Thread: CDS/Chrono Question

I borrowed an old style shoot-through chronograph to measure the speed of some Sako 165gr Gamehead out of my 18” .308 Kimber Montana, in prep for sending in a Leupold CDS form for a new VX5 3-15.

First shot, w/cold barrel and tape over the suppressor, etc. was 2484 fps. Second shot was 2539 fps. Third was 2544 fps. Rain started to pick up so I packed it up. Bullets grouped very well, sub moa at 100m.

Barrel had been cleaned with five pulls of a bore snake prior to shooting, and a proper clean with rod and patches about 25 shots before that.

I’d be very curious to know what anyone’s thoughts are on the speed I should put on the form. I realise that I probably should have shot more but as I mentioned it was getting wet (and a borrowed chrono).

My thought would be just go with the first, cold shot as that’s usually the one that counts, but I don’t want any follow up shots to go high, either. Maybe 2500 or 2510 fps? Is there much difference in drop at say 300m with 2500 vs 2540 muzzle velocity?

Btw, speed on the box is 2674fps.

Thanks in advance

Originally Posted by F Majeur

I borrowed an old style shoot-through chronograph to measure the speed of some Sako 165gr Gamehead out of my 18” .308 Kimber Montana, in prep for sending in a Leupold CDS form for a new VX5 3-15.

First shot, w/cold barrel and tape over the suppressor, etc. was 2484 fps. Second shot was 2539 fps. Third was 2544 fps. Rain started to pick up so I packed it up. Bullets grouped very well, sub moa at 100m.

Barrel had been cleaned with five pulls of a bore snake prior to shooting, and a proper clean with rod and patches about 25 shots before that.

I’d be very curious to know what anyone’s thoughts are on the speed I should put on the form. I realise that I probably should have shot more but as I mentioned it was getting wet (and a borrowed chrono).

My thought would be just go with the first, cold shot as that’s usually the one that counts, but I don’t want any follow up shots to go high, either. Maybe 2500 or 2510 fps? Is there much difference in drop at say 300m with 2500 vs 2540 muzzle velocity?

Btw, speed on the box is 2674fps.

Thanks in advance

Greetings @F Majeur,
Many years ago when scope adjustments were less reliable than today I got around the need to dial my scope on a .308 for target shooting by tinkering with the powder charge. Reducing the charge by two grains shot to point of aim at 100 yards with a 200 yard zero. Based on the load data the velocity dropped by 100 fps. Based on that your velocity drop would result in around an inch lower impact at 100 yards. This is not carved in stone however as I was shooting an unsuppressed rifle (in the 1980's) which may show more difference.
As far as the drop goes 50 fps is nothing as it is the change in zero due to the change in velocity that counts.
To be sure however you need to test what happens with your rifle by shooting at say 200 metres or more. Holes in the target are worth more than a pot full of opinions no matter how large.
Regards Grandpamac.
PS, Your velocity loss for the 18" barrel of around 150 fps is what I would expect.

id take the average but also id check the average drop at 400 with a 5 shot group and check that against your fps to confirm that your cds turret will be accurate.

Hmm, that makes sense. Can they factor that into the CDS dial? Thanks

Good to know, thanks

Chrono’s are great for gaining fairly accurate(but certainly not always) velocity information that trajectories can be calculated from, but as Yesmate has said, info really should be validated at range by doing a tall target test or comparing calculated drop info against actual drop on target.

Good on you presenting the barrel cleaning history before that session. Question is how consistent you can ever be in future ?

Predicting a velocity increase between cleanings is a rabbit hole. And you need a lot more than 3 shots . See my report a couple of months back https://www.nzhuntingandshooting.co....ession-109099/

As noted, the bottom line is your actual trajectory at a range longer than you’ll ever hunt. Use that to reverse calculate your velocity from a good ballistic app eg online JBM and take into account as much as you can like measured scope height above bore, range findered distance, local temp, barometric (station) pressure and any slope where you did the trajectory. Use paper not metal gongs. If youre really fussy w0rk from a tall target using a tape measure not dialling clicks for the trajectory test then do a click adjustment test of your scope.

what you experienced pretty much lines up with what I find in my rifles. The clean bore shot always has a slightly lower velocity. If I sight in prior to going on a hunt I leave the barrel alone, don't clean.

Why get a cds dial? Why not leave the Moa dial on top, work out your drops with a ballistic program and verify them on paper and then write them in the side of your rangefinder on a piece of tape. Whole lot easier to change stuff if you ever change ammo or where you hunt. Temperature and altitude does start making a difference at some point.

You can always go back to your MOA cap if you radically change your cartridge. Meanwhile, a dial calibrated in metres is easy to use, foolproof under pressure and as accurate as most people need,

How far do you wish to shoot? That will dictate your requirements.

As already mentioned - box velocity minus 150fps (assuming 25fps/inch loss) puts you at 2529. Add a few fps for the suppressor, and you're sitting in line with your chrono data, which should be accurate enough depending on how far out you are stretching it. Getting a few shots out further to make sure everything lines up would be ideal i.e. at your max anticipated shooting distance. After you have your dial made up, you can always test out long to see if the first shot (i.e. clean bore) falls low enough to make any real difference.

Originally Posted by Mararoa;[emoji[emoji638

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I find CDS dials pretty genius, much less thinking under pressure. Quick range find, spin the dial quick as and good to go. Less likely to balls up the number of clicks etc. My maths tends to go out the window when I see an animal haha. I just cut down a thin strip of masking tape and wrap it around the turret, then go around dialling up for each distance, then draw a line in line with the zero mark with a thin sharpie, write the distance next to the line. Good to go. Easy to change for a different load and haven’t had any problem in the rain etc although I tend not to go out when its too atrocious.

Originally Posted by gimp

How far do you wish to shoot? That will dictate your requirements.

Such a dry question. Who could truthfully give an answer ?

800m on targets would be typical for 308 as is goes subsonic.
After shooting targets he can stretch or curtail long shots on animals.
We all want to try and do a bit more don’t we ?

Its like asking someone what they want a gun for before they buy their first …

The answer is “whatever I can. I’ll grow into it !”

Some of us think, forward plan and have the ability to give honest answers. Some not so much.