Velocity estimation/difference

J

jonnefudge

New member
CAUTION: The following post includes loading data beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.

Two questions:
1. I don't have my own chrono yet and would like some help with estimating velocity and pressure from this load: 20" barrel, 1/11 twist, 308, 44grains n140, lapua brass, cci 200 (lr), scenar 155 bullet.

2. I read alot of load data and are constantly finding rather substantial differences in velocity from the same loads in the same barrel lengths. Is this possible or just bad measurements?

Any input greatly appreciated!

/jonne
 
Different barrels will produce different velocities with the same loads. I proved this to myself with two 357 revolvers. Both were four inch barrels and the same make and model. One was 250 fps slower than the other using the same loads fired over the same chronograph just seconds apart.
You have no precise way to measure pressure - that's why we have manuals. Just as velocity can differ so can pressures in different barrels. Use published data that matches your components as closely as possible and start low working your way up while watching for signs of excessive pressure. If you get signs of excessive pressure back the load down at least 2% and use that as the maximum in your gun under those conditions.
 
I proved this to myself with two 357 revolvers. Both were four inch barrels and the same make and model. One was 250 fps slower than the other using the same loads fired over the same chronograph just seconds apart.
Click to expand...
It would be interesting to know if a difference between the two could be attributed to a difference in cylinder gap. Did you happen to measure the cylinder gaps in those two guns?
 
Rather than guesstimating velocity try this:

Sight your rifle in at 100 yards.
Fire 5-10 shots at 300 yards with no holdover.
Measure the amount of drop for each shot at 300, add it all together and divide it by the number of shots you took (this gets you the average drop).
Plug your rifle/load info into a ballistic calculator (I use Hornady's free online one) and mess with the velocity until you find the one that provides the same drop as your load.
Now you know what your load is doing on average.
 
250fps difference is quite a bit, and outside the usual range, but obviously, not impossible.

While the cylinder gap could be a small factor, in this specific case, we see the same kind of behavior with all barrels, including rifles.

It is because of the differences in individual barrels and bullets, how they fit, how much friction is created, things that create different results despite everything being "in spec" because specs have tolerances, and things stack up, giving differences, one way, or the other. Barrels are different, AMMO is different, round to round there are variations, things only a chronograph sees.

Which is why we use averages for velocity measurements. The terms "extreme spread" and "standard deviation" are used for a reason.

A friend of mine loved his .270 Weatherby Magnum, until he got a chronograph, and found out his 22" .270 Winchester was actually 20fps faster than his 26" Weatherby Magnum. Again, this isn't a usual thing, but it does happen, and most of the time, we never know it, until we get a chronograph.
 
"...don't have my own chrono yet..." You don't need one either. Accuracy is far more important than velocity. Consistency is too.
Anyway, Vihtavuori shows a Max load for a 155 Scenar(that really doesn't matter) with 43.4 of N140 runs 2648 FPS out of a 1 in 12, 24" barrel. I'd guess(A WHAG at that) your load is running around 2500ish.
You lose velocity due to the barrel length(Lotta arguments about how much per inch.) and the rifling twist. Wouldn't worry about it myself.
 
All I can tell you for sure is you are over Vihtavuori's maximum, so the required warning had to be put in (unless you have other published data that includes your load level). The velocity change in the VV data was about 59 ft/s/grain of charge weight change, so in their test gun using their brass and primer you would get about 2684 fps, but that's in a tight test barrel chamber, not a production chamber, and with the load conditioned to a certain temperature and carefully loaded so the powder stayed back over the flash hole, which maximizes pressure and velocity. If the test barrel were shortened from 24" to 20", you'd lose about 100 fps, according to QuickLOAD, but yours may be lower still, just because it's not as tight a test barrel. But that's not always the case as while the standard test barrel's chamber is tight, it bore is merely nominal, so having a tighter bore can prevent the velocity and pressure difference.

You can try finding velocity by drop difference, but it's not a very sensitive measure. You have to average a lot of rounds per group to narrow the precision.
 
You must log in or register to reply here.
Back
Top