Recently, I've been looking at putting a thompson barrel on an AR-15 upper. The only issue is that the threads don't match. If I were to make a thread 'converter,' ie a piece of round stock that can convert the threads on the AR Upper (1.250x18) to something more reasonable as 7/8-20 or the like, would that cause a problem at all?
AR Upper Thread Converter
- Thread starter OneWound
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MachIVshooter
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Most certainly is not the only issue. Unless you have threaded that T/C barrel and installed an AR barrel extension, then made a collar that holds the T/C barrel at the right depth, this idea is a no-go.
What is it you're trying to accomplish, anyway? It's not like AR barrels aren't available in a host of chamberings, and usually at a lower cost than T/C barrels.
I want an AR with a thompson barrel on it because they are rarely seen.
MachIVshooter
Member
There's a reason.
Are you talking Thompson SMG ribbed barrel?
That would be a .45 ACP, eliminating the gas system and simplifying the bolt/breech mating situation, but is still an advanced project that's going to require professional machining to be right/safe.
Also gonna look rather funny, IMO, but if that's what floats your boat......
Yes I am. And yes I understand that, my main concern was having the barrel walking out on itself after firing, or if I would lose any sort of material strength. My guess is that the adapter would have to be made out of 7075-AI or maybe 304 SS? Was hoping someone would be able to give insight before I did something that was proven to fail.
MachIVshooter
Member
Well, first you would need a .45 ACP AR bolt & carrier to get your dimensions right.
Next, you would want to make an adapter that either threads or pins onto the rear of the Thompson barrel that has an AR barrel extension profile on the outside. If I were going to make such an adapter, I wouldn't use either of the materials you're considering. 4140 PH steel or one of the free machining tool steels like A2 or D2 would be my choice. Heat treated and tempered when finished, of course. They are readily available, inexpensive, easy to machine in the annealed state, and far stronger than 304 stainless (a non-heat treatable grade) or 7075-T6.
Then you're gonna have to figure out your barrel nut. Easiest way is probably going to be a standard nut, less delta ring.
Finally, just come up with a way to attach forearm.
Next, you would want to make an adapter that either threads or pins onto the rear of the Thompson barrel that has an AR barrel extension profile on the outside. If I were going to make such an adapter, I wouldn't use either of the materials you're considering. 4140 PH steel or one of the free machining tool steels like A2 or D2 would be my choice. Heat treated and tempered when finished, of course. They are readily available, inexpensive, easy to machine in the annealed state, and far stronger than 304 stainless (a non-heat treatable grade) or 7075-T6.
Then you're gonna have to figure out your barrel nut. Easiest way is probably going to be a standard nut, less delta ring.
Finally, just come up with a way to attach forearm.
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