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Posted: 10/7/2005 7:42:16 PM EDT
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Recalling the tall/short FSB, tall/short FS post thread(s), has anyone experienced this in a "regular" A2-type barrel? Here's the background: The base rifle is an ArmaLite M15A2; just a standard, fixed carrying handle, 20-inch A2-type rifle. Nothing fancy. With the original club-of-a-barrel, I had absolutely no problems zeroing it IAW standard zeroing procedures. Last year I sent the upper off to Bushmaster and had them install one of their 20-inch government profile barrels. I sold the original barrel before heading to the range. When I attempted to zero the rifle with the new barrel, it shot high, WAY HIGH when the FS post was flush with the FSB outer surface. (About 6" high at most ranges.) I wound up having to back the FS post nearly all of the way out of the FSB in order to get the rifle zeroed. When I asked Bushmaster about this, the person with whom I spoke seemed to think that this was okay. (????) Since the original barrel allowed me to zero okay, I don't necessarily suspect that the rear sight is AFU. And since I don't have the original barrel anymore, I don't have anything to which I can compare the current barrel's FSB. Rather than send the upper back to Bushmaster, I bought one of the 0.040" taller FS posts and zeroed the rifle without having to completely back out the FS post. (The cost of the FS post was cheaper than return shipping for the upper.) Any thoughts? Could one of the shorter FSBs have found its way onto a standard A2 barrel? Could the rear sight be AFU? |
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That's very odd. The BM FSB, and post, should be exactly the same height as the old one. And should be the right height for the standard A2 upper. Even Colts use that height FSB on A2 models. The taller post is for when you have that height FSB and the higher Colt/Mil flattop rear. Sounds to me like BM has some barrels with FSBs out of spec on the low side... Tweak, what's the best way to measure that height? I have calipers, but not sure how to get a good reading from center of bore to that flat without guesstimating or inducing some anglular error. |
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This is a long shot, but also measure the overall height of the FSP itself... remove it and measure it, compare this to a known good one if possible...... Hate to think you would go to all the trouble of trying and questioning your measuring technique for the FSB and not check the FSP also...... better to rule out all the possibilities........ Keep us posted..... |
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Got a chance to measure the FSB this weekend. RRA flat-top 16" middy = 2.665-.750/2 = 2.290" from bore center to flat KPF .22LR 20" = 2.665-.750/2 = 2.290" from bore center to flat Colt MT 20" A2 = 2.685-.750/2 = 2.310" from bore center to flat. Kinda puzzled about that Colt. .020" higher puts it half way between the "normal" and "F" FSBs. I also measured the height from the bottom of the receiver at the rear to the top of the RSB (the flat where the bottom of the aperture rests). RRA with BM handle = 8.833" Colt A2 receiver = 2.824" Both zeroed at 50yds with Q3131A with no problems and without being at any extreme of FSP travel. |
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ar15.com/forums/topic.html?b=3&f=118&t=240120 ar15.com/forums/topic.html?b=3&f=118&t=251786 standsrd FSB height is 2.285" +/- 0.010" |
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