Repairing Bren Mk 1 barrel

[Joe H]

Guys,

I got interested in this thread because as I have recently posted I have a L4A3 parts kit without a barrel and since it becomes increasingly possible as time passes that Sarco isn't going to be able to supply a US made L4 barrel. If this comes to pass I have a 7.62 barrel blank on to which I'm going heat shrink the interrupted threads from a Bren barrel on the barrel blank and then turn the blank to L4 dimensions.

That said, in any Bren barrel the rear section of the Bren barrel around the chamber is only .9" in Diameter. The blank I have is Rc 28-33, very similar to Dutch spec consequently the Yield is around 90,000psi. and the barrel blank has been heat treated.

Here is info I found on the various calibers. Proof on .303 estimated at 33% above normal.

Max chamber psi , Proof psi

.303 British 49,000 , 65,000

7.61x51 Nato 50,000 , 67,500

.308 Winchester 62,000 , 83,000

Considering the chamber as a thick Walled cylinder:

Normal Stress Chamber walls psi, Proof Stress Chamber Walls psi

.303 British 47,300 , 62,850

7.61x51 Nato 48,400 , 65,300

.308 Winchester 59,900 , 80,250


All the proof loads are above the Yield of the un-heat treated steel (62,500 psi). The normal .303 & 7.62 have a factor of safety between 1.29 & 1.31 using untreated 4140 type steel. The max loads on the .308 have virtually no factor of safety using annealed 4140.

All stresses are below the 90,000 psi yield of the heat treated 4140. I guess this tells why why the barrels are heat treated. It also shows the items to be considered when dealing with modifying a barrel.


Joe

[Richard-]

Why can’t you import barrels? Is there a law prohibiting their importation like we have in the US?

Yes, unfortunately MG barrels can only be imported for Government use.

That's very interesting Joe, re: the heat treated barrels.

It never occurred to me to consider that.

I'll contact the barrel blank manufacturer to verify the specs of the steel they used, and the heat treatment process, if any.

[Lee Enfield]

Joe H, if you're capable of threading, why not cut the threads onto a blank (assuming you can get 1 big enough?).

Dispense with the fancy lightening cuts and follow the British mk2 contours.

Just mill out the interruptions from the thread, and time the gas port to the threads.

"Easy peasy..."

---------- Post added at 01:41 PM ---------- Previous post was at 01:40 PM ----------

Joe H, if you're capable of threading, why not cut the square threads onto a blank (assuming you can get 1 big enough?).

Dispense with the fancy lightening cuts and follow the British mk2 contours.

"Easy peasy..."

[Flying10uk]

I haven't had any experience of welding gun barrels but I have had some experience of "emergency repairs" of cracked tool dies by welding just to get a job out on time before a more permanent repair can been made. I have found that for some inexplicable reason MIG welding always seems to give much better results than TIG and the only reason that I can think of is that the welding process is much faster. The problem with welding a higher carbon steel is that the weld it's self becomes very hard and the area immediately around the weld also seems to become very hard and this is often where the joint may fail. I am very wary about welding any higher carbon steel and it's very much a case of keeping your fingers crossed that the weld is not going to fail.

[Woodsy]

Richard, I have done the repairs you speak of a couple of times in the past and the results were satifactory. The threaded plugs need to be a very neat fit and have a bevelled top to give an area for weld penetration. I just used 'as supplied' 4140 bar and TIG welded the plugs to the barrel. I made the threads a tight fit, and allowed about 0.5mm protrusion into the chamber, tidied them up as close as possible to the chamber walls with a long shank die grinder, then carefully tickled out the remainder with a chamber reamer. By this method I have done Vickers, Bren, and Sten barrels with good results. Sleeving the chamber as suggested by Peter is also a good method (I recently did a Garand with a pitted chamber this way) but the junction of the sleeve needs to be no further forward than halfway up the neck or you run the risk of gas cutting. The sleeve also needs to have a very neat fitting thread. To readers in the US, please be aware that we are subject to not only some restrictive import rules (NZ is much easier than Australia but still a pain in the butt) but also the whims of the US State Dept who frequently prohibit export of many items. Another factor is cost as a State Dept export application cost US$250.

[Joe H]

Joe H, if you're capable of threading, why not cut the threads onto a blank (assuming you can get 1 big enough?).

Dispense with the fancy lightening cuts and follow the British mk2 contours.

Just mill out the interruptions from the thread, and time the gas port to the threads.

"Easy peasy..."

---------- Post added at 01:41 PM ---------- Previous post was at 01:40 PM ----------

Joe H, if you're capable of threading, why not cut the square threads onto a blank (assuming you can get 1 big enough?).

Dispense with the fancy lightening cuts and follow the British mk2 contours.

"Easy peasy..."

Lee,

One of the issues is the barrel blank diameter. I can get a new 1.25" dia blank with the correct twist for $50. I also have a number of MKII barrels. To cut the threads you need a larger blank. I couldn't find one with close to the 1:12 twist for other than a custom price. The barrel and thread OD is 1.260". IMO just a little too big for a 1.25" blank plus I can cut threads but for a hobby machinist, like me, those thread details look pretty daunting

Joe

[Lee Enfield]

Lee,

One of the issues is the barrel blank diameter. I can get a new 1.25" dia blank with the correct twist for $50. I also have a number of MKII barrels. To cut the threads you need a larger blank. I couldn't find one with close to the 1:12 twist for other than a custom price. The barrel and thread OD is 1.260". IMO just a little too big for a 1.25" blank plus I can cut threads but for a hobby machinist, like me, those thread details look pretty daunting Joe

I know what you mean, I measured an "oversize" blank I have for a friend with an eye to spinning out a .308 barrel and found the thread diameter to be just a little over the commonly available.

Having said that, I'm not entirely sure the .010" would make any actual difference for a hobbyist as the thread height would be .005" undersize.

[Joe H]

I know what you mean, I measured an "oversize" blank I have for a friend with an eye to spinning out a .308 barrel and found the thread diameter to be just a little over the commonly available.

Having said that, I'm not entirely sure the .010" would make any actual difference for a hobbyist as the thread height would be .005" undersize.

Lee,

I may give it try. I can turn the threads on the end and try them out. The blank is a few inches longer than I need. If it doesn't work out I can just turn them off and go the shrink fit route. If they look good I can continue them and turn off the part I don't need

Joe

[Flying10uk]

Why not have a trail run of cutting the thread on a piece of mild steel first and get your method perfected before starting on the actual job. If you are able to lathe cut small diameter screw threads you should, in theory, be able to do larger threads. The question of whether or not you are able to tackle the job rests partly on the size and quality of the lathe which you have at your disposal. No one has ever really shown me lathe screw cutting although people have given me tips and so I have probably developed my own methods over the years. Personally, I tend to leave the lead screw engaged until the thread is completed taking the tool out of the thread before reversing the machine. Others will disengage the lead screw after each cut and rely on the counter to reengage at the correct position but I tend not to this. It is also helpful to have a digital readout on your lathe for this type of work.

[Joe H]

Lee and Flying10uk,

As usual the devil is in the details. The Bren is a Czech design so it was designed in the metric system. The Bren barrel thread pitch is 7mm that's 3.628 threads per inch. It gives me a headache thinking about what change gears I would need on my Clausing Colchester lathe to get that thread pitch. I'm sure it can be done.

Back to my heat shrink idea

Joe