Range report on M16A1 repro - small problem

[RBruce]

Finally received the last of needed parts yesterday, and completed the build. I decided to replace the four notch front sight post with a five notch one so I could used the USGI sight tool I already have.

Went to the range this morning and the results were good and bad. The rifle functioned just fine, but even with the front sight screwed down all of the way, it was shooting a foot low at 25 yards - no telling at 100.

Should I file down the front sight post? It doesn't look significantly different from the one I took out. What other factors could cause the problem. I'm using factory Privi ammo. Any help will be appreciated.

Information

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[RBruce]

The upper and rear sight is from Midway, and the barrel which came with a front sight base is from Elite Tactical. It is a light weight 20" 1-9. The rear sight is a standard A1.

[browningautorifle]

Is the front base correct? The M4 is taller I think...that would do it. That is, if the receiver is a flat top, the carry handle or sight may be taller. Check with a long straight edge and see where the sight line is compared to your bore axis. Others will be here in a minute, have faith...

[Bill Hollinger]

Is the front base correct? The M4 is taller I think...that would do it. That is, if the receiver is a flat top, the carry handle or sight may be taller. Check with a long straight edge and see where the sight line is compared to your bore axis. Others will be here in a minute, have faith...

^^^^^^^
This
F marked FSB is meant for use with flat-top uppers. The only difference is that the F marked unit places the front post slightly higher, which is often needed with detachable carry handle on a flat-top.

[browningautorifle]

That being said, you can't just get a FSB and cobble it on. The pins are located and drilled mechanically at the factory so they come TDC. Done by hand, well is definitely not advised. They use a taper drill, 10 ton press to set the pins...on and on. Not to preach defeat, the barrel is still useful but I guess one has to specify a carry handle upper for them to be sure what to send. You almost need to write a letter accompanying.

[RBruce]

Turns out that the 20" skinny A1 barrel came with a F marked front sight base. I guess that is the reason for shooting low, although I don't see how .04" could make that much difference at 25 yards. Should the H marked base solve my problem?

[Bill Hollinger]

Sight in at 25 yards and it will be on at 100 yards

[RBruce]

OK, after more research, the FSB I have is marked F which is taller than a standard FSB, thus the shooting low problem. Measured a standard on .750 barrel, it measured 1.907 from the barrel gas port. The F marked measured 1.975. That's a .088" difference, almost 2mm. would this be the cause of shooting low?

[Bill Hollinger]

Yes it will cause low impact point. Sight/gas blocks can be a bear to replace because of the taper pins as Jim stated. They can be real easy too. I tapped the two taper pins out of my Olympic Arms with very little effort and the sight/gas block slid right off after a few taps with a dead blow hammer. It's a toss up because if they are stuck, they are really stuck Take the sight pin down and don't concern yourself with it. We won't tell anyone

[RBruce]

I removed and then re-installed the sight base once, so removing the pins is not an issue (not that it was easy). I need a reasonable source for a standard height (short) sight base, either used or new. Or, perhaps a trade for a like new F marked base.