Gas Block / FSB Pin Location

AR/M4 AR Technical Discussion
1

I’ve seen quite a bit of variation in the location of taper pins relative to the bore centerline.

I’d like to get some confirming opinions on the proper location of the pins.

Seems like most .625" FSB have the pins ~.3125 away from the centerline of the barrel.

This places the center of the pin tangent to the barrel diameter, or about half way into the barrel.

Here are a few examples.

Around half of the .750 diameter FSB and gas blocks have the pins at the proper tangent point and the other half use the old placement for the .625" diameter.

Here is a .750 that looks good.

This one looks a little deep.

It seems like these deep pins defeat the beneficial wedging action that draws the gas block down over the gas port.

So, what is the proper place to locate the pins?

2

Ideally so that 50% of the pin diameter is in barrel and 50% is in block.

Getting too much one way or the other reduces the contact area of the counterpart.

3

Did you take the pictures? The second .750 picture, second to the last picture, is taken from a completely different angle than the one above it, making it look much deeper.

4

No I didn’t take them. That picture may be suspect.

I do have a barrel in my possession that looks like the last picture, with the pin holes forming “tunnels” rather then “troughs” in the barrel.

5

According to forum member, Tweak, the spec is that the pins should not be any deeper than half of their diameter.

My BCM middy (0.750) has ‘full’ holes in the barrel, thought not as deep as the ones in the picture. I don’t feel that the holes are too close to the bore to be of any concern. I can see a potential for full holes detracting from the wedge nature of taper pins but my fsb doesn’t show any signs of leakage and I haven’t had any problems.

6

I have seen some with holes like yours, but not that deep. If the spec is half the diameter of the pin the. I have seen some way out of spec guns.

7

I’ve seen some barrels from major names that one is drilled with just a “scallop” into the barrel and the other, a “hole”

One would think with fixtures, etc available, they would be a bit more uniform than they are.

8

That’s what I was thinking.

I wonder why there are manf pinning soo deep.

Do they just use the same jig for .625 and .750 barrels?