2 piece blank barrle for mg 13

[blckwlfny]

Thanks for the advice. Ill price out a short run and see if I can recoup. Good idea!
On the design: I'm a little ignorant here on tolerances....
I understand that I must keep the piston end "straight"...no taper. As a result I'm running out of material thickness on the outer barrel because of the drastic taper of the barrel itself. As the outside diameter gets smaller, the inside doesn't.
What do you think the minimum wall thickness could be on the outer (long) barrel?

[Blanksguy]

That will depend on the piston-length and diameter designed into what you want to build..

Questions...
1: What is the barrel-diameter (all measurements from front of "flair"....forward.).......at:
A: 3"
B: 6"
C" 6.5"
D: 7"
E: Diameter just in front of "flair" (?).

Regards, RichardS (Temp. in SA, TX.).
US Army, Retired.

[blckwlfny]

Thanks for your patience
Diameters:
@flair 1.10"
3" :1.10"
6" : .96"
6.5: .93"
7": .91"
....the .308 reamer is .480 and it looks like I will have to bore and sleeve an existing chamber...that needs about .580 total O.D.
The barrel extension is 2.7 lbs do you think this mass makes it at all possible to get away with a. 3" stub beyond the flair?...this gives me about .100 wall thickness on the outer barrel for about 3"
...or is mass not the issue?
Thanks

[Blanksguy]

Mass is not the issue.....you are already removing the weight of most of the barrel using the 2-pcs. blank-barrel-assembly.

What is the issue is the amount of unsupported left/right movement and up/down movement if the piston is too short. You want the piston portion to last.....and not leak pressure during firing.

First.........at about 3" above the "flair, you have a measurement of 1.1" for barrel-diameter just in forward of the "flair".....correct (?).
1.1" subtracting a piston-diameter of .68"........would give you .42" ............correct (?). .42" divided by 2 would give you a wall thickness of about .21"........correct (?).
.....just remember that the chamber is for .308 .......... the rest of the barel forward of that is just 8mm rifled-bore.

Last.....remember that the piston needs to have .005"-.010" running clearance (loose-fit).....(IE: The difference between the piston outter-diameter and the piston-bore inner-diameter.).

Regards, RichardS (Temp. in SA, TX.).
US Army, Retired.

[blckwlfny]

Blanksguy, thank you.
After some research, I'm coming at this problem with a different approach so please bear with me. OK, Just to clarify
on these, unlike the barrel extension on the mg 42, the flair encompases the chamber/ battery and the chamber itself extends immediately into the piston stub. The shoulder of the cartiridge sits about 0.5" ahead of the flair. This forces me to have much of the chamber in the piston insert creating a large diameter and/or a thin wall unless it is stepped to accommodate taper

QUESTION 1: What would happen if. I step down the piston after the chamber?

I'm considering a possible design change. It is to mill a damaged L1A1 barrel and make it the entire "piston" by fitting it into a collar made of the mg13 barrel threads. The problem once again is that it tapers as well
At 5" from the end of the chamber, the OD of the L1A1barrel is 0.712...and narrows from there
The chamber is a nominal 0.480
...with no "step down" this gives me a wall thickness at the chamber of .116


QUESTION 2: Assuming that this is surrounded around the middle of the cartridge body by the flair, is 0.116 too thin a wall for the chamber at the shoulder?

Thank you

[Blanksguy]

Unless you machine a new outside (forward-section) you only have a set amount of metal with the original barrel as the forward-section. I believe that you will still have enough metal to make it with a straight-piston.

You should still have enough metal to do the piston as the outside of the barel doesn't "tapper" until you get farther than .5" ahead of the "flair".........your idea to machine a step-down forward of a set distance on the piston will work......but will make that part a bit smaller in diameter with that design. Just insure that the piston does not "bottom-out" at either the end or at the "step".

Regards, RichardS (Temp. in SA, TX).
US Army, Retired.

[blckwlfny]

Great! Thank you for letting draw upon your experience
Got the design set now. Once its all proven, ill post the drawings for all.
Only one loose end regarding the notches around the circumference of the piston that allow the powder residue somewhere to go...
how many notches would you suggest?
Start them how far from the outlet end of the piston?
How deep ?(.150 wall)
And how wide?
...next post will be a field report
Thanks

[Blanksguy]

To answer your questions in order.....

About 5-7.
Start 1/2 diameter below tip.
Not very deep.....just enough to catch carbon, etc..
About as wide as one wall-diameter.

Regards, RichardS (Temp. in SA, TX.).
US Army, Retired.

[blckwlfny]

SUCCESS !!!
The design works beatutifully
It allows for a significantly larger orifice and produces MUCH more bang with the same blanks
I milled a "piston piece" from a scrap L1A1 barrel
Press fit that into a collar formed of the MG-13 barrel threads
Milled out a second mg-13 barrel for the long piece
Threaded the far end of long piece with a 3/8" x 24 thread for the various sized restrictors made from 3/8" set screws
The only trick is that the barrel retaining clip on the bolt carrier must be shortened to just the right legnth so as to keep the piston from rotating out of the threads but not long enough to contact the lip on the front long piece.

[42rocker]

Wow blckwlfny looks like you did a great job. I thought that it was great that you and Blanksguy could work together on this project sharing the info with everyone as you wen.
Thanks to both of you for sharing.

Again Good Work blckwlfny!!

Later 42rocker

[blckwlfny]

FIELD REPORT:
took it to Odessa this weekend. I think I may need to make the orifice a shade smaller. (from 0.1200" to the next size down numbered bit : 0.1160") I had a couple of ejection failures. But otherwise it functioned beautifully through about 1000 rounds.

Does anyone have any suggestions on what to use to lube the mag's to keep the shorter .308 cartridges from jamming? as well as on the cartridges to keep the plastic from sticking?
I'm also wondering what to use to lube the "piston" inside the barrel.
For both, I had used Lucas machine gun oil. It worked in the mag's but its heavier and doesn't spray as a mist so it was messy to apply... and on the piston, it went dry by the end.
I guess both problems could be solved by being able to thin and spray the stuff...thinning suggestions?... alternatives?
thanks

[blckwlfny]

Field report:
After about 2K blanks, the press fit began to slide and the inner stub began to work its way out of the threaded. Collar that fits into the bolt carrier.
I reset the stub and milled three holes through the outer collar and slightly into the inner stub.
(See pic)
I was careful not to pierce the chamber. And left about 3/4 of the original thickness and created a shallow "well"
I then had the holes filled with weld and ground them flush

Another event later and....it seems to be holding.

[42rocker]

Sounds like a good working design if it went through over 2,000 blanks... Way to go!!!

Later 42rocker