New to me Reising, new to me firing pins

[Cort Tompkins]

I recently took possession of an M50 which I bought from a gentleman who didn't want to sell it for more than he paid 15 years ago.  He was concerned about things he had heard/read concerning the "engaged in the business rule".  I told him he was selling at well under market value but he was not dissuaded.  Needless to say I think I got a good deal.

I took possession of the gun two days ago, cleaned and oiled it yesterday, took it out this morning, and broke the firing pin on the second magazine. ?

While I was waiting for the transfer to go through (C&R, so a paper form) I picked up a titanium firing pin so I do have that on hand at least.  I'm planning to do the storied inertia firing pin modification I've read about here.  Keystone currently shows the titanium pins as out of stock ... so if my one remaining pin breaks I suppose I'll give things a go on my lathe with some O1 drill rod.

The previous owner said the gun once belonged to the Sumter, SC Police Department, from whom he sourced a cool department patch.  He didn't share any documentation on that with me and I still need to submit a FOIA request.  As pictured the gun came with a beautiful Federal Laboratories case, two original H&R mags, and two reproduction mags.

I just bit the bullet on 10 more reproduction 20 round mags and 3 Christie 30 rounders, so I am motivated to get the gun running.  The 25 rounds I've been able to put through it so far were great! ? 

[StrangeRanger]

Be sure to take the bolt apart and clean the INSIDE where the firing pin and spring reside.  I am convinced that a lot of the FP failures that are reported are due to accumulated crud jamming the firing pin in the forward position

[Cort Tompkins]

I did a full disassembly and cleaning of everything except the bolt -- a bit of an oversight on my part, it would seem.  When the firing pin broke at my range this morning, I removed the bolt and took out the firing to look at it.  It was ... not clean.  It seemed to be coated in sort of a rusty, oily slurry.  So yes, I think you could be right!

[StrangeRanger]

If you look at the original and Keystone Ti firing pins you will see a design difference

The original pin has a long smooth tapered shape

https://keystonearms.com/product/reising-firing-pin-used-original/

This is structurally sound but depending on the clearance between the pin and its hole in the bolt face, the accumulation of crud in the bolt ID and how far the hammer drives the pin forward it could lead to the pin getting stuck either in the hole or in the crud

The Keystone pin is a constant diameter with a reduced tip

https://keystonearms.com/product/reising-firing-pin-titanium/

This design reduces the operational issues inherent with the original design but has a structural one.  The 90° shoulder at the diameter reduction sets up a stress concentration.  I know of one Ti firing pin, the one in my former Reising, that broke at exactly this spot. The new owner for whom it broke is also a mechanical engineer and told me it broke exactly where his education predicted that it would. If Keystone had cut that shoulder back at 45° it would have been a much more durable design

If you're going to turn your own pins out of drill rod, I strongly suggest that you follow the Keystone design but alter the shoulder angle to 45°

 

[Cort Tompkins]

Thanks for the insights. I was planning on duplicating the Keystone profile mostly because I didn't want to have to cut a taper nearly the whole length of the pin. Converting that 90° shoulder to a taper makes sense. 

[tommyboy]

There’s a listing on GB for some Reising firing pins.

https://www.gunbroker.com/item/1117666297

[Cort Tompkins]

Coming along, coming along. My new firing pin seems to fit correctly in my original bolt and my Indianapolis Ordnance bolt.  I still need to heat treat it and the give it a go. 

This one is O1. I also bought some Stressproof and S7 to try.

[StrangeRanger]

Looks good

[Cort Tompkins]

Whelp, I heat treated the pin and took it out today and ... it broke.  I shot probably a little over a hundred rounds no problem, mostly with some 30 round Christie mags I just picked up for a small fortune.

I then switched to a 20 round Sarco mag which had earlier been giving me some trouble aaaand .... broken firing pin.  With both this broken pin and the original broken pin, the break happened during/right after a specific kind of failure to feed where the bolt was hung up on a round that had only barely started to leave the magazine.  After chambering that round I find the pin is broken.

My bolt and firing pin channel were immaculately clean this go around.  I think I may be having the same type of failure I saw pictured in the inertial firing pin thread, where the pin is sticking way out past the bolt face before it's sheared off.  

I'm going to make another one out of O1 drill rod, but this time I'm going to 1) cut the flat so that the retaining pin doesn't allow the firing pin to protrude more than some tiny amount past the bolt face -- i.e., something like the protrusion of a fixed firing pin on an open bolt, 2) shorten the rear of the firing pin so that it becomes an inertia type, and 3) probably make it a bit softer -- if my process was correct I think this last pin was round 52C which was maybe a bit too hard.

 

Edited July 28, 2025 by Cort Tompkins

[StrangeRanger]

FWIW the FP protrusion spec for a Thompson is .044"-.050", a Reising should be similar

[huggytree]

Mine kept breaking firing pins. Even titanium ones. Did the inertia trick and it ran 100% after.  Mine never ran after market mags well, even after tweaking feed lips.  Factory mags always ran 100%. 
 

it’s a fussy gun for most. I enjoyed mine for its price point 

[Cort Tompkins]

Thankfully I have a bunch of other subguns -- even being well aware of the Reising's reputation for fussiness going in, if this were my first full auto I expect I'd be very depressed right about now.

But, I remain undeterred.  I really think the giant 1/2"+ of firing pin protrusion that's possible with the factory (and the titanium) firing pins is a contributing factor.  In any case, my inertial, minimum protrusion (.044"-.050"!) replacement pin will soon be done and hopefully that'll be the end of this.

And I do hope I can get the Sarco mags running better.  The two original H&Rs and the three Christies I have all seem to run pretty well.

[StrangeRanger]

The guy who bought my Reising monitors this board but doesn't belong.  He's a better materials engineer than I ever was and he doesn't like your choice of O1 for your firing pins.  It doesn't do well with impact loads. He thinks that S7 may be worth trying but his real preferences would be 4340 PHT, 4140 PHT or Stressproof

FWIW he agrees with my suggestion of altering the shoulder angle from 90° to 45° but thinks an even more gradual taper would be better.  You might want to try 30°

[Cort Tompkins]

I started with O1 since it's said to have a forgiving heat treating process and I was planning to heat treat with a torch.  But instead I just bought a cheap Vevor kiln that'll easily get up to proper heat treating temperatures. So yeah, assuming I do this again I'm going to make one pin each out of Stressproof and S7 -- which I ordered when I started this whole mess.  Again I appreciate the insights.  I'm hopeful that limiting the firing pin protrusion is going to be a fix but material choice matters too of course.  

[fartsalot]

Cort T    check your message box

 

[Cort Tompkins]

Alright, so here's my final pin (so far). This is an inertia type and has 0.052" protrusion on both my original H&R bolt and my Indianapolis Ordnance backup bolt. For reference, my titanium Keystone firing pin has a basically unconstrained protrusion of 0.290". I don't have an H&R firing pin to measure but it should be similar to the Keystone, as was my previous attempt.

Side by side comparison with a Keystone pin showing reduced length (for inertia behavior) and modified flat length/position to control protrusion:

Protrusion comparison:

and

 

I'm really hopeful the protrusion modification is going to be the fix. I've set off three empty primed cases with the new pin, so the inertia bit is performing as intended. All Wolff springs.

The new pin is still O1 and hardened to about 45 RC. Before I take the gun out again I'm going to make identical pins from Stressproof and S7. ?

[StrangeRanger]

Extremely well thought out

Using the shorter cross-pin slot to limit the firing pin protrusion was a master stroke

[Frank I.]

If it works, you should start a business!

Unmodified Keystone pins are marginally better than originals. I have broken several of them. 

[Cort Tompkins]

Early tests of the limited protrusion firing pin are positive. I took the gun out yesterday with my least reliable magazines and a new Fatigue-Proof steel pin installed.

I got about 200 rounds through the gun, with many many of the failure types that seemed to quickly break my first two firing pins, and the new pin held up. 

I would have put more rounds through the gun, but my magazine catch retaining screw threads are almost entirely stripped and stopped holding the magazine catch. ? That's an easy enough fix, at least. 

I recorded a short video with some more details here.

I want to do some more testing with pins made from S7 tool steel. If that goes well I'll make up a small batch for sale. 

[Frank I.]

I want to do some more testing with pins made from S7 tool steel. If that goes well I'll make up a small batch for sale. 

First in line to buy!

[19Steve67]

I am interested in purchasing several also.

 

Steve

[Cort Tompkins]

Thanks everyone for the interest. I fixed my magazine release and took my Reising out with the S7 pin this past weekend. I got about 100 rounds through the gun until ... the extractor screw broke. BUT the firing pin was fine. Again using Sarco mags with lots of the types of jams that killed the first two pins. 

After the live fire testing I did a bunch of dry firing and noticed that the rear edge of the flat (what is effectively the firing pin stop) has started to peen over just a bit.  I made the pin relatively soft; I'll be heat treating the next one to be a good deal harder. 

I've got my extractor screw replaced so once I get the heat treating sorted out, I'll do some more testing and then I'll be in a position to get some pins into folk's hands. 

 

[fartsalot]

Thanks for the update on progress with the pins,, your success with this will make my life easier with this broken pin problem.   

Thank you 

Edited August 27, 2025 by fartsalot

[MGMike]

Welcome to the world of Reising SMGs, or anything else that that maladroit idiot designed.  I know, because I've had one for years. He should have been frog-marched into the factory courtyard and executed for conceiving such an abomination.

Yes, it is possible to modify one so that it more or less (mostly less) works, but it's scarcely worth the trouble.  Whenever faced with a fundamental design choice, Eugene picked wrong.

The magazine is incomparably the world's worst, and unrestrained f.p. protrusion is an engraved invitation to breakage. The extractor held on with a screw with a head that must be ground off to fit inside the receiver is utterly incomprehensible. The cocking handle, cleverly hidden where it is accessible only by a prostate exam would be humorous except that it's right next to the barrel where, if hot, the operator can receive a 3d degree burn probing for it.  The studs under the receiver that hold various parts were intended to be brazed on, but by sloppy quality control, came loose and had to be repaired by crudely welding them on. A non-concentric recoil spring guide rod can yield the most perplexing stoppages until it either breaks off or the cause is correctly diagnosed.  The operating rod and its associated parts are so flimsy that they crack, or bend, at the slightest provocation. A multitude of small springs are in rough deep holes, powering even rougher bearing studs that cry for even minimal polishing. What were the workers at H&R thinking? (Or were they thinking at all?) Were the inspectors asleep?

The entire design and standard of manufacture are a blemish on Rube Goldberg's reputation. The Reising makes the French Chauchat look like a precision instrument.  My sympathies are with the poor Marines who were issued this scheisse, and sent to Guadalcanal with it.  No wonder they resourcefully scrounged up anything else they could find (or steal).

M

Edited August 28, 2025 by MGMike

[StrangeRanger]

So do I understand correctly that you really don't like the Reising?