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M3 50 bmg barrel

Rodeo

Plastic
Joined
Jul 26, 2005
Location
St. Louis area
I am interested in building a single shot bolt action 50 bmg rifle using a new M3 barrel and a barrel extention. I think the 3/4" thread length of the original M3 barrel is too short for the unsupported weight of the barrel. I could make these threads longer by reducing the bbl. diameter and cut threads just past the headspacing notches. OR I could simple thread the large out side diameter of the barrel. Either way I want to use the existing chamber. I would like to hear your opinions on either method. Pro and Con
Thanks for all your input.
 
I had a similar decision to make with my M2 barrel. I felt that continuing the original thread forward any more would weaken the chamber area too much for safety. I never considered threading the forward part as you suggest, but did consider clamping on the cylindrical section there to support the barrel. That is what the "John's Rifle Plan" guy did, I think.

I ended up buying a chamber reamer, cutting off about 1.5 inches off the end of the barrel and then re-threading to something like 1.650" or 1.675" x 10 tpi.
 
By the way, if you want any accuracy, I'd recommend getting a commercial barrel and a match or semi-match (if there is such a thing) reamer. The MG barrels and the GI-spec. chamber with its hugely oversized throat area are fine for shooting pulled API and stuff, but mine doesn't shoot too impressively. Accuracy-wise, I mean.
 
Bed the barrel (6 inches, maybe) instead of the action. Then you won't have the action trying to hold up the heavy barrel.
 
Forget using the exsisting threads and cut them off. You'll probably want to recontour the whole barrel.

I've made several single shot rifles and used the existing chamber.There is plenty of meat on the barrel to cut off without any danger.

I recut the thread and had an exellent fitup to the reciever. Using the Maddi Griffen design uses a bushing that is shrunk on at the foward end of the shroud which supports the whole barrel. With the support from the reciever and the bushing, it makes for a pretty accurate platform. I used new never fired milsurp barrels.
Although the chamber is loose as far as cahmber dimensions go, with once fire ammo sized in the chamber and neck sizing the cases, I can shoot one ragged hole at 200 all day long. The key is to tailor the ammo for the specific chamber. You'll have to experiment a bit, but thats part of the fun.
 
I done moved to Japan before I got my reloading press made, so all my shooting's been with some Talon ball ammo that I bought back during a period of poor quality control at Talon. About 30% of it won't even fit into my properly-headspaced chamber, and some fits in with .020 or more play.

In my judgement the original MG thread is already on the small side diameter-wise, the only good thing being that it's so short and is re-inforced by the much thicker metal nearby. I advise against turning it down any smaller. Especially smaller and longer -- that's my opinion. I'm no engineer, and maybe being simplistic, but consider that the latest short magnums are about .550" head diameter and most modern bolt actions have barrel shanks close to 1 inch diameter. Just scale up the proportions. To say nothing of a standard cartridge head in a big 1917 Enfield or large-ring Mauser barrel.

If a barrel was also supported by a bushing up forward in a long tubular receiver, then the original MG barrel thread might be fine if the threaded hole was made to closely match it. I had also thought about that, but my design didn't allow it.
 
Thank you guys for the good information. I think I'll turn and thread the outside of the bbl. to 1.625 X 12 tpi and leave the original threading as is, in order to utilize the new chamber without sacraficing any structural integrity.

More suggestions ? Keep em coming !
Thanks again
 
If you were really good you could use the original pitch on a larger diameter and synchronize the two threaded areas. :D
 
does anyone know the major diameter of the threads on a M3 aircraft barrel. I want to buy one for my project but I want to make sure there's enough meat to turn and thread it t oa new dimension
 
If i'm not mistaken, you can get an outstanding, new match grade barrel from places like Montana Rifleman or Pac Nor for the same price as you would pay for a halfway decent milsurp .50 barrel.
 
If you were really good you could use the original pitch on a larger diameter and synchronize the two threaded areas.
Should be easy enough. Engage the threading dial on any line (even thread). With the tool clearing the old thread, stop the lathe and, using the cross slide and compound, pick up the old thread. Back off the cross slide only to clear the new thread diameter. Disengage the half nuts and move the carriage to position the tool to just touch the new diameter. Move the carriage so that the tool clears the work and advance the tool with the CROSS SLIDE the single depth of the thread. Zero the cross slide dial or set the threading stop if you have one.Back off the COMPOUND to the new diameter and procede to thread as usual engaging the thread dial on any line (even thread).
 








 
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