Originally Posted by cpttango30
The thing I worry about is presure. The faster you push something the more pressure it takes.
So Pressure is a big deal I think we need to address.
Yes and no.
Of greater concern is how long that pressure is pushing the bullet.
To oversimplify things and use psudo-math (I am not up to the exercise to do it all right now.
If you have 40,000 PSI behind bullet X for 16in i will have speed of 2500 fps
If you change the barrel length to 20in it will 3000 fps.
At 26in it will be 4000 fps.
However, the longer the barrel gets, the moer time the bullet will stay in the barrel and longer the pressure will need to be maintained. This can be accomplised with slower buring powders.
However, care must also be taken that the powder is not too slow burning. If it burns too slow and has a relative gentle pressure curve, it may not be able to provide enough of a kick to swage the bullet in the rifled portion of barrel after it leaves the case. If it does get "stuck", even for a split second, it will cause a signficant pressure rise. (Static friction required much more force to overcome than dynamic friction does.)
There are a lot of variables that all play together and can be very diffictult to model. Sometimes the only way to model and determine the actual properties to by empirical analysis with a strain guage on the barrel, a chonograph and case, bullet, chamber, round, etc inspection.
As someone else above also said, be warry of the bullet flying apart. The exact rotational speed that this will happen at is entirely dependant on the composition of the bullet itself. Special bullets do not need to be used, though. A slower rate of twist will do the trick, and likely help keep pressure down as well. [With a 1 in 10 twist, a bullet at 2500 fps will rotate at 180000
RPM. If the you jump that up to 5000 fps, and change the rifling to 1 in 20, the RPM will remain the same.]
In theory, you could have an VERY long barrel and accerate a .17 bullet to escape velocities as long as you have a consistant source of pressure over the time the bullet is in the barrel.