thompson blowback mod

I think it has been mentioned here before, but most don't do it because it requires more maintenance, and doesn't look right anyway.
The bolt handle should be back at rest, then fly forward and back when fired, not the other way round.

Yes if you do it as shown it will jam. The rod must be much shorter to clear the underside of the rear sight and I added a spigot bush to the end of the rod inside the gearbox so it runs centrally and smoothly inside the main spring. All electric blow backs are high maintenance it's just the price. I've had my thompson since they came out, it's just cool to see it do new tricks. But the other comment is true the bolt is in the wrong position at rest. I wont be totally happy till i figure out how to lock the bolt back to prolong life, most manufacturer blow backs have this feature and its a good idea. I would have to say its not for the amateur to try this as it could end in disaster, but worth it to me. I rebuild gearboxes for recreation.

I also looked into both incarnations of the Thompson blowback mod, one which involves cutting a slot in the gearbox and another which utilized a rod fed through the rear of the gearbox to rest on the rear of the piston head.

The first I discounted the first due to it causing what I considered damage to the gearbox. When I examined the second style of mod I came to the conclusion that it was too complex given the tight space to warrant further effort especially when already noted here that the real Thompson operates from an open bolt so the airsoft mod just wouldn't look right.

Been thinking about the bolt position ,might be possible to reverse the system i.e. to have it work on the compression stroke instead of the induction stroke, I should explain that i used a fine cable called craft wire (from hobbycraft) instead of ribbon shown and that I have a lathe and milling machine. You would have to make a pulley and attach the cable directly to the back of the piston. The pulley could be mounted to the back of the gearbox by the thread in the spring guide with a m5 bolt centre drilled hollow to pass the cable through, this would leave the bolt in the right position and would have no rod in the gearbox so much less mass to move. If the cable failed it would probably be less damaging to the gearbox as well.

Best of luck with all that, I'll stick to the path of least resistance and "put up with" an undamaged and working Thompson!

been working on it today, some clearance problems and its fiddly to assemble but the components are there and i think it should work,will post pictures if it does, its actually simpler than it sounds.

No, doesn't work. The power of the main spring just breaks the cord (was using very fine stainless steel cable, very strong stuff no thicker than fishing line) every time. Anything thicker would be to heavy to work, sorry.

A mate of mine did it. Used ribbon. It worked. Right up untill it busted the internals of his gun.