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Posted: 9/1/2004 4:47:46 AM EDT
Hello, everyone. I just wondered if anyone knew what size/length/thread of screw will work to install under front sight post to take out any wiggling. Since changing to LMT BUIS I had to back my front sight post way out to get an IBZ zero (see previous post). I have heard of people installing a set screw in the hole in the FSB under the sight post to take the wiggle out, but have never heard what type will work. Thanks in advance for advice.
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Posted: 9/1/2004 5:27:28 AM EDT
[#1]
Man. That is just an unacceptable situation. You shouldn't need to do these silly mods for these Aftermartket gadget peddlers.
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Posted: 9/1/2004 6:23:53 AM EDT
[#2]
My guess is you have a barrel that has the standard FSB instead of the "F" stamped FSB that is designed for use with the flattop receiver. Your best bet would be to buy an extended front sight post from Bushmaster. This sight post is .040" taller and should solve your problem. The other thing you could do is replace the FSB with an "F" stamped FSB but I would recommend trying the taller sight post first.
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Posted: 9/1/2004 8:02:27 AM EDT
[#3]
I had no idea that bushmaster made different height FSBs for flattop and regular receivers!!  Now that I check for the "F" on the FSB there is not one. There is the "crown" of course, but no "F". Well, how bout that shit? Man, I do NOT want to change my FSB out. I have done it, can do it, know I can do it, been through it, been successful at it, but do NOT want to do it again! |
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Posted: 9/1/2004 8:05:49 AM EDT
[#4]
#1: Bushmaster didn't start the different FSB height BS, Colt did. The reason escapes me. #2: As recommended earlier, get a different front sight post, as that will fix your problem without fiddling around with the FSB. |
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Posted: 9/1/2004 8:11:30 AM EDT
[#5]
'lo all,
The set screw for the front sight is really a DCM/CMP kind of mod. Given that even a little flex in the front sight can throw off your sight picture at 600 yards. To really take advantage of doing that you need an A2 sight with 1/4 x 1/4" adjustments. Otherwise, your money is better spent on ammo. I got the mod for my DCM rifle and it really heped at 600 yards, but my M4-gery is just plain old Bushmaster. Later, JD |
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Posted: 9/1/2004 9:46:17 AM EDT
[#6]
I should have made it clearer, I DO have the .040 taller front sight post already installed and the post is adjusted so the top of it is level with the ears of the front sight base and it is loose because of it. The post is by no means about to fall out, but it does have a bit of play in it. It just seems it shouldnt have to be adjusted this tall to get a 50yd IBZ zero with the .040 installed. In this case, a regular front sight post would fall out before reaching zero.
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Posted: 9/1/2004 10:12:53 AM EDT
[#7]
To answer your original question instead of trying to feed you with useless opinions:
8-36 plug screw (1/4 or 3/8 long). Use an 8-36 tap which is the same thread as your front sight post. HTH 
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Posted: 9/1/2004 12:08:36 PM EDT
[#8]
Sounds like you have some other issue inputing to this problem. The screw may fix your unstable front sight post, but I wonder what else is causing this? |
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Posted: 9/1/2004 3:40:56 PM EDT
[#9]
I wish I knew, the only thing I can come up with is that the FSB is not as tall as a Colt, or what people say is "milspec" or whatever the LMT must be made to the specs of. By the way, I measured the LMT BUIS height compared to my Bushy carry handle and guess what: it is .04 taller. Still, the .040 front sight post should be making up for that easier than it is.
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Posted: 9/1/2004 4:46:11 PM EDT
[#10]
Brownells used to sell a kit to do the mod, and a while back they were showing it in the online catalog as "discontinued", but the instructions for it were still hot linked and downloadable. I have a hard copy around here somewhere... Paladin |
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Posted: 9/1/2004 4:48:12 PM EDT
[#11]
I'd like to know myself and I'll be in that situation very soon. My latest build will be with a Bushmaster barrel & LMT BUIS. I'll update any issues I have (if I remember to). |
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Posted: 9/1/2004 5:33:39 PM EDT
[#12]
Agreed! |
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Posted: 9/1/2004 6:04:49 PM EDT
[#13]
I'm with you guys. AGREED!!!!!!!!!! 
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Posted: 9/1/2004 6:10:24 PM EDT
[#14]
How much of a "kit" is needed to do this? Wouldnt screwing in the appropriate length, size, and thread of screw from the bottom of the hole of the front sight post be enough? This sucks having to deal with this!!!!! 
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Posted: 9/1/2004 10:45:09 PM EDT
[#15]
Another potential solution would be to have the top shelf of your BUIS machined down a bit, allowing the rear sight assembly to seat lower. We're talking .030"-.040" here, not much.
-Troy |
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Posted: 9/2/2004 5:33:00 AM EDT
[#16]
Interesting. I went and looked at my Colt AR15-A3 Tac Carbean and there is an F stamped on the FSB. I never knew there were different site bases for the flat top upper. |
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Posted: 9/2/2004 8:43:55 AM EDT
[#17]
Yeah, I never knew either until all this crap started with my LMT BUIS. Apparently, Colt is the only manufacturer, at least mainstream manufacturer, that machines their FSB about .040 taller, and LMT, ARMS, and some others machine their BUIS to fit those specs. Normally, a .040 taller front sight post is enough to compensate for this, but in my case, even the .040 front sight post has to be backed out to the point that it is loose, level with the ears of the FSB, and the pin under the front sight post barely engages it in order for me to have a good 50yd zero. Just my luck!!! By the way, does anything have to be done with installing a set screw under the front sight post other than taking a screw of the proper size and screwing up into the hole in the FSB under the sight post until it pushes against it?
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