I still say you have to sum your forces which is conservation of momentum(assuming elastic collisions with respect to Kinetic energy), Newton's 3rd law though Newton's 1st law is for an isolated system. For conservation of momentum, the gun, powder, bullet, all are at rest; therefore, the initial momentum is Zero; therefore, the final total momentum equals zero - Hence sum your forces and equate to zero. But you can look at your losses, negative forces, like heat mass transfer to the tube, hoop stress as Mr. Hebbes noted and depending on if the diameter of the tube is less than or greater than 20 times(or whatever the definition is) the wall thickness it looks as there could be traveling bulges in forward and rearward directions which leads to barrel harmonics, possibly multi-generation, vibrating, friction because the event is going to be driven to a steady state and the environment is going to do its best to attenuate the progressive event.

Kind Regards,

Raimey
rse

Last edited by ellenbr; 09/06/08 11:46 AM.