.17 Predator Load Development

[gitano]

At 53,935 PSI, QuickLOAD says I can get this bullet going 4339 f/s. (I suspect faster since I got the 25-grain V-Max going faster than that.) With those numbers, I get the following external balistics:



Still not too shabby with 450 ft-lb of energy at 300 yd and only 2" low.

If I up the MV to 4500 f/s with an attendant max pressure of 61,300 PSI (too high for my blood pressure), the numbers at 300 yd are 506 ft-lb and 1.6" low. Not sufficient gain to go that high in pressure.

At 4400 f/s with an ESTIMATED max pressure of 56,693 PSI - 470 ft-lb and 1.9" low . 56000 PSI is something I can 'live with'. The question then becomes can the bullet go that fast and not come apart? The NECB bullets have jacket thicknesses of 0.016" inches. That is significantly thicker that other .17 cal cup & core bullets, so I expect this bullet to hold together above 4300.

Paul

[j0e_bl0ggs (deceased)]

Flat base using G7 boat tail drag function?

[farmboy]

i do believe the 30 grain is a boat tail

[gitano]

j0e_bl0ggs is correct, farmboy. I incorrectly used the G7 drag function for the flat-based 25-grain NECB bullet. I'll correct that and repost the tables.

Thanks for keeping an eye on things!

Paul

PS - Changed to G6 drag function for 6R ogive and flat base. Actual ogive is 8R, which makes the G6 a little 'conservative', but less so than the G1 drag function. The difference between the G6 and the G7 at 300 yd is 0.3" and about 10 ft-lb. I could have changed it to the G8 drag function which is for a flat based bullet with a 10R ogive. I think mostly splitting hairs between G6 and G8.

Paul

[gitano]

I should probably add a note about the 'reality' of these calculated BCs.

Sometimes calculated BCs are right on the money. Other times, they are off by 0.05. Differences of 0.01 units get completely overwhelmed by factors other than the shape and weight of the bullet like altitude, temperature, humidity, etc.

Now that I am have the Magneto Speed chronograph, it is easier to actually measure the BC at specific atmospheric conditions. Most ballistic calculators these days can calculate the actual BC of a bullet based on two measurements of velocity from the same shot. The Magneto Speed will measure the muzzle velocity, and "sky screens" can measure the velocity at some distance down-range. Given the distance between the two velocity measurements and the difference between the velocities, a true value for the BC can be calculated.

With the other bullets I use (8mm and larger), at the ranges (less than 300 yd) and velocities I use them (less than 3,000 f/s), I am fairly uninterested in ballistic coefficient. It just doesn't matter much until the difference between two bullets get to be more than about 0.05. And trajectory matters even less to me at those ranges. The component of external ballistics that does interest me and is related to BC is impact velocity. Impact velocity determines both delivered energy and the bullet's terminal performance. The higher the BC the greater the delivered energy and the more likely a bullet will have a terminal performance that is consistent with what I was expecting.

When I get a chance to measure some BCs for these bullets, I'll post the results here.

Paul

[farmboy]

Paul pm me found some cheap bullets if you are interested.