[wideminded]

The 1.67-pound piece of fuel tank foam insulation shot out of a 35-foot nitrogen-pressurized gun and slammed into a carbon-reinforced panel removed from shuttle Atlantis.

...

This time, the entire 11 1/2-inch width of the foam chunk — rather than just a corner during previous tests — hit the wing, putting maximum stress on the suspect area.

I'm curious about this gun. Doesn't the piece of foam need to be shaped to fit the barrel ... or not?

[Darksheare]

Not really.
They have a huge pneumatic gun used to fire chicken carcasses into aircraft windscreens to test their durability and resistance to bird impact during flight.
The tube is round, the chicken isn't but the speed they shoot the uncooked beasty at reaches jumbo jet speeds close to 500 MPH or so.

The French bought such a gun off of us a long time ago.
They were horrified when the first chicken shot out of it smashed through their test windscreen and made a huge dent in the back cabin wall. (Full scale mockup test.)
They asked if this was normal, our techies told them:
"Thaw the chicken first."

[TC Rider]

Imagine a plunger that fits the barrel. Anything placed on the plunger will be accelerated.

This is basically how your 'guy shot out of a cannon' at the circus works.

[El Gato]

I'm curious about this gun. Doesn't the piece of foam need to be shaped to fit the barrel ... or not?

It does, but this "gun" has a square or at least rectangular bore. That is one of it's changeable barrels has a none circular bore. It is a smooth bore, so the "projectile" need not rotate. Plus it uses a sabot and a stripper to "hold" the sabot. The sabot provides the seal to the barrel as the nitrogen gas pushes it and the payload down the barrel, but it then stripped off from the payload, so that only the payload hits the "target".

[El Gato]

I'm curious about this gun. Doesn't the piece of foam need to be shaped to fit the barrel ... or not?

It does, but this "gun" has a square or at least rectangular bore. That is one of it's changeable barrels has a none circular bore. It is a smooth bore, so the "projectile" need not rotate. Plus it uses a sabot and a stripper to "hold" the sabot. The sabot provides the seal to the barrel as the nitrogen gas pushes it and the payload down the barrel, but it then stripped off from the payload, so that only the payload hits the "target".