Can changing seating depth change chrono sd?

[kingfamous]

I have never used a chrono to test my loads before. I'm going to be able to use a friends to test some loads and my question is will a change in just the seating depth change the velocities that might change the sd readings on the chrono? What is a great sd number?

[CSACANNONEER]

Yep. Seating depth can change chamber pressure will will affect MV.

[ojisan]

Changes in seating depth will affect pressure and velocity.
To help with a consistent SD, having a consistent seat depth and crimp tension is critical.
You also want to use an appropriate powder that is completely burned every time.
For example, some slow-burning magnum handgun powders really need a strong crimp to get the powder burn up to speed before the bullet starts moving.
A loose or inconsistent crimp with these powders will show big velocity changes.

[Germz]

powder burn time and barrel length are what affect velocity. I think what you are getting at is internal case pressure.

bullet seating depth affects "peak" pressure inside the case. internal pressure rises, then rises again once the bullet engages the rifling. this is a function of the pressure curve.
the closer you seat a bullet to the rifling of the barrel, the higher peak pressure you will have as there is less space for the bullet to move in the throat before it contacts the rifling.
the deeper you seat a bullet, during the initial burn the bullet will have much more room to travel down the throat before it contacts the rifling, at which point pressure will again rise...however not to the levels if you were to seat it further out; the curve will be much less extreme.

at least that is my understanding of internal ballistics in regards to velocity and pressure.

so with that in mind, if two cartridges are seated, one long and one short...assuming they both enter the barrel at the same time, the longer cartridge will produce more pressure and therefor contribute to increase velocity. just understand that [SD and velocity] are not directly proportional. SD is merely a function of the pressure curve.

[donnrcp]

Here's a link to another thread about what's a good SD: http://thefiringline.com/forums/show....php?p=4210052

When I used to shoot 223 from a Savage Model 11, I sorted my brass by make and length in boxes of 50 to the nearest thousandth. That way when crimped using the Lee Factory Crimper, I felt that crimps were as consistent as I could make them.

[kingfamous]

This is for a .308 load using uniformed lapua brass, cci benchrest primers, 43.8gr. varget, 175 smk's weight sorted. 24" bbl with a 1 and 10 twist. All brass trimmed to 2.005" and weight sorted. Current load is 2.790" oal = .015 off the lands. Brass is neck turned and neck sized, and full sized when needed.

[J-cat]

You have a custom short-throated chamber?

[Germz]

This is for a .308 load using uniformed lapua brass, cci benchrest primers, 43.8gr. varget, 175 smk's weight sorted. 24" bbl with a 1 and 10 twist. All brass trimmed to 2.005" and weight sorted. Current load is 2.790" oal = .015 off the lands.

did you determine this with a bullet seating depth tool?

[kingfamous]

no custom barrel. Stock savage fcp-k. Yes, I used a bullet seating depth tool.

[kingfamous]

Will the chrono verify that Im on the node?

[J-cat]

I didn't know Savage allowed you to touch the lands at 2.805" OAL. What a find!

[Fishslayer]

did you determine this with a bullet seating depth tool?

I had to Google bullet seating depth tool. Does this work better than marking the bullet and trial & error-ing the COL? Looks like a time saver but I would still want to check with the brass/bullet combo for a particular load?

I've seen some talking about polishing and looking for the land marks. Does this work better than Sharpie or Prussian Blue?

Almost forgot... as for the OP... Standard Deviation really has nothing to do with pressure or velocity itself. It's a measure of how much variance there is from one round to the next or withing a group of rounds. I learned the formula for it once in a stats class but a $30 calculator can do it easier.

All other variables being equal, since it affects pressure COL can definitely affect SD. A given charge/bullet/COL combo will have a point where the pressure varies the least. That's what you're looking for. Quick & dirty would just be to look for the smallest difference between min & max velocities unless the chrono will calculate your SD for you.

OR....

http://www.mathsisfun.com/data/standard-deviation.html

[Germz]

I had to Google bullet seating depth tool. Does this work better than marking the bullet and trial & error-ing the COL? Looks like a time saver but I would still want to check with the brass/bullet combo for a particular load?

I've seen some talking about polishing and looking for the land marks. Does this work better than Sharpie or Prussian Blue?

Fish get one of these and the appropriate brass adapter. its basically a modified case with just enough neck tension. you push the bullet through the head of the case, through the neck until the bullet touches the lands, extract the entire assembly and measure the oal.

http://www.midwayusa.com/product/570611/hornady-lock-n-load-overall-length-gage-bolt-action

[Fjold]

Very, very simply, SD is a square of the difference in velocity between shots as compared to the average difference.

It makes the biggest difference in long range shooting where a difference in velocity effects the point of impact. I aim for a SD of 5 fps for my 1,000 yard guns but it takes a lot of case prep and sorting, neck turning and Redding S type bushing dies to get close.

[3RDGEARGRNDRR]

Fish get one of these and the appropriate brass adapter. its basically a modified case with just enough neck tension. you push the bullet through the head of the case, through the neck until the bullet touches the lands, extract the entire assembly and measure the oal.

http://www.midwayusa.com/product/570...ge-bolt-action

I just bubba mine by dremeling slots in the necks on a case and chambering the case and a projectile then measuring. I bought the tool for my 308 but the found that the bubba method worked just as well. Have a rod ready to tap out stuck bullets. But for precision get the hornady tool.
Its a cheap fairly effective way to measure depth on a semi auto or your least accurate bolt gun anyhow.