Smith & Wesson M76 Suppressed Hints

[Rekraps]

So, as most of you who own these guns know, using supers with a can presents no issues with cycling. Problems begin when we move into the subsonic or close to subsonic arena. Specifically, 124 grain (only use round nose) 9mm can be barely subsonic, or just over the speed of sound which means it may or may not be as quiet as one would expect, AND it may induce a runaway gun. 147 grain is in fact subsonic and more often than not results in a runaway gun.

The reason is that the recoil (main spring) spring is designed for full power 9mm (115 grain, NATO) which imparts a more robust impulse to the bolt and recoil assembly. On the other hand, the 124 and 147 grain 9mm delivers less of an impulse which may not move the bolt rearward far enough to engage the sear stop. I had issues with both my guns having periodic and sporadic runaways. 

So, the answer is to obtain a separate "can use only" recoil spring and remove coils until the gun behaves correctly. I found that three coils did the trick, then I removed one more for good measure. There is no indication of excessive bolt travel (i.e. it does not hit the rear of receiver) and the gun now works great. I dedicated one of the M76's to suppressed use and left the OEM main spring in the other so they don't get mixed up. 

I initially thought it was the trigger bar, but both guns have had the trigger bars rebuilt by Andrewski so I know they are 100%. 

It's interesting how this particular gun needed work to function while my K's chug along fine with no modification at all. 

[TKTEXAS]

Can you give me the length of the two different springs you have. My 76 came with a 14 1/4" spring and the tip of the guide ride was a bit beat up which I fixed. The spring did not look like any coils were removed.  It ran very well though. I bought a Wolff spring that measures at 15" and it feels good and runs great. Just feels tighter. ROF seems the same, bolt travel good, and runs 124 and 115 grain, so I'm sticking with the Wolff. When I get a suppressor, I'll keep your advice in mind. I wonder what the original factory length was.

[Rekraps]

7 hours ago, TKTEXAS said:

Can you give me the length of the two different springs you have. My 76 came with a 14 1/4" spring and the tip of the guide ride was a bit beat up which I fixed. The spring did not look like any coils were removed.  It ran very well though. I bought a Wolff spring that measures at 15" and it feels good and runs great. Just feels tighter. ROF seems the same, bolt travel good, and runs 124 and 115 grain, so I'm sticking with the Wolff. When I get a suppressor, I'll keep your advice in mind. I wonder what the original factory length was.

Certainly. It will be this weekend. The recoil guide on one of my guns was a bit beat up, so I had two solid billet single piece recoil springs guides made so there will be no more of that. I guess these guns were made to use and throw away and that may be why the OEM recoil guide is hollow. It's also made of three pieces, which is bizarre.

[Rekraps]

20 hours ago, TKTEXAS said:

Can you give me the length of the two different springs you have. My 76 came with a 14 1/4" spring and the tip of the guide ride was a bit beat up which I fixed. The spring did not look like any coils were removed.  It ran very well though. I bought a Wolff spring that measures at 15" and it feels good and runs great. Just feels tighter. ROF seems the same, bolt travel good, and runs 124 and 115 grain, so I'm sticking with the Wolff. When I get a suppressor, I'll keep your advice in mind. I wonder what the original factory length was.

Okay, I was able to get to this today. I have four OEM M76 recoil springs. Unaltered (came with the gun) the springs measure 14.25" (14 1/4") each. The recoil spring in my suppressor dedicated gun, from which I shoot 147 grain round nose 9MM rounds, is 12.25" (12 1/4"). This shorter spring functions very well in the gun and allows the gun to cycle normally, with the sear engaging the sear notch without fail. The OEM spring, with 147's will not allow the bolt to travel far enough to engage the sear. 

Attached are two pics of the recoil spring guide(s) that I had made by a competent fabricator. Solid, single piece billet. They work great and will NEVER bend or get damaged. 

[StrangeRanger]

On 11/12/2024 at 9:10 AM, Rekraps said:

...The reason is that the recoil (main spring) spring is designed for full power 9mm (115 grain, NATO) which imparts a more robust impulse to the bolt and recoil assembly...

NATO spec ammo is 124 gr truncated cone loaded to CIP specs (approximately SAAMI +P)

[StrangeRanger]

FWIW Scott Andrey listed two different recoil springs on his website.  A light spring at 13.5 lbs/inch and a heavy spring at 20 lbs/inch

Burgess sold two springs with each gun, a light for SAAMI-spec loads and a heavy for CIP-spec loads. I do not know but I suspect that they are the same pair

Now to get to the engineering part of this. OAL is an inadequate way to define a spring.  Wire diameter and coil diameter are much more important.  Burgess's springs had the same length, coil diameter and number of coils but different wire diameters. I do not have the the gun any longer so I cannot measure the diameters but if I had to pluck numbers from what passes for memory I would say the pair was either .048/.054" or .054"/.063"

This next part is as counter-intuitive as all Hell but trust me on this, I used to do this crap for a living:  Cutting coils off a spring increases the spring rate.  Think of a coil spring straightened out into a torsion bar and maybe it will make sense.  When you cut coils off a recoil spring you are reducing the initial compression (when the bolt is fully forward) but increasing the rate at which the spring force increases as the bolt  travels backwards.  It's a balancing act. If you cut coils you decrease the total compression (initial plus travel) and probably the initial spring force but you increase the rate at which the spring provides restoring force.  The final force of the altered spring may be either higher or lower than the original depending on the spring how much you cut off of it.

 

[TKTEXAS]

Wow, that's all great and interesting information. It's interesting to me that Wolff makes the M76 spring at 15", so a full 3/4" longer than OEM.  As I said it just feels tight in a good way cocking the bolt and firing, so for the time being I'll stick with it. May I ask who made the guide rod for you and what it cost. Although mine seems fine now, I wouldn't mind having a spare.

[StrangeRanger]

If you care the actual equation for spring rate is:

k = (G x d^4) / (64 x R^3 x N)

 Where:

k = spring rate

G = shear modulus = 12,000,000 psi for steel (sometimes listed as 11,500,000)

d = wire diameter

R = mean coil radius

N = number of active coils

Notice that OAL of the spring does not appear in the equation.

[Rekraps]

17 hours ago, TKTEXAS said:

Wow, that's all great and interesting information. It's interesting to me that Wolff makes the M76 spring at 15", so a full 3/4" longer than OEM.  As I said it just feels tight in a good way cocking the bolt and firing, so for the time being I'll stick with it. May I ask who made the guide rod for you and what it cost. Although mine seems fine now, I wouldn't mind having a spare.

Great info all. Regardless of technical issues, the result of my shortening my OEM spring to 12 1/4” did the trick.  
Unfortunately my supplier will not be making more of the billet recoil guides.  He said the actual cost to mfg the low volume of units makes the sell price way out of line.  
 

I got lucky. 

[Rekraps]

I was also corrected in that NATO spec is 124 grain loaded to +P standards, making it supersonic. So this round would cycle normally, but be way to loud, even with a can. 

[Don.]

Were you having cycling problems using 147’s with a can?  Granted, I have a lowly MK760, but I’ve never had problems with my own 147’s with or without a can.  I’ve used a Bowers VERS9 and YHM R9 with varying barrel lengths.  I guess I got lucky.

[Rekraps]

On 11/23/2024 at 9:48 AM, Don. said:

Were you having cycling problems using 147’s with a can?  Granted, I have a lowly MK760, but I’ve never had problems with my own 147’s with or without a can.  I’ve used a Bowers VERS9 and YHM R9 with varying barrel lengths.  I guess I got lucky.

Yes. I use a variety of 147's and mostly they just did not have sufficient uuuumph to push the bolt back far enough to engage the sear, thus runaway. For me, shortening the spring did the trick. 

[Don.]

 

On 11/24/2024 at 12:02 PM, Rekraps said:

Yes. I use a variety of 147's and mostly they just did not have sufficient uuuumph to push the bolt back far enough to engage the sear, thus runaway. For me, shortening the spring did the trick. 

Interesting.  I wonder if it’s my reloads.  I do load them close to the max charge since 147’s stay subsonic.  It hit me, as I was considering buying some factory loads to try in a Hi-Point I have that malfunctions with a can on it.  I have their heaviest spring they use, but the slide is cycling too fast causing the next round to go almost straight up and get caught on the barrel hood instead of being driven into the chamber.

Yeah, it’s a Hi-Point.  I just bought it for the threaded barrel and the hopes of running a can without a booster.  If factory loads are comparatively weak, then it will probably lose the malfunction.  Looks like I’m hitting Cabelas in the morning for some ammo to test this weekend.

[Rekraps]

Lemme know how it works. This whole subsonic/suppressor world is really a custom work in progress. I have around 10 suppressors, integral and direct mount and they all function differently on the guns to which they are used. Over time, I've decided (to myself) that the subguns of WW2 and post WW2 era were designed for higher 9mm loads, and since then, most commercial 9mm simply is not designed to be used in a heavy open bolt application. That being said, I have some guns that run just fine with commercial ammo. All my pistols and semi-auto guns seem to run just fine.

[Don.]

Picked up some factory 124’s and 147’s yesterday and tried it this morning.  Still getting the failure to feed.  Looks like it’s going back to Hi-Point, lol!

I’m glad my reloads aren’t the problem.  They work in everything else without a hitch.  The HP also runs perfectly without the can, so I suspect it needs more spring or more zinc in the already oversized slide.