338 300gr BC

[trebark]

Trebark,

How do you figure that?

Check it with one velocity and see if it matches up, if not check the 30 fps faster velocity.

In most cases, 30 fps is not worth even worrying about, average the difference and use that velocity for your program.

30 fps spread will not make 1 moa of difference at long range(1000 yards) Not even close.

Here is an example of 30 fps variation at 1000 yards: My 338 AM with 300 gr SMK

3300 fps velocity and data at 1000 yards:
Retained velocity: 2303 fps
Retained energy: 3532 ft/lbs
bullet drop w/ 100 yard zero: 16.75 moa (174.2")
bullet drift w/ 10 mph crosswind: 3.0 moa (31.8")

3330 fps velocity and data at 1000 yards:
Retained velocity: 2327 fps (only 24 fps difference)
Retained energy: 3605 ft/lbs (73 ft/lbs difference)
bullet drop: 16.25 moa (1/2 moa difference)
bullet drift: 3.0 moa (no difference)

Now if you can consistantly hold less then 1/2 moa accuracy you MAY be able to see the difference of 30 fps at 1000 yards with this class of chambering but I know very few that can hold this level of accuracy consistantly shooting from field conditions.

A shooters ability to accurately judge shooting conditions will make MUCH more difference in the real world that 30 fps velocity spread.

Maybe I misunderstood your comments.

It is just my experience that when my loads have a 15+ ES, they rarely match up with software predictions.

 

[Fiftydriver]

You might want to play with the multipul BC feature of your program if it has one. You can set a new BC for each velocity level set as the bullet travels down range. I have had to do this many times.

Looks like you could tweak your BC a bit and be much closer down range as the acutal drops seem to be somewhat consistant as far as the amount of difference from recommended drop.

Make sure you have your Bar pressure set up properly for your elevation and not set for sea level. Also make sure your on with the temp and humidity.

I have at times had to also adjust my zero range to get down range drops to match up. I could really care less if I am supposed to have a 300 zero but in fact I have a 275 yard zero as long as the trajectory trips the down range drop estimates correctly.

Its nearly always required to manipulate the drop charts to match actual bullet flight so you should be able to get it very close. In the end, remember we only want to make a model of the actual bullet flight, what numbers we use to get that really does not matter as long as it matches actual bullet flight.

Also remember to consider up and down drafts, especially at longer ranges. Up drafts and down drafts act on a bullet just as wind does with drift, only in the vertical plain. Its not as dramatic as wind drift but it DOES happen and can really cause problems with developing a drop chart.

 

[canderson]

Thanks Kirby, I will play with the multiple BC. I have been using what sierra recommends. Do you recommend shooting every 100 yds. or 200 yds. to get a good drop table?

 

[wadedc]

Canderson
How long is your barrel and what is the load you are using to get 3200fps?

Wade

 

[canderson]

I have a 32" Lilja. My load is 135.0 gr H50BMG 0.015 off the lands with 300 SMK.

 

[wadedc]

Have you ever tried the 285gr barnes bullet?

Wade

 

[jmden]

canderson,

Here's Litz's measured, banded G1 data for the 300 SMK with the velocities listed as the 'lower velocity limit' for the given bc. Plug 'em in Exbal and see what happens.

.802 3000fps
.7855 2500fps
.7555 2000fps
.7035 1500fps
.665 0fps

Plug these numbers into you ballistic calculator in the (Exbal) 'edit multiple bc's' option and run the numbers with the correct environmentals entered (I assume you've got a very precise, accurate measurement of your scope height above the center of your bore and you have a scope level set up correctly and are using it...?) and see if the numbers the program spits out are close to what you are observing in the field.

Good luck.

 

[Fiftydriver]

Canderson,

The more ranges you can confirm drop at the better but you do not have to go every 100 yards.

If I am setting a rifle up for 1000 yard drops, I will shoot the zero range and then generally 500, 800 and 1000 yards. If all your drop numbers match up at these ranges, it will match up in between as well.

If I am setting a rifle up for serious long range, say out to 2000 yards, I will shoot at 500, 1000, 1500, 1750 and 2000. It also never hurts to record drop numbers for ranges farther then you plan to shoot just because it adds to the accuracy of your drop chart.

 

[canderson]

Thanks everybody. I will try to let yall know what I find out in the next couple of days.