Rockets and tubes

[randombloke]

Recently, I got into a discussion about Gyrojet guns and related topics, wherein someone proposed that:

if you fire a rocket from longer tube (barrel) it actually accelerates faster than when fired from shorter tube (barrel). This is because while in tube (barrel) the pressure from the burning rocket-fuel does not escape to the sides but is actually quite well contained and pushing the rocket (bolt) forwards with more energy. So even with a rocket weapon the lenght of the barrel is only immaterial if the rocket is ignited AFTER it leaves the barrel.

Now I want to know whether this is in any way true, since my intuition is telling me that overpressure in a sealed tube behind a rocket is going to interfere with the exhaust velocity and at best do nothing to the acceleration.
Can any of you folks with more mathematical minds help out?

[echnaton]

I could see either condition being true under specific circumstances. Pressure in the tube could propel the rocket out like a bullet. On the other hand, the design to take whatever advantage there may be utilizing the back pressure, may be more detrimental than beneficial to the function of the rocket.

However this one of those situations where saying "it isn't rocket science" does not apply.