Bullet design and drag models...

[User702]

I shot my first set of hand loads at the range at the weekend (Barton Road, lovely day too) and was pleasantly surprised to find out that:

a) I can still shoot after several months off the rifle and
b) My hand loads worked well enough that every bullet made a hole in the target.

Now, I appreciate that 100m is a bit short for making judgements on, and I know that further work on the narrower band of charge weights is needed, but I could do with some advice on why there appears to be such a difference between the performance of the 155grn SMK 2155 and 2156 bullets at about the same velocity when using different drag models.

From my loads (using 45.1 grns TR140) Quickload suggests a muzzle velocity of 2807 fps with the 2156 SMKs that I am using. That translates (according to Ballistic AE for boing) to a 1000m velocity of 1790fps using the G7 drag model. Any way you look at it, that's still supersonic, so i should not be suffering from any transonic wobble at 1000yrds. Using the data I have for the ammunition I used at the Phoenix (HPS 155grn SMK 2155 @2850fps (apparently)) I get roughly the same sort of velocity at range. However, if I use the G1 drag model, I get substantially different results, enough that the velocity at extreme range is just barely supersonic.

So, to the question: Given that I am trying to develop a load for long range, which drag model should I be using for the 2156 SMKs to best reflect real-world performance?

[ovenpaa]

The #2155 and #2156 have a boat tail so use the G7 model

EDIT - some interesting reading on the subject here http://bulletin.accurateshooter.com/201 ... d-to-know/

[saddler]

So, to the question: Given that I am trying to develop a load for long range, which drag model should I be using for the 2156 SMKs to best reflect real-world performance?

What CHAMBERING is the rifle in?

[User702]

Chambering? .308Win.

Sorry, should have said that earlier.

[Laurie]

From my loads (using 45.1 grns TR140) Quickload suggests a muzzle velocity of 2807 fps with the 2156 SMKs that I am using. That translates (according to Ballistic AE for boing) to a 1000m velocity of 1790fps using the G7 drag model. Any way you look at it, that's still supersonic, so i should not be suffering from any transonic wobble at 1000yrds. Using the data I have for the ammunition I used at the Phoenix (HPS 155grn SMK 2155 @2850fps (apparently)) I get roughly the same sort of velocity at range. However, if I use the G1 drag model, I get substantially different results, enough that the velocity at extreme range is just barely supersonic. [User 702]

Errr ... that's way out. Like you've just matched or even exceeded the 1,000 yard performance of a 7mm short magnum with a good 180gn VLD!

Running the two SMKs through Bryan Litz's PM Ballistic Solver II program suggests you used a G1 BC in a G7 program - not allowed! Don't ever use G1 anyway for long-range ballistics as the results are invariably over-optimistic. G1 relies on a completely different shape 'standard projectile' from a modern HPBT match bullet.

The G7 BCs for the pair are 0.214 (#2155) and 0.229 (#2156). At 2,807 fps, the former is down to 1,070 fps and the latter at 1,136 fps under standard ballistic conditions. On a low lying range, in cold, high atmospheric pressure conditions that we might see at this time of the year, they'll go noticeably slower still due to denser air and the #2156 will be subsonic too at this distance. In any event, any bullet whose average MV just leaves it supersonic (by a mere 10 fps here) at the target may see some very inconsistent performance, partly through transonic turbulence which starts as it drops below 1.2MACH and gets worse as it approaches the speed of sound, but also and usually worse the effects caused by bullets at the top end of the MV spread just staying supersonic while those at the other end drop through the sound barrier just in front of the target.

[meles meles]

Hmmm, we thought this was thread about hunting transvestites...

[User702]

Steady now! What you do in the comfort of your own sett.....

Laurie, thanks for clearing that up, or at least making things clearer. I did check the data i had with the on.line jbm ballistics calculatlr and got pretty much identical results to yours, so my thouts now are that the boing app is getting its knickersnin a twist somehow. I think i may drop the developer a mail.