Damn M4-2000 latch broke so I drilled it out in the mill, and set about designing a replacement.  About then the shop picked up and the CNC mill is on duty for a while.

In tinkering about I noticed that the hole for the latch spring is but 0.004" or so from a 4-40 tap drill.  Open 'er up, tap it, boom.  Set screw retention.  I only posted this because it could be done by anyone with a mill.  I removed the old latch pin with a normal 1/16" HSS drill, no fancy shit involved.  It beats waiting for CS from Palmetto, it's MUCH faster than sending to ECCO and no need to change mounts.

This shit is Ghetto.  GHETTO.  But I bet it gets me through until I can mill a proper latch.  :)

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I figure I'll safety wire the screw.  Thia can hasn't left this upper in 4yrs anyway.

I like the way you think! Some fellas are scared to death of a few scratches...I see them as tools that get used...not abused...then repaired if they break.

I remember a guy/build on Silencertalk who made his own AAC "T" mount. He also used a screw.



Understand, though, you'll need to harden and temper the latch mechanism, if you make another.

My prototype will be from 4140ph, I'll see how how that holds up and go from there.

The thing I loked about my screw-mod is that once removed no one would ever know it's there.

It won't.   Even in full hard untempered condition,  I wouldn't expect much life out of a 4140 part.  I'd recommend a tool steel like S7 or 440C in a hard temper.

S7 was going to be my choice.

What kills the latches?  Is it the mounting action, or is it the hammering from the muzzle report?

I'd replace that Allen head screw with a grub screw/set screw an leave it.

Cool fix. Especially if you rarely remove it

Thanks.

McMaster has brass tipped set screws.  I've not used this yet, but if it proves successful that's what I'll replace it with.

If I was a real psycho I could grind the whole thing from some Rex AAA that I have...

Or a piece of MoMax M42

Haha.  I've made a few small parts from HSS and cobalt parting blades and other tools.  Definitely not my favorite thing to do.

S7, 440C and 0-1 are what I usually use if I need hard, sometimes A2 or D2.  But the heat treatment is annoying, so if no more than Rc45 is necessary, 17-4 is my go-to,  mostly because I have oodles of it always, it machines beautifully and heat treatment is a snap.  I make a lot of high use fixtures and limited use plug gauges from it.  But I wouldn't use 17-4 for an AAC latch, either.

As far as I know, the OEMs are a MIM part, and while I don't have a Rockwell or other hardness tester,  the way they behave with files and saw blades tells me they're probably mid-high 40s Rc.  14/18 Starrett or Morse band saw blades go through them pretty easy.

Id just go with 17-4PH H900 because it’s machines so easy and strength is goo then sweet talk a Nitride finishing shop into tossing it in the next run for a couple bucks.

I think I have some...

Nitriding is only going to increase wear resistance, not overall strength.
I agree w/ @Sixtysixdeuce  that 17-4 isn't going to perform how you'd like and nowhere near what a carpenter steel will do. S7 will be stronger, harder, and have more elongation, than 17-4, as hardened or full temper. This is why it's chosen for riveting dies, bending dies, shear blades, etc.

It's not expensive stuff, relatively easy to machine, and can be treated by the home machinist.

I have heat treat oven that's not a problem.

I don't see why the duty for a ratchet pawl in this application is so demanding?

The number of them that fail completely and the wear I see on the ones I convert tell us that they take a lot of abuse which medium hardness alloys are not up to the task of handling.

These don't get locked down hard like the pawls on an ASR mount, or held in place with tremendous spring tension like the YHM QD tooth collars.  It's a poor design in an otherwise good line of suppressors.  

I would still expect a tool steel latch to fail in time, but I think it will have a much longer service life than something with a hardness in the 40-50 Rc range.  

Of course, the down side of a harder, stronger latch may be that it wears the muzzle device instead.........

Do you have any data for this?  I ask in genuine curiosity because it goes against my experiences.

My factory pawl/latch has the teeth seemingly cut-off, as if dragging it over the mount (which is hard as hell btw) filed them down.  It seems to me that toughness may be preferred to absolute hardness in this case.  

I'll post pics in the AM.

Hardness correlates to wear/abrasion resistance, toughness is resistance to fracture.  There are lots of very tough materials with poor wear characteristics.  That's why we don't make knife blades or other cutting tools out of materials like titanium and Inconel.

Its been a while since material sciences, and I get what you're saying but it's not adding up in my head.  I will, however, defer to your experience.

It works!  Ran a few mags through it this weekend and it stayed tight.

Stainless steel is tough, difficult to cut, drill, machine, compare to carbon steel, but easier to get scratched, galled, deformed.
Carbon steel is harder, difficult to deform, but prone to fracture.

Draw a file over a piece of tungsten, nothing is gonna happen, smash it with a hammer, it will break into pieces.
Draw a file over a piece of stainless steel wire, it will take some material off, smash it with a hammer, likely it will be flattened a little.

Nope.

Carbon and stainless steel mechanical properties are all over the map.  A36, your basic hardware store hot rolled carbon steel, is about as soft and gummy as it gets with a low 36 KSI yield strength , while low temper 440C is an extremely hard stainless, around 63 Rc with a TYS of 275 KSI.  440C is commonly used for bearings and blades.

The mechanical properties and machining characteristics depend entirely on the alloy and, with martensitic or PH grades, the temper.   Most hardenable alloys, whether carbon or stainless, have fairly low properties and poor wear characteristics in the annealed state.  PM high speed carbon steels, such as D2, may cut like butter and be easily scratched & deformed when annealed, but tempering can get them to a very rigid & wear resistant hardness, well over 60 Rc.

Other alloys achieve only moderate hardness but are extremely tough and very fracture resistant.   Among these would be spring steels like 1095 and 17-7 stainless.

Also, tungsten is not especially hard, about 31 Rc.  You're thinking of tungsten carbide.