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Thread: Trapdoor is .357 mag?

  1. #1
    Boolit Master
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    Trapdoor in .357 mag?

    I've read where someone converted a Springfield Trapdoor to 38-55. Was wondering if anyone has tried .357 magnum.

    I did some bolt thrust calculations and come up with 3948 psi for .357 mag and 5041-5644 psi for 45-70 Gov. (Range is based on 25-28k psi for 45-70 because I find both these listed for max pressure.) That puts .357 at least 1093 psi inside the 45-70 Gov. bolt thrust. So it's doable as far as bolt thrust.

    Just wondering if anyone has tried it? I have a barreled receiver I got from a member here almost two years ago that had two divots ground at the breech of the barrel yet the receiver is undamaged. The barrel could be repaired but I thought "what if"? Oh... and Numrich has Star rolling block barrels in .357 for $40! ...Hmmm...

    Here's what the barreled receiver looks like.


    And unscrewed.


    Good receiver.
    Last edited by tbx-4; 02-07-2018 at 12:06 PM.

  2. #2
    Boolit Master Moleman-'s Avatar
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    Sounds like a sweet idea! Would the extractor have a little sideways leg on it to reach the smaller rim?

  3. #3
    Boolit Master
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    Yes. One would have to be made like an original but with L shape to reach under the rim of the small case. It wouldn’t have to be much of an extension. One could buy a drill rifle extractor and do a little TIG welding on it, they’re inexpensive. I happen to have a friend with a TIG welder. Or, one could just hack something from a piece of steel.

  4. #4
    Boolit Master Char-Gar's Avatar
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    I would not do it. The Trapdoor lockup is a very weak sister.
    Disclaimer: The above is not holy writ. It is just my opinion based on my experience and knowledge. Your mileage may vary.

  5. #5
    Boolit Master Moleman-'s Avatar
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    The bolt thrust is about 1/5th less with a 357Mag than with trapdoor 45-70 loads so it will be easier on it than shooting it with a 45-70 barrel. About the only thing I'd be concerned about would be the firing pin dia/clearance/protrusion.

  6. #6
    Boolit Master

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    Quote Originally Posted by tbx-4 View Post
    I've read where someone converted a Springfield Trapdoor to 38-55. Was wondering if anyone has tried .357 magnum.


    Numrich has Star rolling block barrels in .357 for $40! ...Hmmm...


    Sounds like a great idea -except for your choice of a barrel! Numrich has a reputation for selling some really poor quality barrels, some decent ones sometimes also but it's a ****-shoot. For just a very few dollars more you can get a really GOOD quality barrel blank from Green Mountain and make a real shooter out of that thing.

    http://www.gmriflebarrel.com/bx357-2...ned-bbl-blank/

    I have used GM barrels on several projects and I couldn't be happier with them, top notch all the way. My little "Baby" HighWall scaled down 1885 used a $39 Gun Smith Special raw blank in 22 Mag and it's a real tack driver, those Gun Smith Special deals they have are some of the best deals anywhere!
    Last edited by ReloaderFred; 02-08-2018 at 11:59 AM.
    Statistics show that criminals commit fewer crimes after they have been shot

  7. #7
    Boolit Master uscra112's Avatar
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    AMEN! Numich buys reject lots to sell, I am totally convinced. Been there, burned the T-shirt out of sheer embarrassment.
    flectere si nequeo superos, acheronta movebo

  8. #8
    I have one if those Baby rolling block barrels from numeric. They are basically a contoured blank with a 1/4 inch cone breach unchambered. Outside is rough turned but the bore is great, minimal tool marks and smooth. If you need dimensions let me know I'll take some. The thing about numeric is to be willing to call and talk at someone till you get a return label to ship JUNK back occasionally. I've only had to do it 3 or 4 times. The last being a new 357 that had one whole side of the bore rusted out while full of cosmoline. I like the green rivers gunsmith blanks too, only problem is they don't make them in 358 at a price I can afford for my dink-around projects.

  9. #9
    Boolit Master
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    fireby,
    I ordrerd one and have it in my hot little hands now. It’s just as you say, a little rough on the outside but very good bore. Nothing a few turns on a lathe and a little emery cloth can’t fix as far as the out side goes.

    I have measurements of the TD barrel shank and the damaged barrel with another good barrel to compare as well. Thanks for the offer.

    Just need to take it over to a friends place to turn it down and cut some square 10 tpi threads.

    Here’s the TD barrel and the .357
    Last edited by tbx-4; 02-08-2018 at 03:23 PM.

  10. #10
    Boolit Bub
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    If that original barrel has a good bore it can be salvaged. I've done several over the years by cutting off the torched part, sleeving part of the chamber with an extension to bring it back to the original length then rechambering. The relief for the hinge and extractor cut have to be made. My best shooting trapdoor was done that way and it's a winner.

  11. #11
    Boolit Master Jedman's Avatar
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    A 357 trapdoor sounds really cool.

  12. #12
    Boolit Master
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    Hi tbx-4: I think you got that barrel from me awhile back. If I remember right it had a nice
    bore, but a screwed up chamber from someone trying to make a lamp. You could drill it out
    and put a stub chamber in it. Or maybe cut the chamber off and recut the threads and rechamber
    it back to 45/70. Then make a 357 barrel and make a switch barrel rifle. A lot of possible combinations
    to try. I'm not a machinist or I would have played with it. but it was beyond my capabilities so I
    sold it. Keep us posted on this project as I like to see these old guns put back in shape.
    webfoot10

  13. #13
    Boolit Master
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    webfoot,
    Yes sir, I got this from you! Finally getting around to doing something with it.
    I like the idea of a switch barrel but how would you do that? Unscrew the barrel every time you changed calibers?

    But then fixing the old barrel is just the seed for another TD build!

  14. #14
    Boolit Master
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    Deadeye,
    The damaged barrel has a bore that looks unused. No kidding... but it’s been cut down to 27.5”! Would make a nice carbine barrel though.

  15. #15
    Boolit Master
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    Deadeye,
    Sent you a PM.

  16. #16
    Boolit Bub
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    PM answered.

  17. #17
    Boolit Man
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    About 25 years ago I saw a beautiful Trapdoor cav carbine that had been converted to .357 on the shelves of a now-defunct gun store in Kansas City (CR Specialties, if any of you remember it). It looked like it worked, but I never saw it fired.

  18. #18
    Boolit Master
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    Well, I'm pursuing the .357 Trapdoor project but I forgot about this thread so I'll up dated it.

    After getting the Numrich barrel I needed a a little tool to make this happen... a lathe. So I started looking on craigslist but didn't see anything I liked. Called some machinist buddies and one thing led to another. I ended up with an old Atlas/Craftsman 12" lathe for $300. I cleaned it up and fixed a few things then started making chips.

    Digging through the box of old HSS tooling it came with I found one that looks like it was used for cutting square threads. Practiced cutting square threads on some scrap stock till I broke the tip. Had to re-grind the tool and it came out ok.
    Using another tool that I sharpened, turned down the .357 barrel shank to match the Trapdoor. Then started cutting the threads. The lathe is over 75 years old yet it still does a decent job.

    The lathe. I added the chip pan.


    Setting up change gears for 10 TPI.


    Threads finished, parting barrel stub.


    Taking the edge off the breach end with file and emery cloth.


    Not too bad.

  19. #19
    Boolit Master
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    After filing the relief for the hinge I marked location for extractor.


    Trial fit to receiver.


    Found that the extractor is offset to the outside, if I made one a mirror image that is offset to the inside it will engage the .357 rim.
    I just need to whittle a piece of 1/4" plate down to a new extractor. Using a little bench grinder and files I did just that.



    And done. Trying to hold that little piece and file on it is hard on the 'ol fingers!



    Here you can see the offset.

  20. #20
    Boolit Master
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    Ordered a .357 reamer and cut the chamber.


    Before I cut the relief for the extractor, but with the chamber cut, I put it all together and went out back and shot some .38 sp through it. Sound like a quite .22 with about same recoil. Tried some .357 mag and now it sounded and felt like a rifle. The .38 will be great for plinking and those who are "recoil sensitive". The rifle is coming out on the light side at 6.5 lbs. I like that.
    Stay tuned there's more to come..

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check