Which is best 9mm or 45 cal round.

I've read many forums claiming one over the other and many choosing alternate rounds to get the exact desired effect.  Speaking predomantly about those who use service arms for protection like police officers.  A great deal of focus is on penetrating power and some departments take in consideration over penetration, considering most of their gun play will be in urban environments with houses and perhaps bystanders near by.  The thing which is most often left out of the conversations is the laws of physics.

As a general rule in the laws of physics (Newton's), an object will remain in motion until acted upon by another object of equal or greater force.  Many other factors are also determinant of the reletive penetration power of a round or caliber of round.  It's reletive speed at impact, the weight of the round in grams, the diameter of the round and the ballistic coefficient of the round base off of its characteristics.

A 9mm round is smaller and generally can be made to travel at very high speeds, it will have less surface area than the larger round 45 cal. given these facts coupled with a reletive ballistic coefficent "meaning the demention and air dynamics characteristics and shape", the 9mm round generally will have more penetration power.  Reason being, upon its reletive speed at inpact will be generally higher than the 45 cal round and the resistance density of the target acts upon less surface area of the round and it carries the existing residual energy with it as it travels.  On the other hand the 45 cal round having a slower rate of speed but greater diameter exerts greater force upon initial impact because the velocity and weight transffers through kientic energy upon impact at a greater rate than the 9mm.  However, most of the energy is lost due to surface resistance to the median and because of lower speeds upon impact.  The equations change with the ballistic coefficient of the round however.  Whereas, a hollow point shall transfer the kientic energy at a greater rate than a jacketed ball ammunition; primarily due to expansion within the median, which add greater resistance.

Therefore, if I were a officer in the field in and urban environment, I would chose a round based on the greatest energy transfer with the least amount of penetration value.  I would be likley carring a 45 calibur hand gun.  However, if I was a soldier doing house to house urban assult than I might choose the 9mm round.  Reason being, I might have to shoot through something to hit my target.  That isn't to say the 45 calibur round will not penetrate a door or wall at close range, however as stated above much of the energy is transferred upon its initial impact verses the 9mm round.

I would give stopping power to the larger 45 calibur because of the same reason law enforcement people would choose the calibur in the urban settings.  There is a balance of both by changing the type of ammuntion based on its ballistic values.

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