Range Report 7mm 175smk dope vs chrono

[turbo54]

Ugh...

Scope passes full range tracking test with flying colors.

Dead fucking nuts 200 yard zero.

Dead nuts hits on paper, with 1.5" 5-shot group at 600 yards requires 2.9mrad elevation.

JBM with Litz data, with DA input according to my calibrated Kestrel (3200 ft today) says this means my MV is ~2850, which is right about what I'd expect with my barrel length and load.

Over the chrono (CED M2) this afternoon, I got:

2702
2705
2703
2709
2702

WTF?

284 Winchester (28.25" Krieger, recoil lug to crown)
175smk
54gr H4831sc
2.950" COAL

 

[jbailey]

Re: 7mm 175smk dope vs chrono

was the chrono in bright sunlight?

Even with the screens up, chronos suck in sunlight.

 

[turbo54]

Re: 7mm 175smk dope vs chrono

It was about 6:30-7:00 this afternoon/evening. There was no cloud cover, but it wasn't "bright".

The light was obviously coming down at an angle.

This chrono won't read at all if the sky screens aren't casting an obvious and clearly defined shadow...this chrono is a pain in my ass, but when it reads, it's always been pretty accurate, even when comparing it to a friends Oehler 35.

 

[George63]

Re: 7mm 175smk dope vs chrono

I quit using the chrono - I get MV only from JBM, bullet drop is more consistent

 

[turbo54]

Re: 7mm 175smk dope vs chrono

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: George63</div><div class="ubbcode-body">I quit using the chrono - I get MV only from JBM, bullet drop is more consistent </div></div>

I think I'm going to give up on my chrono. It's a PIMA, and what matters is where the bullet hits the steel/paper.

As an engineer, and a downright technically minded fellow, it just seems like I should be able to measure MV, look up the experimentally-derived BC (thanks Bryan!), punch in the DA, and print out a dead nuts dope chart.

I've tried to make this happen. It just doesn't work like it "should".

I'm into cars too. I gave up on racing the "dyno" a long time ago. It's time I quit racing the chronograph.

It's *probably* good for comparing speeds of two loads, back to back, one right after the other, but that's about it.

Fuck it.

 

[264win]

Re: 7mm 175smk dope vs chrono

Like yourself, I have been so frustrated with numerous chronographs that I just gave up on them.
I establish the load I want and then go shoot it at distance to find my trajectory.

 

[MTETM]

Re: 7mm 175smk dope vs chrono

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 264win</div><div class="ubbcode-body">Like yourself, I have been so frustrated with numerous chronographs that I just gave up on them.
I establish the load I want and then go shoot it at distance to find my trajectory.</div></div>

I am with you guys...
I have given up on velocity from the chrono..I only use it when working up a load looking for velocity plateaus, I don't care what the actual velocity numbers it reports are, I just want to see the velocity trends with different loads.

When we find a load that shoots at 300 we shoot at 300, 500, 800, 1000 on paper to verify actual drops and vertical of the load, then the data goes into Cold Bore with Bryon's G7 to give drop charts... We just change the velocity numbers in the software until the reports meet the actual drops..

 

[KYpatriot]

Re: 7mm 175smk dope vs chrono

It is still giving you data that you can use....for instance, it is giving you the ES of the load and it appears to be giving consistent velocity measurements relative to each shot. So, seems it would still be useful for load development.

Since you have a pretty good idea what your velocity ought to be based on the load and barrel length, I agree your chrony data is low...but one thing I have learned to respect is the terrain effects on the wind.

For instance, shooting across rolling terrain like I have can result in fairly significant vertical wind components. These present on the target like a variation in velocity. It can be easy to overlook since we are more used to seeing wind in the horizontal and the vertical deviations are typically smaller than the horizontal wind devs, but can still be significant out past several hundred yards.

I am having good success with the magnetic chronograph. So far it has been great, and it doesnt care if there is light or not.

 

[RobG]

Re: 7mm 175smk dope vs chrono

I wouldn't sweat it too much. You know about my issues dealing with mine so I won't go into it. I stopped worrying about the actual speed and look more at the ES/SD numbers which are more important to me. As long as it does what you want you should be good to go.

 

[sandwarrior]

Re: 7mm 175smk dope vs chrono

The only thing I use a chrono for anymore is determining those type cases that need a good one to tell you that you are getting way too hot without knowing, i.e. Ackley cases.
Other than that, I find that without setting up given repeatable scientific conditions you aren't going to get given repeatable velocities. Quantity of light and variation of light angle makes for a lot of variation in velocity reading.

As some of you noted, the best thing to do is go out and get the dope for your rifle by firing it. Then you'll always know under given conditions what it's doing.

 

[KYpatriot]

Re: 7mm 175smk dope vs chrono

Sand I agree with you, but with this caveat:

If you are going to "back into" the velocity data you need to be very sure you have all the other variables locked down....temps, pressure, sight adjustment (not just the sight setting but KNOWING that your scope clicks are consistent or accounted for across the range), wind, range error, etc. Tolerances stack, sometimes they offset and sometimes they add.

If these variables aren't tight your derived velocity data will disappoint you when you use it on another day for that first round hit we all strive for. A good direct measurement is always preferable, but if the quality of the chrony won't allow that then everything else needs to be as accurate as possible for good derived velocity.

"A man with one watch always knows what time it is, a man with two watches is never quite sure"

The best solution is to get a better watch!

 

[sandwarrior]

Re: 7mm 175smk dope vs chrono

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: KYpatriot</div><div class="ubbcode-body">Sand I agree with you, but with this caveat:

If you are going to "back into" the velocity data you need to be very sure you have all the other variables locked down....temps, pressure, sight adjustment (not just the sight setting but KNOWING that your scope clicks are consistent or accounted for across the range), wind, range error, etc. Tolerances stack, sometimes they offset and sometimes they add.

If these variables aren't tight your derived velocity data will disappoint you when you use it on another day for that first round hit we all strive for. A good direct measurement is always preferable, but if the quality of the chrony won't allow that then everything else needs to be as accurate as possible for good derived velocity.

"A man with one watch always knows what time it is, a man with two watches is never quite sure"

The best solution is to get a better watch!</div></div>

KYpatriot,

I totally agree with you. Going out and shooting on two very different days is going to give you two very different sets of results. Using one or the other set of data to figure drops is going to make for a WTF?!? day. It is imperative you have the atmospheric factors down to a "T" or it's the same point as not taking any of the conditions into account with a chrono. Each time you shoot in given conditions, that should be checked against a baseline you have built up over time to what each load is doing. Or, better yet, your one load is doing.

 

[turbo54]

Re: 7mm 175smk dope vs chrono

So, I cranked out 3 sets of dope cards at various DA for 2750, 2800 and 2850 using JBM, and Litz BC.

On my way to ASC, I stopped at Thunder Valley, got a great 100 yard zero, and then doped in to get headshots on an IPSC @ 1000 (no, I can't do this with great consistency).

The 3000ft dopecard that matched was 2800. I continued to make great hits on everything I shot at using that card.

Went to ASC, where the DA was ~5500, so I used my 5000ft card. I had NO troubles with Dope, scoring FRH @ 1220 on a large target, then a SRH on the tiny target @ 1220.

The bullet sure sails across some SERIOUS airspace when engaging those targets at ASC.

I sure love that 7mm 175smk!

The fellow next to me was running a 243 with 115dtacs. For wind (10mph FV), he wasn't far behind, but as we worked past 1000, we stopped seeing his misses but mine showed up right nice.

 

[The Lawn Ranger]

Re: 7mm 175smk dope vs chrono

Out of curiosity, what is Litz's G7 BC for the 175 smk.

I am itching to run those in my 284.

 

[turbo54]

Re: 7mm 175smk dope vs chrono

His book lists them @ .64x (note this is the only example I'm aware of where the measured BC is higher than advertised by the manufacturer which is .607).

They fly real nice, and word on the street is they transition to subsonic very well.

 

[ncboiler]

Re: 7mm 175smk dope vs chrono

I'm thinking I'm going to give these 1st priority when I get my 7wsm back over the 162 Amax.

 

[turbo54]

Re: 7mm 175smk dope vs chrono

Good call.

Barrels seem to be picky about the 162s. Some just won't shoot them.

The 175 is very, very easy to tune, and runs great. Not *quite* as flat, but a significant BC advantage. According to Litz, .599 vs. .64

 

[RobG]

Re: 7mm 175smk dope vs chrono

I run the 175 in mine. I have run it touching the lands to .015 off with no noticeable change. They certainly do not seem to be finicky like some bullets out there.

 

[KevinU]

Re: 7mm 175smk dope vs chrono

Are the slots on the blue sensors exactly 24" CTC?

 

[turbo54]

Re: 7mm 175smk dope vs chrono

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: KevinU</div><div class="ubbcode-body">Are the slots on the blue sensors exactly 24" CTC? </div></div>

I've never measured. The instructions indicate to fully seat the sensors on the "boom", and lock them down, which I did...and I've never taken them off.

ETA: The chrono has ever yielded velocities that matched a 6' Oehler 35 on the same day, so....

 

[tspearsrph]

Re: 7mm 175smk dope vs chrono

FWIW.....

I had a CED chrono. Actual drops indicated it was reading slow. I bought an Oehler 35P and set up the 2 CED screens between my 3 Oehler
screens. CED consistently ready approx. 200 fps SLOWER than the 35P.
Actual first-hand experience.

Tom