Originally Posted by

**sdfgeoff**
It goes something like this: the world isn't perfect, and solenoids are horrible.

The hope was to accelerate a solenoid to a low speed, and use the impact of it with the bb to accelerate it the rest of the way. Unfortunately, momentum transfer can only maximum double the velocity - even if the impactor is a hundred times heavier than the bb. So to get a bb at 50m/s, you have to get the impactor to 25m/s.

From there you can do the required calculation for force over the distance. To get to 25m/s in 0.01m, you have to accelerate it extremely fast. Assuming a constant acceleration (and thus an average velocity of 12.5m/s, you only have some 0.00083 seconds along the 1cm stroke. To achieve 25m/s in 0.00083s, you need some serious force.

I couldn't find my original spreadsheet, and I recall taking a different approach to solve it, so when doing the math the way mentioned above, I now get some 125kN of force:

Very comforting to know I can't get the same answer twice with different approaches. I'm more convinced by my previous one than this quick-and-dirty approach, so let me know if there's something I'm overlooking (It's probably obvious, but I can't see it at the moment).

Then there's the fact that a solenoid does not provide linear force, far from it. A solenoid rated at 1kg provides barely 200g of force at the far end of it's stroke. One idea we had was to use a neodymium magnet to increase the force without needing to add more electrical energy. However, I'm still not convinced you can get within an order of magnitude of enough force.

Unfortunately this math applies to every direct acceleration method - you need a crazy amount of acceleration. Either this can be done using air to turn a very low velocity spring into a very high velocity air-stream (as in a normal AEG), or using a spinning mass with enough angular momentum to not take too much of a velocity hit when the bb gets accelerated (ie the mass of the spinning bits needs to be several times the mass of the bb).

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