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10/20/2017 1:01:18 AM
9/22/2017 12:11:25 AM
Posted: 8/22/2005 3:50:02 PM EDT
Just finished up an M4gery build and it's shooting way, way too high. It's a flattop with detachable carry handle, std FSB. I know there are .040 taller posts but I need one that's at least twice that much taller.

The top of the post is higher than the ears on the FSB and it doesn't engage the detent and it's still about a 4 inches high at 50 yards. I tried a different rear sight and got the same results.

Anybody know where I can get an extra tall post?
Link Posted: 8/22/2005 3:54:28 PM EDT

Originally Posted By yfs200:


The top of the post is higher than the ears on the FSB and it doesn't engage the detent and it's still about a 4 inches high at 50 yards. I tried a different rear sight and got the same results.



Ever heard the saying, "You can't get there from here".

If both, the new and old rear sights were all the way down, then you need to pull the barrel and give it a roll test. Chances are either the muzzle has a nasty muzzle bur kicking the bullet up at exist, or the barrel is just way bent.
Link Posted: 8/22/2005 3:58:10 PM EDT
Brand new Bushmaster barrel on a CMT upper. I'll pull it apart and put it back together to see if it changes anything.
Link Posted: 8/22/2005 4:11:39 PM EDT
[Last Edit: 8/22/2005 4:13:38 PM EDT by Dano523]
Make sure you didn't streach/cant the upper barrel socket by over torque when you installed the barrel.

Also, check the face of the upper socket, and the socket side bearing face of the barrel extension to make sure you don't have a piece of debre that may be causing the cant.
Link Posted: 8/22/2005 4:26:56 PM EDT
Got the barrel off. I made sure not to overtighten it when I put it together. No debris between the barrel and the upper, no burr. I pulled a rod through slowly to try to feel for changing resistance. It pulled smooth. Maybe I should have tightned the nut an extra tooth? The barrel fits extremely tight into the upper socket.
Link Posted: 8/22/2005 10:21:40 PM EDT
Link Posted: 8/23/2005 10:23:40 AM EDT

Originally Posted By yfs200:
Just finished up an M4gery build and it's shooting way, way too high. It's a flattop with detachable carry handle, std FSB. I know there are .040 taller posts but I need one that's at least twice that much taller.

The top of the post is higher than the ears on the FSB and it doesn't engage the detent and it's still about a 4 inches high at 50 yards. I tried a different rear sight and got the same results.

Anybody know where I can get an extra tall post?



So you haven't tried the taller post? I was in the same boat so to speak, but with my post on the last thread that the detent could "catch" on the front sight, I was shooting rather high at 50 yd.

I got the taller post and everything's fine now.
Link Posted: 8/23/2005 2:49:37 PM EDT
[Last Edit: 8/23/2005 2:50:56 PM EDT by yfs200]

Originally Posted By Tweak:
you may have that magic combination of the wrong height FSB combined with a milspec height upper and a non milspec height (taller than milspec) carry handle.

was the other rear sight the same brand as the original. the FAQ covers this in detail.



Interesting. Upper measures 1.723" from bottom to bottom of the slot on top. 1.84 from bottom to top. I have a carry handle I bought off the EE. It was sold as an Armalite. I have no way of confirming. I borrowed a Midwest Industries flip up rear sight and got the same results.

I thought Colt was the only manufacturer of taller FSBs?
Link Posted: 8/23/2005 11:10:43 PM EDT
[Last Edit: 8/23/2005 11:12:45 PM EDT by Tweak]
Link Posted: 8/27/2005 9:31:58 AM EDT

Originally Posted By yfs200:
Brand new Bushmaster barrel on a CMT upper. I'll pull it apart and put it back together to see if it changes anything.



I measured the "up" deck height down to these heavy barrels(not cl of bore) on hand. Listed from shortest to tallest(F marked):

PRI folder 1.900(unfrikinbelievable $180 pos! making it TOO SHORT took a mail order degree?)
Colt Non-F 1.914
BM HBARS 1.920, 1.920
CMMG M4 1.939
Colt F marked 1.974, 1.972

All are nominal 0.750 at the FSB.

With the PRI, non-F Colt's, and BM FSB's I have needed taller front sight posts when using a Colt A2 upper and RRA A3 uppers using the ARMS 40A2 and 40L-SP BUIS. Anyone interested in building Arfs should just have a taller front sight post on hand, as you will eventually need it. HTH.

Paladin
Link Posted: 8/27/2005 10:53:51 AM EDT
[Last Edit: 8/27/2005 11:06:43 AM EDT by j3_]
I had a barrel that gave me that problem. I found taller sight post but it put the top above the ears and and it kept getting bent. I finally gave up on mine and put in on and old A1 upper.

Some of the KNS pins are taller and DPMS has a taller pin. It was thin and would bend easily though.
The sight post DPMS sent me is .1 higher that an A2 not what it says on the web site.
www.dpmsinc.com/10Expand.asp?ProductCode=FS-08
Link Posted: 8/27/2005 5:41:19 PM EDT
As I stated before, the barrel fit into the upper socket extremely tight. When I took it apart it probably wasn't any tighter than 20 ft.lbs. I think it may have become looser after a few round down the tube. When I put it back together I tightend it an extra tooth. That put it at around 60 ft lbs on the torque wrench. I'll test fire it tomorrow.

Hopefully this will put it close enough for the .040 post. I think it will. We'll see.
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