Help needed - AR-15 FSB pin working loose during firing

[BerksCountyDave]

I just got home from putting about 150 rounds through my rifle. After I was done, I saw that the front FSB pin had worked itself almost 1/2 way out. Here's what she looks like:





As you can tell from the pictures, the FSB is chopped down. I can't for the life of me figure out why that would make the pin come loose, though. The pin slides in and out with just finger pressure now, which of course is not normal. Anyone have any ideas on how to remedy this?

[SlimShady]

That sucks Dave. I'm not real sure there's a lot you can do for that aside from trying to locate a slightly oversized pin to replace it with. It sounds like for whatever reason your holes have somehow become enlarged. Maybe during the cutdown procedure too much pressure was put on the sight base? I honestly don't know.

Look up Donley's Guns in Quakertown and give Dave a call, he will most likely know what you need to fix the problem as well as have the parts you need to do so.

[d90king]

I cant tell by the pics but is it tapered? How was is reamed? It could be reamed for a tapered pin and that one isnt...

Who cut it down?

[BerksCountyDave]

I cant tell by the pics but is it tapered? How was is reamed? It could be reamed for a tapered pin and that one isnt...

Who cut it down?

I don't think the pin is tapered. I can slide it right out by hand, and it looks to be the same diameter throughout. However, it will only slide in one direction, so maybe it is?

I cut the FSB down. I didn't remove the FSB though, so the pin is the original pin.

The upper is an LMT, if that matters for whatever reason.

When I slide the pin in (it only goes in from the right), it will not come out the left. So I'm thinking maybe sliding it in and giving it a whack with a punch may seat it properly?

[RoyJackson]

No telling why it's moving (although my guess is the hole was slightly oversized or the pin slightly undersized from day one).
I suggest you tap the pin out and measure the diameter with a micrometer (calipers are not accurate enough and are too dependent upon the user's skill). Then measure the hole ID (which is going to be trickier...use gage pins if you can).
If the pin is tapered (by design), you could tap it back into place then stake it. Lack of staking may be why it's moving.
Another alternative is to drill the hole out to match a roll pin. Roll pins are spring steel and will slightly compress when driven in to a properly sized hole. The springy aspect of the pin provides the tension to hole the pin in place.

What brand of rifle is this? Was it from a kit build? Did you buy it new or used (used is always suspect).

[BerksCountyDave]

No telling why it's moving (although my guess is the hole was slightly oversized or the pin slightly undersized from day one).
I suggest you tap the pin out and measure the diameter with a micrometer (calipers are not accurate enough and are too dependent upon the user's skill). Then measure the hole ID (which is going to be trickier...use gage pins if you can).
If the pin is tapered (by design), you could tap it back into place then stake it. Lack of staking may be why it's moving.
Another alternative is to drill the hole out to match a roll pin. Roll pins are spring steel and will slightly compress when driven in to a properly sized hole. The springy aspect of the pin provides the tension to hole the pin in place.

What brand of rifle is this? Was it from a kit build? Did you buy it new or used (used is always suspect).

It's an LMT upper. Bought it new.

[arjohnson]

I would take the pin and try and make it bow slightly, put a little bend in it, smack the center of it between vice jaws. Then you will have to tap it back in with a hammer and punch, should work like bending a door hinge pin in your house to keep a door from swinging freely.

[BerksCountyDave]

I would take the pin and try and make it bow slightly, put a little bend in it, smack the center of it between vice jaws. Then you will have to tap it back in with a hammer and punch, should work like bending a door hinge pin in your house to keep a door from swinging freely.

That may be worth a shot.

I can't say that I'm OVERLY concerned because:

1. The rear pin is rock solid.
2. This FSB will be replaced with a low profile gas block in a couple of months.

[d90king]

[QUOTE=BerksCountyDave;869934
When I slide the pin in (it only goes in from the right), it will not come out the left. So I'm thinking maybe sliding it in and giving it a whack with a punch may seat it properly? [/QUOTE]

It sounds like it is... If that's the case you do need to seat it. Here is a video that might help. I think it is as simple as that. If memory serves me most FSB are tapered. In the first place the video pops up ignore the down load message and say no thanks it will take you directly to the Brownells video. Go to the last third of the video to see him seat it...

Hope it helps.

http://www.wonderhowto.com/how-to/vi...barrel-267095/

[BerksCountyDave]

It sounds like it is... If that's the case you do need to seat it. Here is a video that might help. I think it is as simple as that. If memory serves me most FSB are tapered. In the first place the video pops up ignore the down load message and say no thanks it will take you directly to the Brownells video. Go to the last third of the video to see him seat it...

Hope it helps.

http://www.wonderhowto.com/how-to/vi...barrel-267095/

I took calipers to the pin and it appears to be the same diameter end to end. However, it only goes in front the right side so I think the hole in the FSB is tapered. I'll slide the pin back in and give it a tap with a punch.

Thanks for the link, I'll check it out.