Has anyone made a post sample of the SIG MPX?  Due to the high bolt speed you get a ton of trigger slap on semi and full auto.   This is normally prevented by the Trigger Bridge, but that part will not allow the hammer to go far enough back to catch the full auto sear.

There isn't enough material thickness in the Trigger Bridge to shave it down to allow the hammer to go far enough back to catch the auto sear, but I am thinking that maybe a bit can be shaved off the rear of the hammer to allow clearance.

Also, I actually need one of the earlier Trigger Bridge parts that works with the milspec AR-15 trigger group, as my gun came with a Timney trigger and the Trigger Bridge that comes with it will not be retained in the gun without a Timney Trigger pack, which obviously won't work for full auto ....any idea on a source for that part?  (No, I haven't called SIG yet, but they have a nasty habit of discontinuing parts the minute they update gun models, LOL)

Sven
Manticore Arms

I actually like the 900-1000 RPM cyclic rate, I find it very good for short, fast bursts, but I also seem to be the exception to the rule.  

We haven't really had any issues with back gassing, at least in the limited shooting with a suppressor.

The trigger slap is awful, no doubt, and to be honest if I had known the slap was going to be that bad I would not have made it a post sample.

That being said, looking at the tail of the semi hammer, the full auto hammer, the auto sear, and the trigger bridge, I think there is enough room with a little mill and dremel adjustment to each piece to make them all work together and eliminate the trigger slap.  At the very least, it doesn't have slap with the bridge installed and will still shoot semi (no auto) while I puzzle out a solution.


If I find a solution I will be happy to share!

Sven
Manticore Arms

@whitedingo

Oh, awesome, thank you for the tip!

They are listed as out of stock at the moment on the Hiperfire webstore right now, but I will reach out to them and see what they have to say.

My other fallback was to potentially use a spare captured trigger pin and drill an additional cross hole and use that as a hammer bridge, but have it catch on the back of the hammer where it curves instead of right at the rear where the trip is.  I figured if it breaks it would be an easy replacement, and it would allow me to let the trip function while precisely controlling the stop point of the hammer reset so it doesn't slam the trigger too hard.

The hardest part about this stuff is just finding the knowledge as it isn't out there and readily available.  I would be really interested to see how a factory SIG MPX full auto SMG solves the issue, I am guess they have their own specific hammer and trigger bridge to reduce slap (or maybe SIG just didn't care about trigger slap on select fire guns...that somehow wouldn't surprise me. )

Sven
Manticore Arms

@burntoutbarrel

The MPX is inherently a bufferless gun.  It uses a trip lever extension that allows the shortened bolt carrier to be able to trip the auto sear.

Not sure if that answers your question how it works or what you are asking.

Sven
Manticore Arms

I was looking at the MPX bolt, and was wondering if anyone has tried ramping it, kind of like how 9mm AR-15 bolts are ramped.

I think part of the problem of the trigger slap is the same as with some AR-15's, the angle at the bottom underside of the bolt carrier just, well, sucks.  A more gradual ramping plus rounding the top of the hammer may take a lot of the hard slap out of all of it.

I already have way too much time and money in this stupid MPX, so maybe I will just ramp the carrier and see what happens.  

Sven
Manticore Arms

A CZ EVO and SIG MPX are very, very different beasts on the inside, and unfortunately would not be of help on this particular issue.  

If he does have a full auto MPX I would be curious to hear if he had a solution to the trigger slap issue.

Sven
Manticore Arms

@amphibian


What shot timer are you using to measure ROF?  I am actually looking for one to do just that!

Yes, I did see your post- just so I understand, the hammer is supposed to have enough clearance on the trigger and disconnect with those relief cuts that it does not contact at all, or it limits the contact?

Is the issue here that the bolt carrier is literally physically pushing the hammer into the trigger and disconnector, or is this more a hammer is getting hit so hard by the rapid rotation from contact with the bolt that it is swinging down and rebounding off of the trigger and discoonnector, and relieving it gives just a little more space for the hammer spring to de-accelerator the hammer and not make it slam into the trigger and disconnector quite as hard?


ETA - also, you are correct, the PMM piece is basically just a linear linkage, not a lever.  I just couldn't think of a good way to describe it in my previous post.

Sven
Manticore Arms

I removed some material from the back of the hammer today and it ran fine on full auto, but I was getting a dead hammer after a single shot on semi.  Comparing the profile of the full auto trigger system disconnector, it had a little more material at the lower front and top rear than the SIG OEM semi disconnector, so I carefully removed some material on the full auto one to match the profile of the SIG semi one (while still leaving the full auto tail).

The disconnector was *just* barely catching on semi when hand operated, and after the mods there is considerably more engagement.  I am hoping to get out to the range tomorrow and test fire, and if all works I will post some pics of the mods I made.

If the trigger bridge eventually cracks I will either mod the trigger parts like Amphibian has shown, or get a Hiperfire full auto capable pack, or just let it sit in the back of the gun safe and curse SIG for yet another "SIG design decision" like so many of their others (side note:  my second sig red dot crapped out today.  That is two for two on ones that came with MPX's. Par for the course with my experience with "SIG quality products" so far, LOL).

Sven
Manticore Arms

I helped a friend who is an SOT convert his MPX and his ran 100% on semi and full with the hammer and disconnector adjustments.

For some reason I would get a dead hammer about 1 in 3 times on semiauto, I ended up installing a heavier Wolff XP hammer spring and that solved the issue (not sure if the hammer was rebounding forward and skipping the disconnector, or maybe getting hung up and not releasing form the disconnector, but either way, the stronger hammer spring fixed it)

Sven
Manticore Arms

@burntoutbarrel

If you go to the Wolff Gunsprings webstore just go through the AR-15/M-16 springs nad look for the XP hammer spring.

MidwayUSA also carries them IIRC.


Sven
Manticore Arms

LOL, I have the same type of box!

Honestly, you can usually just bend the spring legs and feel the difference.  The Wolff Xpower springs are the same color as OEM trigger srpings, they are not color coded, so that is your first hint.

Sven
Manticore Arms

If you are referring to Amphibian's picture, from the side the M-16 hammer looks exactly the same, the rear "tooth" that hooks on the auto sear is still there, it is just cut away in the center to allow it to site like a cowboy's legs around a horse (with the horse being the disconnector that only operates on semi).  The point is to prevent the tail/tooth of the M-16 hammer from slamming into the disconnector when recocking, that is what causes the trigger slap.  It is hard to explain in words, but look for an animation of how the full auto M-16 works and it will make sense.  on semi, the projection on the mid rear of the hammer catches on the disconnector, on full auto, the rear hook/tooth on the hammer is catching on the sear as the selector pushes the disconnector out of engagement with the hammer.  

As for the disconnectors...there is no way I am going to some sketchy solvent trap webstore! you are going to need to post pics.


As I said earlier in this thread, a full auto M-16 trigger parts kit has the correct disconnector, and they are available from a number of reputable resellers.  You are going to need that trigger parts kit anyhow to get the correct selector and hammer and trigger and the auto sear  to make it all work in the first place as a post sample (I am assuming you have an FFL 07/ SOT 02, if not, don't even think about doing this!), none of the semi parts in the MPX trigger group will work.

Sven
Manticore Arms

Totally understand.  I too was a fan of understanding all this stuff well before I had an FFL and SOT.



In short, the MPX uses an M-16 trigger group for full auto, and will function just fine with that (you also need the trip rod from Parker Mountain Machine for the bolt carrier to trip the auto sear on full auto.)  

Here is a great animation of the workings of the M-16 trigger group on semi and full auto.  Note that on semi, the hammer hooks and is held back by the disconnector.  on full auto, the hammer is held back in a different spot on it by the auto sear.  Once you understand that, you get the concept of how the M-16 trigger group works.

https://www.youtube.com/watch?v=75ec-J6lTDE&t=82s

The only real difference is modifying the trigger group for comfort to prevent the hammer slamming back so fast that it hits the disconnector and causes "trigger slap" on your trigger finger.  There is a piece that fits just above the rear of the disconnector call the "trigger bridge" by SIG that the hammer slams into instead of the disconnector.  It just requires a bit of material be removed from the top rear of the hammer to allow the hammer to cock back far enough to catch on the auto sear before the hammer slams into the trigger bridge.

I did find my gun needed a heavier hammer spring to get everything to function 100%, not exactly sure why, but it prevented the hammer from not locking on the disconnector 100% of the time (I suspect the hammer was bouncing forward before the disconnector caught it, and a heavier hammer spring slowed down the hammer enough to stop that.


That is all I can do to describe everything here, hope that helps!

Sven
Manticore Arms