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Yugoslav 9mm question


StrangeRanger
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Got Uzi and I finally got the 9mm Para barrel for my Yugoslav M49/57 together.

It simply refuses to run on US spec 9mm; it short strokes and drops the empties back in the ejection port.  I know that Euro spec 9 is a bit hotter than ours but I also know that a lot of SMGs such as the Swedish K are designed to run on very hot ammo, far above the standard spec.  I suspect that this is the case with the Yugo.  Does anyone know for certain what ammo the 9mm version of the M49/57 was designed to use?

Edited by StrangeRanger
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What a great question. Although you know I am no Yugo 49/57 expert, I do know that circa WW2 open bolt 9mm guns used (as you both know) blowback to operate and the bolt weight/spring tension were determiners of the cyclic rates. Inherent in the design was an assumed charge (or impulse) if you will from the round that provided the energy to make the whole thing work. 

So, most WW2 9mm (European) was hotter in order to ensure operation.

Now, my PPSH (Russian) has NO issues with 7.62 x25, and the 49/57 was designed for this round (as you know) so my take on this is that with the 9mm conversion you'll need to run 9mm+ rounds, or 9mm subgun rounds only. 

I find it interesting that my MP40 sometimes behaves exactly like you describe when I run American spec 9mm through it. Short cycle, rounds falling into the receiver etc.

My $0.02

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12 minutes ago, ron_brock said:

SR,

Have you tried running with S&B?  Just curious if you’ve tried and if it’s reliable 

Ron 

I don't have any S&B 9mm. Given that PPU is Serbian I figured it would be the most logical standard ammo to test in the Yugo (and I have a couple of cases of it])

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That could be.  I’ve not shot S&B 9mm, but I know when it comes to 45acp in subguns it’s hard to beat and I believe it runs considerably faster (hotter) than others.  

Not sure how it compares to vintage 9mm subgun rounds tho 

Ron

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The M49/57 was issued to the Yugoslav army in 7.62x25 but they also offered it for export sales in 9 Para so there's some 9 ammo somewhere that it will run on.  It's also entirely possible that they used a different spring in the 9 Para version

If we can get a couple of other issues resolved I may try some Egyptian subgun ammo in it next month at our plate shoot.  It was made to run the Port Said K clones and is extremely hot. I'm pretty sure it will work but I also want to try some current PPI Euro spec 9 which is warmer than SAAMI spec but nowhere near "K spec."

I was just hoping someone could confirm what the Yugo really needs and save me some experimentation.

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1 minute ago, StrangeRanger said:

The M49/57 was issued to the Yugoslav army in 7.62x25 but they also offered it for export sales in 9 Para so there's some 9 ammo somewhere that it will run on.  It's also entirely possible that they used a different spring in the 9 Para version

If we can get a couple of other issues resolved I may try some Egyptian subgun ammo in it next month at our plate shoot.  It was made to run the Port Said K clones and is extremely hot. I'm pretty sure it will work but I also want to try some current PPI Euro spec 9 which is warmer than SAAMI spec but nowhere near "K spec."

I was just hoping someone could confirm what the Yugo really needs and save me some experimentation.

Ah, but we rely on you and GotUzi to experiment and solve our issues so we don't have to do so!

I wonder if the bolt of the guns chambered in 9mm was substantially lighter than the one issued with 7.62 x 25? Are you running the OEM bolt? If so, then total mass may be the culprit. 

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Same case head, same bolt weight, you'd never machine two different bolts unless there was no other way.  Easy to swap out springs and make other simpler mods.  I have a case of that Egyptian  stuff, it's not that hot, but it's worth trying.  

My 9mm kit and stack of MP40 mags still sits in waiting from the creek after 15 years.....for you guys to figure this out.  LOL

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There are light and heavy bolts.  I have 2 of the former and 1 of the latter.  I've been running the light bolt.  The ROF in 7.62 is a very Shpagin-like something north of 900 RPM.  There's a minor dimensional problem with the heavy bolt which we are addressing.  I'm hoping it slows the 7.62 down to 750 or so.  It would probably be hopeless even with SMG specific 9 Para

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Light and heavy from the same country/ gun design, or different bolts from different countries/ models of guns?   What are the actual weights?   That was the old adage about the bronze sten bolts, that they were different ROF, but it didn't pan out that way  as I recall?  You're probably going to end up with different springs at the end of the day? 

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The Yugoslav army manual lists both bolts for the 7.62x25 M49/57.  the light bolt is 450 g/16 oz. the heavy is 550g/19.4 oz.  The light is listed as an alternative part.  There is no mention of 9mm anywhere in the army manual so apparently both bolts were intended to be used with the 7.62 ammo.  The spring that came with the light bolt parts kit is identical to the one that came with the heavy bolt kit

The M49/57 is basically a Beretta 38 series copy receiver/bolt with a FCG from a Shpagin.  The heavy bolt is probably intended to provide a Beretta-like ROF; the light definitely mimics the Shpagin

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21 hours ago, StrangeRanger said:

The Yugoslav army manual lists both bolts for the 7.62x25 M49/57.  the light bolt is 450 g/16 oz. the heavy is 550g/19.4 oz.  The light is listed as an alternative part.  There is no mention of 9mm anywhere in the army manual so apparently both bolts were intended to be used with the 7.62 ammo.  The spring that came with the light bolt parts kit is identical to the one that came with the heavy bolt kit

The M49/57 is basically a Beretta 38 series copy receiver/bolt with a FCG from a Shpagin.  The heavy bolt is probably intended to provide a Beretta-like ROF; the light definitely mimics the Shpagin

Interesting as we know the kinetic energy of the 9mm is less than that of the 7.62 x 25 so it would seem (dangerous assumption) that the lighter bolt would be the designated 9mm pairing. 

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I think the light bolt was probably just about mimicking the Shpagin ROF .  If it had been for the export guns only it never would have been in the Serbian language manual

And forget about kinetic energy of the bullet, it's not relevant to what happens to the bolt.  Look instead at conservation of momentum.  The momentum (mass x velocity) of everything going forward (i.e. bullet and gasses) equals the momentum of everything going backwards (bolt, empty casing and some portion of the spring mass).  Except for a very slight difference in case mass which is insignificant when compared to the mass of the bolt, the backward moving components are the same with either round so whatever difference is seen in the rearward velocity of the bolt will be proportional to the differences in momentum between the momenta of the bullet plus powder of the two cartridges

Using metric units for the purpose of clarity (the unit of momentum in the Imperial system is the otherwise meaningless pound-second) approximate values are

7.62x25 bullet: 5.5g x 518 m/sec = 2.85 kg-m/sec; powder: 0.9g x 1220m/sec = 1.10kg-m/sec.  TOTAL: 3.95 kg-m/sec

9x19 bullet: 8.0g x 380 m/sec = 3.04 kg-m/sec; powder: 0.5g x 1220m/sec = 0.61kg-m/sec.  TOTAL = 3.65 kg-m/sec  

So only about 8% higher bolt velocity.  But the energy imparted to the spring is the kinetic energy of the bolt (which goes by Newton's 2nd Law 1/2*M*v^2) is about 16% higher than with 9x19.  The spring compresses at a constant rate in lbs/in so the distance it compresses is proportional to the force compressing it, in this case the kinetic energy of the bolt.  That 16% difference explains the short stroking on 9mm.  The energy imparted to the cartridge case by the ejector is likewise going to be 16% higher which explains the failures to completely eject.

All else being equal a "SMG spec" 9mm would need to be about 8% or about 100 fps (30m/sec) faster to operate reliably in the Yugo

And yes I did pay attention in Statics and Dynamics 201

 

 

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1 hour ago, StrangeRanger said:

I think the light bolt was probably just about mimicking the Shpagin ROF .  If it had been for the export guns only it never would have been in the Serbian language manual

And forget about kinetic energy of the bullet, it's not relevant to what happens to the bolt.  Look instead at conservation of momentum.  The momentum (mass x velocity) of everything going forward (i.e. bullet and gasses) equals the momentum of everything going backwards (bolt, empty casing and some portion of the spring mass).  Except for a very slight difference in case mass which is insignificant when compared to the mass of the bolt, the backward moving components are the same with either round so whatever difference is seen in the rearward velocity of the bolt will be proportional to the differences in momentum between the momenta of the bullet plus powder of the two cartridges

Using metric units for the purpose of clarity (the unit of momentum in the Imperial system is the otherwise meaningless pound-second) approximate values are

7.62x25 bullet: 5.5g x 518 m/sec = 2.85 kg-m/sec; powder: 0.9g x 1220m/sec = 1.10kg-m/sec.  TOTAL: 3.95 kg-m/sec

9x19 bullet: 8.0g x 380 m/sec = 3.04 kg-m/sec; powder: 0.5g x 1220m/sec = 0.61kg-m/sec.  TOTAL = 3.65 kg-m/sec  

So only about 8% higher bolt velocity.  But the energy imparted to the spring is the kinetic energy of the bolt (which goes by Newton's 2nd Law 1/2*M*v^2) is about 16% higher than with 9x19.  The spring compresses at a constant rate in lbs/in so the distance it compresses is proportional to the force compressing it, in this case the kinetic energy of the bolt.  That 16% difference explains the short stroking on 9mm.  The energy imparted to the cartridge case by the ejector is likewise going to be 16% higher which explains the failures to completely eject.

All else being equal a "SMG spec" 9mm would need to be about 8% or about 100 fps (30m/sec) faster to operate reliably in the Yugo

And yes I did pay attention in Statics and Dynamics 201

 

 

That my friend is EXACTLY what I meant! :)

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The whole energy vs. momentum thing can get complex and confusing.  I got bit on the arse by it when I made my initial ROF assumption.  I was thinking in terms of momentum which would mean the heavier bolt would reduce the ROF from around 900 RPM to 900*(450/550) = a too good to be true 736 RPM.  The correct number is an energy-determined 900*(450/550(^0.5)) = 814 RPM still substantial but hardly earth shattering.

Been too long since I did this shit for a living🤣

 

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Except the spring doesn't compress at a constant rate, and as we know different powder burn rates and a lot of factors other than just the raw energy numbers have a substantial effect on the whole system.  Guns never work on paper due to all of the many other factors that are difficult to measure or predict.  Add in the human component and even the best gun can fail.  We ran a subgun at the creek once with a bunch of us shooting, until one guy (with a soft shoulder) stepped up and stopped the gun dead since absorbed so much recoil the gun would not cycle.  We tried different stances, etc. and it never ran while in his hands.  None of the rest of us had the problem.  I wish I could recall which gun it was?  He ran all the other guns just fine?

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Are you seriously claiming that a constant pitch coil spring does not act according to the equation f = k*x where f is the spring force, x is the compression distance  and k is the spring constant expressed in lbs per inch of compression?

You've just disagreed with every machine design text on the planet

 

 

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On 3/20/2023 at 11:56 AM, StrangeRanger said:

I think the light bolt was probably just about mimicking the Shpagin ROF .  If it had been for the export guns only it never would have been in the Serbian language manual

And forget about kinetic energy of the bullet, it's not relevant to what happens to the bolt.  Look instead at conservation of momentum.  The momentum (mass x velocity) of everything going forward (i.e. bullet and gasses) equals the momentum of everything going backwards (bolt, empty casing and some portion of the spring mass).  Except for a very slight difference in case mass which is insignificant when compared to the mass of the bolt, the backward moving components are the same with either round so whatever difference is seen in the rearward velocity of the bolt will be proportional to the differences in momentum between the momenta of the bullet plus powder of the two cartridges

Using metric units for the purpose of clarity (the unit of momentum in the Imperial system is the otherwise meaningless pound-second) approximate values are

7.62x25 bullet: 5.5g x 518 m/sec = 2.85 kg-m/sec; powder: 0.9g x 1220m/sec = 1.10kg-m/sec.  TOTAL: 3.95 kg-m/sec

9x19 bullet: 8.0g x 380 m/sec = 3.04 kg-m/sec; powder: 0.5g x 1220m/sec = 0.61kg-m/sec.  TOTAL = 3.65 kg-m/sec  

So only about 8% higher bolt velocity.  But the energy imparted to the spring is the kinetic energy of the bolt (which goes by Newton's 2nd Law 1/2*M*v^2) is about 16% higher than with 9x19.  The spring compresses at a constant rate in lbs/in so the distance it compresses is proportional to the force compressing it, in this case the kinetic energy of the bolt.  That 16% difference explains the short stroking on 9mm.  The energy imparted to the cartridge case by the ejector is likewise going to be 16% higher which explains the failures to completely eject.

All else being equal a "SMG spec" 9mm would need to be about 8% or about 100 fps (30m/sec) faster to operate reliably in the Yugo

And yes I did pay attention in Statics and Dynamics 201

 

 

You should read my write up on trying to run a suppressed M1A on sub rounds.... under the M14 Forum. Similar thoughts.....

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The tighter you compress the spring the rates  change in a non-linear fashion, add in friction on the guide rods and haphazard contact with the rod as well as other unknowns and the math becomes theory.  If you put the gun in perfect vacuum chamber will it work then, since the feather falls as fast as the  BB in that circumstance and you've eliminated a bunch of variables.  

I agree with all the math and theory's but in practice they are only just that.  If it were that simple you're gun would be working by now, would it not?  LOL

The reality is it will never be fixed and run by any amount of math and formula's on paper.  It will take someone with gunsmithing and mechanical ability to make it function.  A pencil (even with a big eraser) and calculator cannot make it happen...  ever.

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13 hours ago, johnsonlmg41 said:

The tighter you compress the spring the rates  change in a non-linear fashion, add in friction on the guide rods and haphazard contact with the rod as well as other unknowns and the math becomes theory.  If you put the gun in perfect vacuum chamber will it work then, since the feather falls as fast as the  BB in that circumstance and you've eliminated a bunch of variables.  

I agree with all the math and theory's but in practice they are only just that.  If it were that simple you're gun would be working by now, would it not?  LOL

The reality is it will never be fixed and run by any amount of math and formula's on paper.  It will take someone with gunsmithing and mechanical ability to make it function.  A pencil (even with a big eraser) and calculator cannot make it happen...  ever.

Very true, however what I learned from tying to get my M14 to run with a can and subs is that the math told me ahead of time that it would NOT work and sure as shit and shinola, it did not. No amount of spring fiddling, bolt shaving and gas tube plugging would make it work because the math showed requirements far in excess of what could be done. 

Now, if it were close, then perhaps... but the calcs saved me a ton of sweat and tears.

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It can work, but the mods involved to make an M14 work with subs are not worth the time and effort when there are much easier and cheaper alternatives to putting that bullet down range quietly.  At the end of the day you buy a crappy AR in .300 BO and press the easy button. 

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On 3/16/2023 at 9:24 PM, StrangeRanger said:

Got Uzi and I finally got the 9mm Para barrel for my Yugoslav M49/57 together.

It simply refuses to run on US spec 9mm; it short strokes and drops the empties back in the ejection port.  I know that Euro spec 9 is a bit hotter than ours but I also know that a lot of SMGs such as the Swedish K are designed to run on very hot ammo, far above the standard spec.  I suspect that this is the case with the Yugo.  Does anyone know for certain what ammo the 9mm version of the M49/57 was designed to use?

 

I would pick up some 124gr NATO spec ammo to use as your baseline for function testing.  If you don't get consistent function with that ammo, then you'd probably need to start making hardware adjustments.

BTW...where did you get your 9mm barrel?  Did it come complete and ready to drop it?  I've got a tube Yugo 49 and it would be nice to have the extra (9mm) caliber option.

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The barrel came with the gun and it was a very, very long way from ready to drop in.  The chamber is very tight and will not accept all brands of factory ammo. The lugs that are supposed to set headspace and lock it into the receiver had to be welded up front and rear and re-machined.  It had an extended muzzle which was threaded for a suppressor but was too large in diameter to fit through the hole in the end of the muzzle cap and the shoulder for the muzzle cap was in the wrong place.  I have no idea who made the barrel which is a good thing because I might have given it back to them - as a suppository.

It has been such a mess that I may be back-burnering the 9mm project for a while and concentrating on getting it running 100% in 7.62 Tok with the heavy bolt

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