Gun is an FM9 16 inch upper.

1) Ran perfectly with 0 malfunctions on endomags and Anderson multi-cal lower, with 8 ounce solid buffer and 10 ounce moving weight buffer. Always a standard carbine spring.

2) Put a sylvan arms mag block in and started running Glock mags. OEM, some Pmag, A few Promag. None choked noticably.

Ran the Sylvan arms/Glock mag setup first on the solid 8 ouncer, then 7.5 oz Macon Armory buffer. A handful of failure to feeds, maybe a couple of jhp rounds hit the feed ramp in every 100-plus shots. I attribute it to the feed cone being less than stellar.

3) Grabbed a PSA complete lower for it, using the carbine spring that came installed in that, which is about .5 in longer than the one in the Anderson lower. Same 7.5 oz Macon armory buffer, Sylvan mag block, Same Glock mags.

It’s now a jam-o-matic. First outing with this setup I was lucky to get 6 problem free shots off. Mostly they came every other or every 3rd shot. But this time no FTF. I get the case being ejected and then a new round going in and a click on the next shot, like the hammer didn’t reset.

Or a round going in to be chambered but the old case not being ejected so the case and new round are jumbled in the chamber together.

Or I’ll get ejection and a new round in the chamber, but the ejection is weak. Case basically drips straight down out of the ejection port.

Gonna be a bit before I can test fire again. But I’m hoping it was the new spring being too strong to cycle this bolt properly?! Trigger works and resets consistently in function checks. Upper was perfect until now. The weak ejection made me think it’s all symptoms of the bolt not going back far enough. I figured the occasional weak ejection is more likely that than bolt traveling too fast. As in, bolt not moving back fast enough for the ejector in the Mag Block to kick a round out with proper force.

Why not go back to the Sylvan adapter?

Are you running it ‘wet’?

Allot of the AR variant manufacturers have a special statement in their manual about ensuring to run  them wet.

Perhaps a slick finish on components has worn off or factory prelube.

There's a lot to unpack here...



Sounds like the Sylvan Mag adapter presents the rounds to the chamber at a slightly different height or angle. I think you're right about the feed cone. Feed cone fix from Macon Armory would probably resolve this.



This part right here is the tough part.You said there's no hammer reset, but you get a click.  Something had to reset for it to click, otherwise it should just be a slack trigger like pulling the trigger after dry firing. I think we need more investigation for this one.

Is the bolt not going fully into battery because a cartridge did not fully seat in the chamber, and the "click" is the hammer hitting the bolt below the firing pin, sending it into battery? This could be another symptom of the feed cone problem.

The extractor may not be snapping over the rim, causing a failure to fire, and the impact of the hammer on the back of the bolt snaps it over the rim. This, again, could be part of the feeding issue, robbing the bolt of necessary forward momentum to finish chambering the cartridge.

Next range trip, don't change anything. Leave it like it was when it was failing. Take a shot and STOP.  Carefully check after each shot fires and look at the bolt. Is it out of battery? Pull the bolt back carefully. Is the cartridge NOT under the extractor?

If one of those are happening, you could try a stronger recoil spring just to troubleshoot. Although not optimal, a little stronger recoil spring may give the bolt enough of an extra boost so that it at least runs. The actual "fix" would be the feed cone reprofiling.

If neither if those issues are present, I'd inspect the trigger parts for unusual wear, bent pins, out of spec disconnector, or upside down disco. spring.



This sounds like an ejector alignment issue between the lower and the bolt. Adjusting the ejector slightly should help. See the picture in the link.
I doubt the spring has anything to do with it. I've used various carbine springs, wildly different lengths and strengths without failures. 0.5" is very little.



Weak, inconsistent ejection is probably due to ejector alignment. Note that a heavy buffer can give the appearance of weak ejection. My 27oz rig has what some would call "weak" ejection, dropping the empties about a foot away, but it's consistent and error-free. If it's inconsistent, it's probably ejector alignment.

As someone above suggested, too - give it a little extra lube on all the friction surfaces just to be sure.

Thank you for all advice. Sitting there playing with it this morning I saw the bolt catching on the magazine going home and I think there may be some variance between this and my last lower that seats the magazine block too high or gives it a bit too much room to shift upward when firing, putting the magazine in the path of the bolt and slowing it down.  I swapped lowers with another gun and have that setup ready to try next time out. On this one the bolt goes home every time without catching on the magazine.

Good catch.  So the PSA lower mag catch is holding the sylvan arms adapter a little higher, causing the mag feed lips to touch the bottom of the bolt... a different mag catch may fix the problem if the adapter is not adjustable.

Good luck!


__________

My site: Blowback9.com

My FM9 is a complete FM9.  In working through LRBHO issue with mine with FM and talking to people who mixed FM9 uppers and lowers with other brands, it sounds like it's best to go all in with FM upper and lower, or go with another brand that is more forgiving for franken9's.

Thing ran like a sewing machine on Endomags but I’m the greedy asshole who can never settle and decided to monkey with it to try to get it to work with Glock mags. Then when I got it to do it, I changed the formula again.

If this upper and older  PSA lower function well with the Mag block on the next outing they will stay married forever. Otherwise maybe reverse the plan to put the Anderson unit in reserve since I know that one ran well.

Newest trip was repeated failures to eject with a case stovepiped in the chamber area next to a fresh almost chambered or fully chambered round. Then found a loose “gas key”. Bolts not staked from factory. Now headed home to red loctite them and put the 8 oz kak buffer that gave me 600 trouble free rounds on Endomags back in.

This was on the psa multi cal lower with 7.5 inch Macon buffer and standard spring. I am hoping it’s as simple as the loose gas key permitting bolt rotation that threw everything off.

Can’t find another reason the thing went from 99% trouble free to a paper weight.

Cases extracted out of the chamber but not getting ejected all the way out. New rounds making it into the chamber.

99% with Endomags?  Could it be an ejector issue?  Endomags as you know, have their own ejector.  Dedicated lowers have their ejector and if it isn't nearly riding against the bolt they don't run too well.  If this has been addressed and I missed it, sorry.

I have the sylvan arms block with the ejector built in and it worked fine for a couple hundred rounds. Nothing seems to have changed about it. After I got the carrier key torqued back down and loctited I hand cycled a few snap caps through and they spat out consistently, making me *think* I have a bolt velocity problem and/or I had the loose carrier key causing issues more than a mechanical problem with the ejector. 8 inch Kak buffer that never failed me in there now waiting for the next trip.

Thinking more about it I’m wondering if the lose key was allowing the bolt to rotate on firing and snag the magazine lips or ejector, slowing it down.

It would make sense as one of the longest strings of malfunction free fire came when someone tightened them for me, then after 6-8 shots it started hanging up again. And I took the bolt back out and the screws
were loose again.

Can confirm after my newest trip, problem definitely relates to how the magazine block interfaces with and presents magazines to the upper. And not specific to a particular lower. I tried the magazine block on my Anderson and PSA lowers and ran endomags through after also swapping buffers and springs back and fourth. As long as it was Endomags in the gun it ran 100%. The mag block ran like crap on both lowers. My promags, Pmag, and oem glock mag made no difference in consistency. None worked.

Endomags gonna get the dust blown off them and come back out and the Glock mags are gonna go back in the parts bin with the block.

It’s a bit irritating as I ran out and bought Glock mags after the first trip went well but something changed and they haven’t worked since. There is an allen screw tinier than any key I own in the side of the block near the ejector. Only thing I can wonder is if it adjusts the ejector vertically. The ejector doesn’t really look like it’s meant to be bent.

Otherwise I’m at a loss and probably just gonna run the Endomags until I can pick up an FM9 lower down the line rather than blow more ammo trying to make the conversion block work.

After going over the thing repeatedly for problem areas I found a chipped off piece of metal from the face of the ejector in the block. Top left tip when facing it, the part closest to the shell. Tiny piece, but positioned just right to not interface with the back of the shell consistently. Bolt looks fine but for some reasons the ejector chipped. Maybe defective metal? Only possible reason I can find that the thing performed decently in outing #1 and then suddenly became a paperweight. I did have carrier key screws back out too, I'm wondering if the bolt rotating or moving more than it should in its channel somehow caused damage to the ejector???

I donno. Now have a Stern defense unit coming
to try out. If the ejector on the Sylvan is replaceable and the Stern works may try to get Sylvan to send me an ejector so I can fix and then dump the Sylvan.


I know you’re technically not supposed to be able to use the Stern on an Fm9 upper without modifying the bolt but I’ve seen a couple youtubers have success just by making sure it’s mounted as low as possible in the magwell, so giving it a shot.

https://www.1911forum.com/attachments/b3bfff08-dc64-4078-bd59-2a0635ecf24f-jpeg.618538/

https://www.1911forum.com/attachments/81987181-dbab-407d-992b-729045f55b68-jpeg.618539/