Sneaking in a quick Bren question for Mr Laidler...

[Son]

One of the elder statesmen over at gunboards is looking at making his Bren cheaper to use for trimming the cactus outside his front gate. In answering another poster's Bren Q, he added his problem to do with making the Chinese 7.62X39 kit cycle....

"Yes there are 4 positions, but the No2 position is reccomended. Only when the gun gets really dirty do you go to three and four. But then we are not shooting ours in a war, so we can clean them regularly.
However I am still trying to make mine work with 7.62x39. Have the chinese barrel, gas regulator and other bits to make the AK mags fit I do have spare return springs and may eventualy try shortening one. The gun will load and fire but not keep going. However it still works fine with .303

Oh by the way the gun DOES NOT ruin brass."

Any thoughts?

Information

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[Peter Laidler]

Mmmmmm. There are two major things that make a Bren stop Insufficient energy available to unlock, extract eject and cock. The second is insufficient energy to feed, load, lock and fire. Cause of 1 is insufficient gas and cause of 2 is insufficient stored energy in the return spring. From that, the answer is that we need to know at what point it actually stops. Any further thoughts Son?

One point that isn't well known regarding Brens is the friction imparted on the breech block by the action of the locking ramp on the piston extension. The ramp forces the locking cam at the back of the bolt to bear hard on the underside of the body. This is deliberate friction that gives the Bren its slow (?...., well, relatively slow....) rate of fire. It eats up a lot of the stored energy held within the retutrn spring. So on the face of it, the spring can seem perfect but is too weak to overcome this additional friction. That's enough of the physics lesson!

[Brit plumber]

I'm thinking tha this is a Semi Auto conversion as he says it loads and the fires. If it is, (I'm not fully conversant with the SA conversions) does that not mean theres a 3rd spring to overcome when the parts are forced rearwards by the gas pressure? If I was this guy, I would go to the 3rd gas setting to see if it worked and maybe even no.4 if required.

Also, which Mark is it, whos the manufacturer and whats the serial number? I ask as depending on the Bren, the gas cylinder to bipod sleave may be worn and allow some gas to escape. The .303 may be ok but the 7.62mmx 39 dosnt have the power to lose and operate satisfactorily.

[Peter Laidler]

BP is right. The only thing that can make this guns working parts move to the rear is a huge blast of gas. If they ain't going far enough it means that they ain't getting enough gas. But as we showed with some simple experiments with the AK gas system (brilliantly thought out and an example of pure mind boggling simplicity.......), there comes a critical point, due to boyles law when the m/v of the bullet is SERIOUSLY compromised.

There is a simple laboratory experiment where you can show the volume of gas generated by an amount of propellant. I'm not a chemist but used to enjoy this bit of chemistry. But above my head!

[Son]

Yes- I was telling him that the problem would be insufficient gas. I mentioned enlarging the gas system but he said the kit was supposed to be a proven item, and the gas port and regulator were already much larger than their .303 counterparts. I said that my own workings with a similar but more modern weapon regards rates of fire we discovered there was only a couple of RPM difference between a totally worn out (very weak) return spring and a new one, and that cutting a return spring down wouldn't do much except endanger it's ability to return to lock.

Brit Plumber- thankyou for your input, The gun is full auto. I too question the ammo's ability to do the work...

Does the Bren, like the similar action in service currently, like to be soaked in oil to function... I am wondering if a combination of lack of lube to minimise the friction of unlocking and also gas leakage might be his drama.

I have had no experience with Brens, so anything anyone can offer here is appreciated.

[Peter Laidler]

The Bren doesn't run on a lot of oil Son. It's really the piston extension slides in the body, the locking cam surfaces and importantly, an oil coating on the inside top surface of the body where the top rear surface of the breech block frictions.

You COULD tell him to get his head out of the sand regarding the perceived notion that it's a tried and tested system............ It's clearly not that well tried and tested because it ain't working. If the breech block and piston aren't going fully to the rear and touching the buffer plate, then there isn't enough gas. It's really as simple as that. He could try to eliminate some of the not so obvious areas, like fitting a new gas cylinder and piston but as I said, it'll be insufficient gas.

Next time you're here in the land of the Poms Son, we'll get you in the Armourers shop for a day with a few Brens and L4's

[Kev G]

As the 7.62x39 AK conversion was utillised by the Chinese on ZB's and Brens it would seem to be a proven modification design.
Although the gas porting (what diameter is the exit hole on the gas regulator housing ?) is enlarged to compensate for the lesser cartridge there is also slightly less energy required to cycle the gun.
The 7.62 x 39 round being shorter there is a second bent ground into the piston post about 18mm rearward of the 303 one so slightly less energy required to compress the return spring to enable the sear to engage or strip the next round from the mag.
If it is a 'genuine' Chinese conversion using the equivilent military AK round and carbon free and not leaking gas it should be good to go.
My concerns would be "supposed to be a proven item"........?

ATB Kevin

[tankhunter]

I would be most interested to see a GENUINE Chinese converstion to 7.62 x 39mm. To examine how they got over the combination of difficulties in getting a Bren series to function correctly. There definately ARE a number of variables as to how a Bren would function properly. Some covered here already. Gas bleed, Worn componants Etc. all pretty obvious really. My concerns would be. The 7.62 x 39mm Round is a LOT less powerfull then the Original 7.92mm/ .303" class of round it was DESIGNED around, in the first instance. With a significantly weaker round to overcome friction/ gas loss/ worn componants, spring pressure , Etc Etc.
It is indeed quite a bit to ask of such a modest round! It has been sugested by Kev that the Chinese did indeed overcome the problem. I would be very interested from a Technical viewpoint to see how that was achieved.
I like Peter, am FIRMLY of the opinion the weapon most OBVIOULY needs gas to correctly function cycle. IE: Has the gas post UNDER the gas block in the actual Barrel been enlarged?
I am certainly NOT a Mathmatician! But working out on paper all the other variables. IE: Mass, spring pressure, wear, gas loss pressure, Etc & loading them in the equasion would be mind boggling to me!
It IS asking a LOT from a significatly lower prressure round to RELIABLEY function cycle a Bren! Im not saying it cannot be done, but i have my own doubts. There may possibly be some drastic machining done internally to relieve weight, Etc? Without seeing one, it's difficult to state for certain. Agreed?
We have the opposite end of the spectrum with the FG 42 dont we? a TOO powerfull round for the intended role of assualt rifle! BUT, If it had been made in 7.92mm KURTZ at the time..........
OR, Indeed, THEN, a working postwar coversion into 7.62 x 39mm IF, there were enough FG 42's about to convert without ruining one because of it's rarity!.......No Problem!

[Kev G]

[MGMike]

Given the fact that this is a "kit" derived from a Chinese-made or converted barrel, I think I would start by very carefully examining the chamber in strong sunlight and, if necessary, making a cerrosafe chamber cast. If the chamber is rough, or scored or of improper profile it could be causing enough drag on the fired cartridge case to emulate insufficient gas.

M