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Posted: 10/25/2025 10:55:18 PM EDT
[Last Edit: 10-8DoWork][Edited]
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I’m chopping a 14.5 Simunition barrel down to 10.5. There is no rifling past about ~9” so I’m just cutting a metal tube off. I won’t have to be concerned with a crown or anything. Once I get the barrel cut to the desired length, I need to use a lathe to reduce the barrel diameter where the muzzle device installs so I can thread it to 1/2x28. What is the outer diameter I need to thread 1/2x28? Is there any trickery to it or is it just half inch OD? I found a document that says .498” OD. Is there an acceptable range? Do you guys have any experience with this? I’m not a machinist and just want to check with guys who have done this kind of stuff before I cut. |
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Originally Posted By TGWLDR: .600" O.D. .498" major diameter for 1/2"-28 threads. I presume you mean .6" OD for the muzzle device to butt up against. OP - there are plenty of diagrams with all the dimensions you should need. If you aren't experienced at thread cutting you may want to have them cut by someone who is - it can be tricky. |
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Originally Posted By mirion: Maybe the OP is planning on using a die versus lathe cutting the threads, not a good idea. Why is this not a good idea? I’ve watched some videos of people doing real barrels this way. This is not for live ammo. It’s simunition. Also the part I’m threading has no rifling, no crown, and never makes contact with the projectile. It’s not a critical piece. The threads just need to be there to thread on a tube that matches the same length as my work guns suppressor. |
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Originally Posted By Cycolac: I presume you mean .6" OD for the muzzle device to butt up against. OP - there are plenty of diagrams with all the dimensions you should need. If you aren't experienced at thread cutting you may want to have them cut by someone who is - it can be tricky. Originally Posted By Cycolac: Originally Posted By TGWLDR: .600" O.D. .498" major diameter for 1/2"-28 threads. I presume you mean .6" OD for the muzzle device to butt up against. OP - there are plenty of diagrams with all the dimensions you should need. If you aren't experienced at thread cutting you may want to have them cut by someone who is - it can be tricky. Yes, the O.D. of the barrel in relation to the threading in order for a device to mount. Possibly I misread the OP  OP, your thread dimensions for 1/2"-×28 will be as follows: Major: 0.4989"-0.4924" Minor: 0.4602"- 0.4488" And definitely, as stated above, a task best left to the experienced. Edit: I am far from anything but a hobbyist machine operator, I'd definitely consult a knowledgeable individual. |
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Originally Posted By 10-8DoWork: I’m not a machinist and just want to check with guys who have done this kind of stuff before I cut. And here we go !! Cut barrel down with a saw about .050 longer than your finished length. Chuck barrel in lathe. Use right cut facing tool to face the barrel to finished length. Use radius nose to turn area to be threaded to .498-.499 max OD Use right cut facing tool for the shoulder corner after turning the barrel to the required OD for the threads. Use right cut tool that is set for the thread pitch angle of the end of the barrel where the threads start and then add the external thread chamfer. Set machine to the lowest speed setting (like 25-40 rpm's), place 1/2x28 die facing correctly into the die holder. Verify tailstock chuck is in plane and parallel with your barrel. Place die holder into tailstock chuck, and verify the locks are slightly loose to allow it to slide along the bed rails. Slide tailstock close to the barrel, unlock the tailstock quill, and adjust quill to the rearward position, and lock tailstock to bed. Quill should move freely back and forth, but not be sloppy. Coat with oil if it's dry and run it out, then back in. Verify tailstock chuck is in plane and parallel with your barrel again. Start machine and verify barrel is turning in the correct rotation, and gently move quill close to barrel. Critical you have your hand on the off button, and foot on the brake. Add cutting oil onto barrel and die, and slowly move die onto the barrel. As it cuts, it will slowly pull the quill towards the barrel. Stop the machine (turn off) and let it rotate about 1 revolution when the die gets close to the barrel shoulder. Remove die tool from quill, move tailstock back, remove die holder from die, get die handle and continue threading the last bit by hand until it hits the shoulder. Blow off chips, slowly back off die using die handle. Blow off chips again, and use a 1/2x28 nut or your muzzle device to see how bad you fucked it up. Deburr ID of barrel with a chamfer tool mounted in the tailstock chuck while the lathe is still at the lowest RPM setting. Bring it in close, lock down tailstock, and use the quill to just lightly kiss the end of the barrel with the chamfer tool. OP.....before you do the above on a barrel, practice on some aluminum or brass bar stock a few times. Watch some YouTube videos OR....just have a reputable shop knock it out for you, like ADCO. . |
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I don’t think folks understood that you would never, ever, put anything other than a FH on a sim barrel. Concentric threads don’t matter at all so long as the plastic projectile won’t hit the FH. It isn’t going fast enough to worry about anything else. Never use simunitions thru a can unless you want to really screw it up or it can come apart to clean. Sounds like OP is using a drop on upper which uses 9mm Sims (actually 7mm diameter). A .223 drop in bolt and .223 sim rounds is a far better setup. |
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