Failure to fire SW MP-15 OR (AR-15)

[mommicked]

My S&W MP-15 OR (flat top) seems to have developed a failure to fire tendancy. It will not fire about 20% of the time (1 in 5 rounds). I have tried 2 brands of ammo, PMC and Federal. It seems the same in either. The gun has around 1-200 rounds through it, probably around 150. It has a new trigger (jard) and a new extractor rubber thingy (d-fender) and a one piece gas ring. All primers are indeed dented, though perhaps not as heavily as I'm used to on big game rifles.

Am I not cleaning something properly? Is the gun defective? Do I not have enough firing pin protrusion?

Mommicked

[papajohn]

I'd have a hard look at the trigger mechanism, it sounds like there's just not enough force to reliably set them off. If there are dents in the primers and it fires 80% of the time it's not a bad gun. Maybe a stiffer spring would be enough to make it reliable.

[miatakix]

+1 to what PJ says. It is almost certainly the case that your hammer spring is too weak. I'd contact Jard and tell them as much. When I researched AR triggers a while ago, I came to the conclusion that the only stone cold reliable triggers out there are the standard single stage triggers (either as is or gently worked over by a competent gunsmith like Bill Springfield that isn't going to ruin the trigger by polishing through the hardened surfaces) and possibly the Geiselles, which are less proven because of their relative newness, but seem to have a sound design for reliability's sake and a good track record so far. The Geiselles have been 100% for me so far.

I doubt firing pin protrusion is your problem. Still, it's not a bad idea to measure this from time to time, inspect the tip of your firing pin, and otherwise inspect your firing pin. I try to keep the channel it rides in in the bolt carrier group clean, too.

[mommicked]

The trigger I used (I installed it) came with a 3 lb spring, and I swapped it out for a 2 lb pull spring. I don't think the little pull weight spring has much to do with the spring that activates the hammer. The original firing pin had a little crud built up on the front face of it. I cleaned it, then I dropped in a titanium firing pin.

When I use this single stage trigger, it feels almost like an old mauser two stage trigger. There is a little takeup, or hump, then it breaks cleanly. As I have played with it a bit, I've noticed the pins seem a little small, and are wallowing in the frame. The takeup coincides with the pins wallowing in the hole, then it breaks like glass. Best as I can tell, though, these are the original pins that came with the gun I believe. (I may be wrong, been known to happen). PJ-you have several of this same gun, does it use a large or small pin trigger? Would you mind measuring the pin diameter for me? Is it possible mine is a large pin, and I installed a small pin trigger? Would this cause the low percentage of fired rounds/high misfire rate?

Thanks for the help guys. I'm used to big game rifles, and completely new to the AR platform.

I've done a bit of work on Marlins, Mausers, and Rem 700s, and done trigger jobs on all of these and Ruger 77's as well. This would be the first real 'problem' trigger I've encountered so far if it is indeed the trigger.

[miatakix]

Ah, titanium firing pin, that is likely exacerbating whatever problem you have with the hammer strike. Their lower momentum from their lower weight likely takes something away from the primer strike.

There are upsides and downsides to a titanium firing pin. If you haven't already, you might want to weigh the pros and cons of one:

http://www.fulton-armory.com/QMI.htm

I was thinking about how it could be the hammer pin seizing up, not just the hammer spring, but strange excursion could do as you say, I imagine, too. Seizing may be an issue, if you didn't put a drop or two of some good quality oil on the bearing surfaces.

I seriously doubt the S&W has large pin holes. You would know it, the hammer wouldn't even probably stay put. Maybe your receiver or pin is just a little out of spec. Mic your pin to see what is what. Slightly oversized pins can be had to account for receiver wear, you may want to look into this if your hammer pin is in spec. Also, anti-rotating pins may be a good option for you if your receiver holes are a little big to start with, so that they don't wear out and get extra big in the future. I'm pretty sure that slightly oversized anti-rotating pins can be had, just can't remember exactly who markets them...ah, looks like KNS precision makes some, see here:

http://www.knsprecisioninc.com/mm5/m...egory_Code=KNS

[papajohn]

I think Ryan's right about the pins, if they're a little undersized that's one thing, but if they were the wrong size you'd know right away.

I also concur about the TI firing pin, I don't think it has enough mass to set off the harder primers. It's not a mechanical failure, from what you've said. Proper lubrication is still important, but a shot of CLP in the side of the bolt should be plenty, even dirty. I bought a case of the PMC Bronze ammo, and haven't had any issues with it at all.

I have yet to shoot my M&P O-R off a rest, but in the meantime I don't think the trigger needs upgrading. I have a match lower on my Del-Ton, and it's spoiled me rotten, but my primary M&P has a pretty decent trigger right out of the box. Which explains precisely WHY it's my primary M&P!

[mommicked]

Pins came out at .154, pin holes are .155. I think I may order some non rotating pins. When the trigger is manipulated, the pins actually move (visibly, from the side of the empty gun). This movement gives the otherwise very smooth trigger a two stage feel to it, that I don't think I care for. (I like single and two stage triggers, but this feels like a sloppy two stage, if that makes sense). I contacted the nice folks at Jard, and they said that they would send me a more powerful hammer spring.

They were making this trigger with a speed hammer and a heavy hammer. The one I got looked like a hybrid between the two. It has a small hole drilled in it (for no reason I can see as nothing goes there) that I may fill with molten lead to add a little more weight. It can't hurt if a light strike is indeed the problem and the heavier spring doesn't fix it. The folks at Jard were very fast to respond, and cordial. But then, in my experience, folks from Missouri and Iowa and Kansas usually are.

[miatakix]

Cool I wouldn't worry about adding any weight to that hammer until you try the new pins and spring and see if those alone do the trick, first. Let us know how it works out for you.