difference between posts, but saying "zeroing is not the issue" is false in regards to the OPs context when his question was "Will I be able to zero the rifle?"

We've established the answer is most likely yes, with few exceptions, your shelf explanation w/pics demonstrating the differences that may or may not affect him. That info is great, I'm not contesting that, but if you are going to call me out by quoting me and say that it is irrelevant when it addresses the OPs root concern, then I need to buy you better coffee and cheer you up better. I'm sorry for making you feel defensive.

[Zach S]

but if you are going to call me out by quoting meConsidering the fact that I didn't feel the need to add your name to the quote, as you did in your post, is it really calling you out?

I'm not trying to be confrontational
Really? I didn't notice. Perhaps the next line will explain why...
Did you read the OPs actual question? I cited it for you.

Yes, I did read the OPs question. I simply pointed out that zeroing the rifle isn't the issue, explained what the issue is, and how to fix the problem if it arose...

Since you work in IT, you should be well aware that there are often other issues behind someone's root concern.