AR Trigger issue

I have a M&P-15 lower and the pin in front of the trigger keeps backing out, when this happens it allow the gun to fire when the trigger is pulled and fires again when you let off the trigger. I can work the pin back in but it comes back out after several rounds. This is very dangerous; What can I do to fix this issue?

Have you tried flipping the pin end for end and putting back in?

I have not done anything yet, just discovered the pin issue last night.

Flip the pin as suggested. If that does not help, go online and get a different set of pins. Lots of companies that make AR parts.
mt

If flipping the pin dose not work get a no walk pin set for $10-15, this will eliminate the problem. Another cheaper option is a drop of locktite at each end of the pins. The last option is probably not the best solution, but I have done it for truck guns and it works.

If the pin is walking out and that is a milspec trigger then the trigger is not installed correctly. The hammer spring sits in a groove and holds the trigger pin in place. The hammer has a piece of wire under tension inside of it that holds the hammer pin in place. The pin that is walking out on you is the hammer pin so the piece of wire inside the hammer that holds tension on that pin must not be installed correctly or is damaged. You can take out the hammer and look in the very middle of the hole and you should see piece of spring wire in it, that little piece sits in the middle groove of that pin. There should be two grooves on each pin, the trigger pin is held in place by the groove on the end and the hammer pin is held in place by the groove in the middle. You can either replace the hammer if it is lacking the wire retaining spring thingy inside of it. Or you can buy anti walk pins and either of those options will fix your problem. I have never needed anti walk pins on standard designed triggers. If you are using a one piece drop in trigger you may need the anti walk pins to keep everything in place depending on the trigger.

when you push in that hammer pin it should click into place, it wont be very noticeable but it will click and should not come out without noticeable force pushing it out when the hammer is not under pressure. If it does not click or seat properly then the pin will never stay in place and is very dangerous and will cause accidental discharges. When that pin comes out it allows the sear to disconnect and will fire.

Open the gun up, take the safety off, pull the trigger while you put your thumb on the hammer so it doesn't slam into stuff, and look inside. You should be able to see a metal spring arm on top on each side, as shown in the picture. If not, the gun is put together wrong. If you do see both arms, you've got other issues.

Possum has it. A properly installed trigger does not require antiwalk pins, locktite or anything else. If it's an aftermarket trigger that does require those things then it ain't the right trigger.

They make a anti-walk pin that has a C-clip at each end if it keeps doing that.

Guy’s I took the trigger out and found if you take either the trigger pin or the hammer pin and line it up on the holes it will drop straight through; is this right? The only thing holding either one in is the spring tension.

They should be a little tight. They shouldn't need to be pounded in witha hammer,but they shouldn't just fall through either. In any case, there is a little groove on one or both ends of the pins, and the trigger spring sitting in that groove is all that keeps it in. Your best bet would be a no-walk pin set. They use a little bar that screws into and connects the pins on both sides of the lower receiver.

Me thinks you're the recipient of tolerance stacking. Basically a milspec data package spells out aN acceptable range of dimensions for parts. In this case the holes may be upper end of the range while the pins are on the lower end. However the ranges were formulated that it should still work properly if the spring tension holds the legs in their respective grooves properly.

Knowing this I see at least two viable options. Shop for new pins that are a better fit or more likely upgrade your springs.

Better yet scrap the stock trigger and buy a geissele or alg (budget geissele).

I'm with possum man as well. Only because I had the exact same thing happen to me on one of my first builds. I now refer to this pic every time I install one.

take the hammer out. look through the hole that the pin goes through. You should see a wire right in the middle of the hole. That is the part that sits in the middle groove and keeps that pin in place. If you cannot see the wire in the pin hole, you need a new hammer. If you do see it and it is not protruding enough to clip into the groove on the hammer pin, you need a new hammer. Or the you need new pins. That looks like a palmetto state trigger group judging by the back of the hammer, i have some of the same. SO you need to figure out what isnt seating properly but the fix is gonna be a new hammer, new pins, or a new trigger group. Anti walk pins will also fix this issue and may be the cheapest option. Here is an option for an awesome trigger priced great, I love them and own several of the two stage varmint triggers. For the money they cant be beat in my opinion.
https://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_ID=1339

If the wire inside isnt protruding enough to seat onto the pin you might could take a pick and try to reach into the pin hole and bend it out enough to seat onto the pin. Personally i wouldnt trust it and i would rip it out and toss it and buy a new trigger if that was the issue.