300BO build issue

[Darkstar5012]

I threw together a simple 16" 300 BO rifle for hog hunting. It's my 5th AR build and the first one I've had problems with. Tested it out today and it is firing when I chambers a round. It happened 3 rounds in a row so it's not a fluke... It has a cheap (ARstoner/midwayusa) BCG. This is what I think is causing the problem. Any thoughts? Could it be a headspacing issue? I'll take any advice or knowledge I can get. Thank you.

[Bob Oso]

try a different H buffer

[buruli]

Ammo. Soft primers. Bolt slams home, inertia sends firing pin forward into soft primer.

[Darkstar5012]

It does have a standard weight carbine buffer. I'll try an H buffer. Thanks.

Ammo was factory, Remington I think. 125gr. It has a $70 BCG so it may have skipped QC somewhere along the way. The firing pin was clean and moved freely in the BCG but it may be out of spec(?). I'm almost certain a quality BCG will fix the problem.

[solidfreshdope]

Is the trigger resetting?


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[Darkstar5012]

I'm not sure if the trigger is resetting or not. I had limited time and was just trying to test for function. I chambered the first round by racking the charging handle, no issues there. My first shot it double fired and didn't chamber a third round. I racked the charging handle and if fired on its own right after going into battery but didn't chamber the next round. I dropped the mag ,checked the chamber and wrapped it up for the day.

[russ69]

Not enough info but if you are loading a single round from the magazine and the gun fires upon chambering, that is one set of issues but if the second round from the magazine fires, that is another issue. Which one is it?

[Grumpy_Hippie]

If it's doing it at the range it'll problem do it at home.
Make sure it's unloaded !!!
Load a empty magazine.
Pull back and lock the bolt open.
Release the bolt catch so the BCG goes into battery.
Pull the rear pin, check the hammer position.
If hammer is cocked, issue with bolt.
If hammer had dropped, issue with sear.

It's only metal, you can think it

[caliguy93]

Originally Posted by buruli

Ammo?? Could be soft primers, and/or or defective firing pin spring. Bolt slams home, inertia sends firing pin forward into soft primer.

There is no firing pin spring in an AR.

In normal operation of AR, it is perfectly normal for the primer to have a mark from the firing pin when chambered and ejected without firing.

So you have one of two things that might be happening..

Either the firing pin is going too far forward from the inertia due to a defective firing pin or defective bolt or carrier.

Or

The ammo you are using is defectively too soft for the AR operation and is discharging when it normally wouldn’t.

Easiest way to test it is to try various brands of ammo.

Another way to test it is to swap out the entire carrier.

Borrow from a buddy or ask someone at the range next time you go if you can borrow their bcg to trouble shoot if you don’t have multiple ARs that you can pull one from. It uses a standard 5.56 bcg

Remember to make one change at a time when trouble shooting.

If it’s the ammo, easy fix.. if it’s the bolt you can easily swap the entire bcg for under $100 or try to determine what part is the actual defective part and swap out either the bolt or the firing pin or the carrier

Just whatever you do, please do not list the defective bolt for sale once your replace it as it can eventually kill someone on accident

[caliguy93]

Also check the disconnect or spring in your trigger group. The little spring that goes inside your trigger.

That spring has one larger coil On one end than the other.. the larger end should go inside the trigger.

If you have that upside down, it’s possible your hammer is following the bolt and causing the discharges.

You can check the next time it double fires by dropping the mag and detaching the upper after it stops firing on its own to check if the hammer is up or down. If down, likely not the issue but check the spring anyways.

Or you can simply check the spring in an Unloaded and safe environment in your home.

If it’s up, it’s definitly a trigger issue

[81turbota]

Originally Posted by Darkstar5012

I'm not sure if the trigger is resetting or not. I had limited time and was just trying to test for function. I chambered the first round by racking the charging handle, no issues there. My first shot it double fired and didn't chamber a third round. I racked the charging handle and if fired on its own right after going into battery but didn't chamber the next round. I dropped the mag ,checked the chamber and wrapped it up for the day.

To check trigger reset -
Make sure rifle is CLEAR no ammo no mag
Charge rifle
Pull trigger, hammer will drop
Keep trigger depressed, charge rifle again
Release trigger, you will hear a dull clunk
Pull trigger, hammer will fall

If all of that checks out I would wager your fcg is fine.

[Darkstar5012]

Upon disassembly I noticed the hammer pin had walked out. I'm still gonna get a better BCG but I'm confident the pins walking out are what caused all this. I feel like an idiot for not noticing it sooner. ill toss some KNS anti rotation pins in it and go from there. Thanks for all the suggestions everyone.

[BajaJames83]

the BCG is fine. you must have assembeled the lower wrong.
those oins do not walk out when properly assembeled no need for the KNS pins

[Darkstar5012]

It is assembled correctly but it is an 80% and I drilled my trigger/hammer pins to .156 by mistake when I finished the lower. The pins are a bit loose compared to my other builds and can be pushed out of place fairly easily. I think KNS pins are what I need, but you're absolutely correct. If the lower was 100% in spec then they wouldn't be needed.

[caliguy93]

I think they actually make .156 pins

[Darkstar5012]

Originally Posted by caliguy93

I think they actually make .156 pins

Right on . I'll look into the oversized pins. Thanks for the heads up.

[solidfreshdope]

Sounds good. Always safety check your lower before pinning on the upper.


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