B.C. variances for Accubond bullets?

[azsugarbear]

Currently shooting a 300 RUM with 200 gr. AB's at 3,120 fps. Went out a couple of weeks ago to dial in my scope to get accurate drop table for this load. Using the Excel program provided by Dave King on this forum, I was able to publish a drop table that was very close to actual performance. I only had to correct a couple of clicks at 500 yds. However, in order to get an accurate drop chart that reflected the actual bullet path, I had to go back to the software and change the B.C. of the 200 gr AB from the published B.C. of .580 down to a B.C. of .520. /ubbthreads/images/graemlins/confused.gif Now the software prints a table that is dead on and reflects actual bullet flight out to 700 yds (farthest I could shoot that day). Has anybody run into this before with B.C.'s for the Accubond? I didn't expect it to be off by that much.

 

[Michael Eichele]

That EXCEL calculator doesnt factor barometric pressure, only altitude. If you are having unusualy high pressure on the days you are shooting, yeah it could appear to be way low.

Also, what is your rate of twist??

 

[Michael Eichele]

Also,

The differance in clicks between .520 and .580 at 500 yards is only a couple as you stated. If you are using a 100 yard zero and udjusting from there, that could throw you off IF it is slightly off. Alot of shooters think they have a 100 yard zero and dont. In other words, you may be trying to zero on a 2" circle at 100 yards and if youre hitting 1/2" high of center, it seems like a perfect zero. In reality, a 1/2 off at 100 yards makes for neccesary adjustments at 500 yards.

Try a 300 yard zero. Then goto 500 yards and check that against a calculator. 1/2" off at 300 is still 1/2 off at 500 yards. 1/2 at 100 could be inches at 500 yards making you belive your BC is lower than it should be.

 

[azsugarbear]

Both comments are appreciated. Rifle has 27" Lilja barrel with 3 groove and 1 in 12 twist. I've punched paper out to 1000 yds. & bullet appears to be stable (nice round hole). My zero is 200 yds - could be off by 1/4 inch. Scope is mounted on flat bases (read: no 15 degree rail). Not sure what barometric pressure was on that day, but weather does not usually change much in the deserts around Phoenix. Weather conditions were 75-80 degrees, clear skies, 5 mph wind from 3 O'clock position.

 

[Michael Eichele]

I also shoot a 300 RUM Lilja 12 twist barrel 27-1/4". I have the regular groove and not 3 groove. I dont believe the grooves will affect the BC that adversly. I get right at .590 with the 200 ACCUBOND. The farthest I have shot this load is 650 yards and .590 works best for me.

Phoenix is usually clear bight and blue. This means higher than average pressure is the norm. Average meaning that average for the rest of the country.

What I can say for certain is even 1/4" low at 200 yards for a zero (182 yard zero) is enough for you to percive over 1-1/4" error at 500 yards. If you calculate .520 instead of .588, you come up with about 1-1/4". This may very well be your problem. I would say your BC is much closer to .580-.590 than you think. It only takes a SMALL error for you to percieve a huge differance in your BC.

The barometer most likely didnt change enough to make that big of an error at 500 yards. The fact remains that the program youre using is either based on 29.53 or 29.92 Hg. with no way to adjust it. If it is calibrated with 29.53 and the pressure the day you shot was 30.2, that will only increase your margin of error.