Man, glad my LMT upper was done right (the hole, not the straight pins). Jeesh.
I think you nailed it Randall.
Even a favorite mfg here punches those pins completely through the barrel on .750" FSB.
They should have modified the forging way back when to make it right.
This is why the pins end up centered on the flats on 0.625" diameter barrels and you need to drill at the lower edge of the flats on 0.750" barrels to have the pin end up centered at the edge of the barrel.
^^ This explains it ^^
Can someone please post a photo of a F-style FSB “correctly pinned” to a .750" gas block? Specifically, the flats and where the tapered holes are drilled? I just want to get an idea on how it will look on a .750" gas block. Many thanks!
You may not be able to tell if its correctly pinned by looking at where the pins are on the fsb. I’ve been told that the Colt spec is that the pins shouldn’t pass into the barrel more than half of their diameter.
Even pinned as mentioned above, while I agree there is some ‘pull down’ effect, I don’t don’t think its enough to be of any real value. Fsb to barrel fit is what matters. Because the axis of the pins are constrained to the axis of the holes in the fsb, they cannot act like a pure wedge between the fsb and barrel. If the outside faces of the fsb were counter bored, then the pins would be free to act like a wedge and really squeeze the fsb down onto the barrel.
After much research and deliberation I have concluded that a) no two FSB installations are the same, and b) unless you’re using a .625" FSB, the center-line of the pin holes will transect more than 50% of the barrel at its apex. There is just not enough meat on the flats of a .750" FSB to drill these holes any other way.
Also, I see nothing wrong with the way LMT has drilled these holes as complete tunnels rather than channels. IMO, the only advantage of using a tapered pin is in assembly/disassemble of the FSB to the bbl. What I do object to in that photo, however, is that they would assemble a complete barreled assembly AND THEN give it a dunk in the park tank! I am really surprised to see that…
Incorrect.
You can drill the pins centered in the edge of a 0.750" barrel and they will be right at the lower edge of the flats of the FSB forging.
Just look at any colt barrel.
Unfortunately, I don’t have a Colt barrel to use as a reference guide. This is why I requested some pics of a FSB pinned to a .750" bbl (at the gas block).
To add some information to this thread, my LMT 14.5" has the pins drilled properly, i:e centered on the edge of the barrel.
I’ll dig through my drawer of used parts and see what I can turn up…
All my pinning jobs the last week have been for 0.625" barrels or I would have already posted a picture of how the 0.750" ones come out when done properly.
The picture above is how they were doing it in 2010.
I’m glad to hear that the newer ones are improved.
Any chance it is parked under the fsb now?
How about taper pins instead of straight pins?
Greatly appreciated. All I need is a photo of the FSB pinned to a .750" bbl. I just have a hard time wrapping my pea-sized brain around the idea that the pins are just too low in relation to the flats (not the bbl).
I’ll try and remember to take a picture of a colt barrel/fsb tonight or tomorrow morning. 50% of the taper pin is through the barrel and 50% through the fsb.
Just looked at my BCM uppers and they all have the pin centered above the midline of the flat.
What is the diameter of the bbl at the gas block? Does your BCM have a F-style FSB? (sorry…I have to ask).
At one point a few years back, someone from Barfcom contacted LMT, and claimed LMT replied that they had no plans to switch from straight pins.
It is a standard mid-length and rifle with a .750 barrel.
(Gonna burst your bubble here ;))
In fact it was made/test fired in late March 2010, so I guess I striked lucky, and that location of the pin/drill/hole varies from one barrel to another (I hope they’re not just eye-balling it…). They are straight pins, I re-checked precisely and I’d say they are 1/3 out of the barrel, not 1/2, but way better than the picture posted.
Honestly, has anyone had any issues with LMT barrels due to the lack of parkerizing under the FSB and taper pins?
Since I got it I put a drop of EWL where the FSB and barrel meet while it sits muzzle up in storage, every once in a while, just for the heck of it, but I highly doubt I would ever see any issues if I didn’t.
It’s a tool, not a trophy. Some people can’t figure that one out tho…
Just to update an old thread…fwiw
I finally got motivated to finish this upper. After great consternation, I decided to use the mil-spec tapered pins over straight pins for the FSB installation. The only question remaining was where to index these holes? As hard as I tried to imagine it, the F-style FSB with a .750" bbl just didn’t look right to me in the “proper” position.
I indexed off the bottom of the bbl and set my DRO to zero. So far so good…
Then I touched the flats using a center drill, just to create a point of reference. The dimple under the bayo lug looks gtg but the one under the sling attachment looks “low” in comparison.
Compared to the squareness of the front flat, the rear flat is noticeably contoured. I moved the y-axis .050" and touched off one more time…(note the 2nd dimple)
Here’s where “You be the Gunsmith” enters the equation (thanks to my degree in SOPE – see post title). In the spirit of compromise, I decided to split the difference between .000 and .0050.
I love pictures and I love playing with my new camcorder in HD mode 
Center drill…
Next up, #30 drill bit (.128"). Now you can see how close the hole is to the bottom edge of the flat…and this is before the taper reamer!
Both through-holes complete. On to the reamer…
Taper pin in place. As a side note, I should have used a tap handle wrench to finish these holes. They went waaaay too easy with the B’port in low gear…and the blue tape I used as a “stop guide” on the reamer got jacked up with the cutting fluid. lol
Now you be the judge. I’m open to comments, criticism, complaints, complements (yeah, right…) or whatever. I know I cheated by .025 on the “proper” location of the pins but it was a judgement call. From the looks of the hole placement in relation to the flats, I’m okay with the results…good or bad, right or wrong. I hope this thread helps others with their future FSB installation.
– ale
Great post Ale!
Solid writeup and pictures.
Could you detail how you fixtured / indexed the barrel and FSB to ensure it was straight with no “cant” ?
Was this done on a stripped barrel or barreled upper?