JBM Ballistics App Questions

[Tesoro]

I use the Ballistics app exclusively. The other day I was shooting 6mm 103gr at 1k and wondered how the trajectory would change with an off weightl bullet. So I put in 102.5 g and the trajectory was the same. And then I put in 104 gr and the same?? With it being such a complex program taking into account a dozen variables it seems that minor bullet weight changes would also be part of the end calculation. Note: I used the same BC so does this overshadow the minor weight change of the bullet? What am I missing here?

 

[jelenko]

If the bc is the same, the trajectory is the same

 

[Tesoro]

But BC is calculated with mass being one of the 3 factors, along with drag coefficient and area. If two bullets have the same drag profile but one has a heavier metal in the core then how can they have an identical trajectory?

 

[Doug Beach]

Two bullets of identical bc, launched at the same velocity in identical conditions, will have identical trajectories & wind drift regardless of weight. One could be 100 grains, the other 500 grains.

 

[jdh47]

Velocity plays as well.

 

[jelenko]

But BC is calculated with mass being one of the 3 factors, along with drag coefficient and area. If two bullets have the same drag profile but one has a heavier metal in the core then how can they have an identical trajectory?

Right - the BC would be different - but the OP is using the same BC.

 

[Tesoro]

Ok..I emailed JBM and got a quick reply: the weight entered is only used to calculate the energy. The BC would have to be changed per the bullet weight.

 

[cwood59]

think significant digits..............

 

[6ShotsOr5?]

Different bullet masses change the internal ballistics - resulting in a different muzzle velocity. Also since the bullets should have the same diameter after being engraved by the barrel for 20+ in, the heavier bullets are longer, and will likely have a slightly higher BC. But you aren’t changing either the MV or the BC when you run the ballistic calculation.