Range Report JBM and the transonic point

[ReaperDriver]

I was using JBM the other day to run my load against my iphone ballistic program and verify what I was seeing for drop on the range. For my .308, 175SMK load on JBM, I ran both the G1 numbers and Litz's G7 numbers with the exact same variables.

My issue is that the G1 calculations show the bullet stays supersonic at 1K yards but the Litz selection shows it goes subsonic at about 980 yds. My chrono'd MV is 2610 for this load and I assumed that was fast enough to stay supersonic at 1K. Now I'm not sure. Anywon have any insight or suggestions?

 

[Pete E]

Re: JBM and the transonic point

Repear Driver,

To add to the confusion, the area of transonic unstability starts once the bullet falls below approx Mach 1.2, so really you want enough velocity to stay above that at 1000yards.

Further, to be totally watertight and in an ideal world, you need to consider the extreme spread of your velocities produced by your load... This is to ensure that the lowest velocity you may get will still be above Mach 1.2 at your choosen maximum range.

Regards,

Peter

 

[BryanLitz]

Re: JBM and the transonic point

That's part of the problem with G1 BC's: they're optimistic on retained velocity. Even if you adjust the G1 BC to match your drop at long range, the program will still over-predict retained velocity. This is due to the mis-match in shape between your bullet and the G1 standard projectile.

Pete is also right, in order to be safe from the transonic 'nasties', you want to be above ~ Mach 1.1 to 1.2, which is ~1230 fps to 1340 fps. Somewhere between these velocities is where you're bullet is likely to start having trouble, IF it's going to have trouble. Some bullets, depending on air density, temperature, and rifling twist, may survive right down to Mach 1.0 (1116 fps) or lower. The rules that determine flight quality/stability in this regime for each particular bullet are not well understood, so the best policy is to try it, see if you're bullet holes are round, how well accuracy is maintained and take good notes.

We know the 168 SMK fails, but the 175 SMK is a whole different bullet, most importantly the boat tail is different. I think I've heard of guys shooting the 175's to ranges where velocities are trans/sub sonic with relatively good success.

Give is a whack.

-Bryan

 

[Pete E]

Re: JBM and the transonic point

Bryan,

Can yu explain a little about your G7 drag figures? I know they are supposed to represent a more realistic represention of bullet performance, but how do I use them in JBM? Change the BC to your lower one and then select "G7" in the drag function box?

Regards,

Peter

 

[ReaperDriver]

Re: JBM and the transonic point

Thanks Bryan and Peter. Good stuff to think about. My LIMFAC right now is I'm running this out of a 20" stick, so I'm starting to max out on my load right now getting it above 2600 fps. I have gone a bit above my current accuracy load that's giving me 2615, but I've noticed a leveling off of velocity (IOW, very slight gain with more powder) and a definite opening up of the groups. I'm not getting pressures signs yet, but I'm not sure I can go much more before I do looking for that next accuracy node (if its exists).

I guess there's only one way to find out....

On a related note: Should I be using G7 or G5 BCs for the 175 SMK? They seem to fit the G5 definition better, but the Military is using G7 numbers for their M118LR, so now I'm not sure.

 

[Lindy]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Change the BC to your lower one and then select "G7" in the drag function box?</div></div>

Yep.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Should I be using G7 or G5 BCs for the 175 SMK?</div></div>

I start with the G7 BC from Bryan's book. I don't know if the G5 would work better or not, but I have no source of G5 BCs.

 

[DannySeesUSMC]

Re: JBM and the transonic point

ReaperDriver, are you only using a factory load?

There are too many better options to be using a 308 with a 175gr. bullet, unless its not your choice on what you use.

 

[ReaperDriver]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Change the BC to your lower one and then select "G7" in the drag function box?</div></div>

Yep.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Should I be using G7 or G5 BCs for the 175 SMK?</div></div>

I start with the G7 BC from Bryan's book. I don't know if the G5 would work better or not, but I have no source of G5 BCs.
</div></div>

I went back and re-read the G7 BC thread and it looks like the consensus is that 175 and 168 SMKs and AMAXs are best if using G5 BCs. Someone posted the G5 BC for the 175SMK @ .331. I'd like to find the G5 for the 168 SMK and the 168/178 AMAXs.

Mr Litz - if you're still here, does the bullet library on JBM with your "Litz" listings use G5 or G7 numbers for the .308 SMKs and AMAXs?

 

[ReaperDriver]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DannySeesUSMC</div><div class="ubbcode-body">ReaperDriver, are you only using a factory load?

There are too many better options to be using a 308 with a 175gr. bullet, unless its not your choice on what you use.

</div></div>
Nope. These are handloads. I am concentrating on the 175gr in hopes of developing a 1K load that my rifle likes. WHat are your better options for that range?

 

[Lindy]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">...does the bullet library on JBM with your "Litz" listings use G5 or G7 numbers for the .308 SMKs and AMAXs?</div></div>

G7.

 

[DannySeesUSMC]

Re: JBM and the transonic point

Berger 155.5, Sierra 155, Lapua 155...even a mild load at 2850fps they show up at 1,000 yards with over 1200fps velocity.

I don't know if you would want to go the route of those guys on here shooting 190-210gr bullets.

To stay soft shooting you can also do 185gr. Berger at 2550fps and its still over 1250fps at 1,000 yards.

 

[ReaperDriver]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DannySeesUSMC</div><div class="ubbcode-body">Berger 155.5, Sierra 155, Lapua 155...even a mild load at 2850fps they show up at 1,000 yards with over 1200fps velocity.

I don't know if you would want to go the route of those guys on here shooting 190-210gr bullets.

To stay soft shooting you can also do 185gr. Berger at 2550fps and its still over 1250fps at 1,000 yards. </div></div>

I already tried a small sample of the Sierra 155 Palmas (2155) with not a lot of luck - although I don't think I got them fast enough. I have some 155 Scenars on order to try out and I'm going to go back and try the 155 Palmas again and push them faster and see what I get.

I'm sure I'll get flamed for saying this..... but I guess my mindset for using the 175s rather than the really gucci lightweight 155 VLD bullets is I'm trying to emulate the skill required to try to shoot at that range with what Mil snipers are using in current mil-spec loads. Sure I could shoot paper and steel at range with a super high BC bullet and get hits, but it almost feels like cheating using the laser flat shooting bullets and guns like the various 6mm's in "Tactical" matches. Granted, I'm sure I'll never need the terminal ballistics of the 175 over the 155 VLDs at 1K. But it will be gratifying to know that I CAN shoot maybe the less optimum wind-bucking bullet that the professional guys are using on two-way ranges.

 

[DannySeesUSMC]

Re: JBM and the transonic point

No matter which 308 round you get working best for you it is still at a disadvantage compared to those guys with rounds screaming along at 1,000 yards with 1,600fps velocity...definitely don't feel like you are cheating.

The few USMC snipers I got to know a little bit in Iraq pretty much told me average ranges would be 300-600 meters...very few 800 meters...this was working around Camp Korean Village with people always trying to go from Syria into Iraq. They admitted more than a few occasions of hits on elbows, shins, etc. I don't think it's comparable to what guys are doing on ranges at all.

I'm betting they would be very happy with a lighter bullet that has flatter trajectory and opens up consistently & rapidly.

 

[Sig685]

Re: JBM and the transonic point

The transonic area goes from about Mach 1.2 to 0.8 or so. As the bullet decelerates and reaches Mach 1.2, the Center of Pressure, which was at the rear of the bullet starts moving forward. As the velocity approaches Mach 1, the CP is starting to have greater effect on the dynamic (not static) stability of the bullet, to the point where in certain cases, the bullet will begin to show a coning or precession motion that can quickly get out of control and the bullet will tumble. This depends on the bullet and some are more prone to it than others.

Regardless if a bullet survives the transonic passage or if the bullet is in that area when it gets to the target, it is not a good thing for accuracy. I totally agree with Peter E. when he says stay above Mach 1.2 at the target and that is exactly one of the parameters I use when working up an LR load. Staying supersonic is not enough, you need to be above Mach 1.2 for the utmost in accuracy.

I cringe when I hear people saying their bullets are at or just above Mach 1 at the target, that is actually the area of maximum pressure and disturbance. I think if you can't be above 1.2 at the target, you're better off coming in at Mach 0.9 or less if your bullet does not keyhole when going transonic.

 

[Pete E]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ReaperDriver</div><div class="ubbcode-body">
Granted, I'm sure I'll never need the terminal ballistics of the 175 over the 155 VLDs at 1K. But it will be gratifying to know that I CAN shoot maybe the less optimum wind-bucking bullet that the professional guys are using on two-way ranges. </div></div>

If you ever get on that "two way range", connecting with the bad guy at the far end is going to be the main thing...make that connection, and they'll never know 20grn difference...If you really want to increase your "authority" over that range, you need to step up to .338LM which will give you certain advantages over either .308win loads mentioned..

 

[ReaperDriver]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DannySeesUSMC</div><div class="ubbcode-body">No matter which 308 round you get working best for you it is still at a disadvantage compared to those guys with rounds screaming along at 1,000 yards with 1,600fps velocity...definitely don't feel like you are cheating.

The few USMC snipers I got to know a little bit in Iraq pretty much told me average ranges would be 300-600 meters...very few 800 meters...this was working around Camp Korean Village with people always trying to go from Syria into Iraq. They admitted more than a few occasions of hits on elbows, shins, etc. I don't think it's comparable to what guys are doing on ranges at all.

I'm betting they would be very happy with a lighter bullet that has flatter trajectory and opens up consistently & rapidly. </div></div>

If that's true, Any idea why the Mil went with the 175 SMK instead of one of these lighter bullets screaming along well above 1600fps at 1K?

 

[Lindy]

Re: JBM and the transonic point

The military moves very slooooooooowly. Those high-BC thirty cal bullets weren't available when the military standardized on the 175 SMK. Somewhere down the road, they probably go to one - if it's made in the U.S.A, which the Lapuas aren't.

 

[DannySeesUSMC]

Re: JBM and the transonic point

1600fps velocity at 1000 yards is more like the 6.5mm and 7mm calibers...so any 308 round wouldn't be cheating when you are comparing it to them.

You would have to be shooting with very high pressure a 208 A Max or 210 Berger at a very high altitude to keep 1600fps at 1000 yards with a 308.

 

[Pete E]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DannySeesUSMC</div><div class="ubbcode-body">1600fps velocity at 1000 yards is more like the 6.5mm and 7mm calibers...so any 308 round wouldn't be cheating when you are comparing it to them.

You would have to be shooting with very high pressure a 208 A Max or 210 Berger at a very high altitude to keep 1600fps at 1000 yards with a 308. </div></div>

Would you get that 1600 fps with the proposed heavy for caliber bullet (That might be a 210) out of the 300win mag that the US Military is looking at?

 

[DannySeesUSMC]

Re: JBM and the transonic point

1577fps at a 1000 yards for the 220smk at 2850fps that I read about as being the new military 300WM load...thats at 1,000ft above sea level.

If that bullet opens at that velocity it's very similar to 44 magnum loads that have been used to take lots of huge animals around the world for a long time.

 

[SAS_Sniper]

Re: JBM and the transonic point

Hi Reaper,these are the results, with sierra 175 and , below, a-max 178

 

[DannySeesUSMC]

Re: JBM and the transonic point

If you make it 4000ft altitude at 85 degrees temp. it changes things. Also using the G7 b.c. numbers from Mr. Litz shows roughly 50fps less velocity with all conditions.

In perspective...the 300WM load with 220smk at 2850fps muzzle velocity would now be doing 1753fps at 1,000 yards. So using the G1 b.c. numbers and the change in conditions added 176fps.

 

[308sako]

Re: JBM and the transonic point

.

 

[ReaperDriver]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 308sako</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DannySeesUSMC</div><div class="ubbcode-body">If you make it 4000ft altitude at 85 degrees temp. it changes things. Also using the G7 b.c. numbers from Mr. Litz shows roughly 50fps less velocity with all conditions.
</div></div>

Reaper and his NV crew are shooting at approxiamtely this altitude and well the desert temps are hotter than Juneau AK...

</div></div>

 

[Grump]

Re: JBM and the transonic point

My sources put the Desert Sportsman's club steel range at about 3300 ft ASL.

Much of this discussion gets even more favorable for 1,000-yard velocities remaining above transonic when you factor in the change in speed of sound at higher altitude AND at higher temperature. That's why I like JBM having that column that reads Mach.

 

[Lindy]

Re: JBM and the transonic point

The speed of sound changes only with temperature, not altitude.

 

[Pete E]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">The speed of sound changes only with temperature, not altitude. </div></div>

Isn't it affected by the density of the air?

 

[Lindy]

Re: JBM and the transonic point

More than you ever wanted to know about that:

http://en.wikipedia.org/wiki/Speed_of_sound

If you want to skip the useless stuff, page down to "Practical Formula for Dry Air." Note that the formula is a function only of temperature. Enjoy the details if you wish.

 

[LoneWolfUSMC]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">The military moves very slooooooooowly. Those high-BC thirty cal bullets weren't available when the military standardized on the 175 SMK. Somewhere down the road, they probably go to one - if it's made in the U.S.A, which the Lapuas aren't.
</div></div>

It will be interesting to see if the 155 Sierra Palma's get any interest from the military in the future.

 

[Renegade]

Re: JBM and the transonic point

Ballistic Calculators are nice toys to play with, but a poor substitute for hard numbers. They do not seem to be good for much more than a general sight in, IME. Getting actual drop numbers & fps at various ranges and then curve fitting for unknown distances has been much more accurate for me.

 

[LoneWolfUSMC]

Re: JBM and the transonic point

I beg to differ.

Shot a match this weekend with a round I didn't have hard dope for. Just estimated courtesy of JBM and Ballistic FTE (cross referenced).

The key, as stated before, is having good data to enter. In this case the data I had was chronograph numbers from my rifle with that round as well as correct environmental data.

This match was only to 530 yards, but the dope was dead on. No elevation corrections needed.

In some cases you can get extremely accurate dope from the computer.

 

[Lindy]

Re: JBM and the transonic point

If you shoot all the time in the same place, or places with similar conditions, you don't need a ballistic computer.

If, however, you shoot mostly in one place, and get deployed to a place with a very different environment, then a ballistic computer can get you closer on the first shot.

No ballistic computer can get you dead on every shot, because there are too many variables. For example, if your .308 load has an SD of 15, then you're going to be plus or minus 0.2 mils at 1000 yards just in the two-thirds of the shots whose muzzle velocities are within one SD of the mean.

But it's better than a wild guess.

 

[LoneWolfUSMC]

Re: JBM and the transonic point

I like my ballistic pocket brain....but I would trade it in a heartbeat for an experienced spotter.

 

[BryanLitz]

Re: JBM and the transonic point

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Ballistic Calculators are nice toys to play with, but a poor substitute for hard numbers. They do not seem to be good for much more than a general sight in, IME. Getting actual drop numbers & fps at various ranges and then curve fitting for unknown distances has been much more accurate for me. </div></div>

If you guess at all the inputs, then sure, the output will be sketchy at best.

However, if your inputs are truly accurate measured values, then the output will be correct.

Ballistics is not a new or magical science. The equations of motion are old and well established. There are many reasons why shooters may <span style="font-style: italic">perceive</span> some error between calculated vs actual drop, but those perceptions are usually a result of some incomplete or incorrect understanding of how something works.

For example, consider your scope adjustment. If the computer says you need 30 MOA to be zeroed at 1000 yards, and you dial 30 MOA on your scope and hit 2 MOA low, most shooters would shrug and say "computer was off by 2 MOA". However, if all your inputs were right, it's much more likely that the scope only <span style="font-style: italic">actually</span> moved 28 MOA as opposed to the 30 MOA it was supposed to. So the shooter dutifully enters 28 MOA into the logbook as the 'correct' elevation for 1000 yards. It works for that shooter and scope, but it might not be the true drop.

There was a time in the recent past where Renegades belief was shared by many, if not most shooters. The belief was common because it was real hard to get computer programs to match real world dope. Well, it's not that the programs were faulty, it's because it was hard to feed them good inputs. Now that laser rangefinders, Kestrals, and more accurate BC's are available, you can feed the programs more accurate inputs and it will return more accurate output.

I'm not saying the programs will be <span style="font-style: italic">perfect</span>. Like Lindy said, there will always be some non-deterministic variables like MV variation, BC variation, and some error in every other input (atmospherics, scope height, etc). However, if you're careful and have an accurate load with low MV variation, the combination of all these errors won't cause you to miss by much, even at long range.

It's the 21st century. I think it's safe to say that science works (at least for a problem as basic as ballistics).

-Bryan