abinok,
Agreed, there are a number of things that make determining actual BC's difficult by measuring drop.
1 - I use the Oehler 43 at the muzzle and the Oehler 35P set at 300 yards.
2 - Or the M43 and acoustic target.
3 - BORELINE drop from muzzle at 100 yard intervals on out to 600 yards, minimum, on an 8' sheet of plywood, SAME POA.
There are issues with ALL these methods that cannot be overlooked if you are after REAL BC numbers.
Pressure and temperature measurements are essential to any of these methods.
Measurement to the "tenth of a foot" is absolutely critical when using the acoustic target, and you don't get that without a steel tape... rangefinder won't cut it by a long shot. It however is very accurate in the determination of BC's.
With two chronoghraphs, the screen spacing must be adjustable and able to be calibrated to each other. The first chrono closest to the muzzle will read a slightly higher MV than the second chrono when set just feet apart. The chronos must indicate this or something is wrong. A bullet with a BC range of .5 - .55 at say 3000 FPS MV will loose a set FPS for every 1 ft. traveled. If you set a chrono up at 300 yards, which was reading 30 fps slower than the chrono at the muzzle when they were only 4' away from each other, those chrono's are not even close and will yield inaccurate BC's.
If you use just one chrono to test at two ranges in order to get the BC by averaging the sample fired at the muzzle, then averaging the sample fired over the chrono set up down range, you must be sure MV will be stable throughout the whole test, and not climb on you as the barrel fouls or heats up. In other words, don't waste your ammo on the test unless you can shoot a minimum of 20-30 rounds and see the last 10 shots average the same MV as the first 10, and the second 10. If you shoot less than 10 shots at the muzzle and 10 shots downrange at 300 yards or more, I think you'll have too small of sample size for a reasonable average. Using a tape measure to establish the distance between the MV chrono and downrange chrono placement is recommended.
The method of determining BC using actual drop eats up even more ammo but, does have the side benifit of practicing some wind doping skills and also eliminating any possible calibration/tracking issues with your elevation turret, as you will not be dialing but rather aiming at the top edge of the plywood and meausuring actual drop from the top at each 100 yard interval.
I sight my rifle in at 100 yards after taking some scope height measurements. Distance between the scope tube and bolt + half the diameter of the scope tube + half the diameter of the bolt = scope height. Scope height + boreline drop from muzzle to 100 yards = how many inches low I zero in at 100 yards. This effectively sets the reticle plane parallel to the boreline, and all we measure and compare from here on out is actual DROP, path has nothing to do with this.
You need to measure and record MV or you will never get the BC this way.
If you can set up a vidio camera off to the side of your target to record the POI for each shot number in the sequence, associating each MV with each shot's POI will greatly help to further paint a clearer picture.
Be very careful to zero your rifle EXACTLY!! If you are 1/4" above or below where you should be at 100 yards, you have this 1/4" effectively dialed into your scope the whole way out to 600 yards and farther, so it will appear you have a much higher BC when in fact you do NOT!! If you should see the group printing 3" below POA at 100 yards, make sure it's EXACTLY 3" below! A 3 shot group here, and at each 100 yard interval will tell you nothing, so shoot enough rounds at each to find a CLEAR and obvious center in your groups!! Forget about the horizontal in your groups and find the center of the vertical dispersion. And for any LR shooting to be consistant, the rifle must recoil identically throughout your drop testing... don't change your hold or your rest setup.
I take the pressure, temp, and MV and enter it into the RSI ballistic program for a look at the boreline drop column to compare it to what I found in the test. The BC inputed in the program is the only "unknown" variable at this point, and manipulating this number until the drops match will produce a now "known" BC. This program will also tell you what the BC would be if this was standard sea level conditions in which this bullet was being shot, it is not however, but it will show how much temp and pressure effectively alter the BC.
This all said, I prefer to test using seperate chrono's in order to compare the BC of each individual shot. Takes less ammo and time, and is highly accurate.
If I had a few 30 cal Wildcat bullets, in a matter of minutes I could tell if they flew flatter than the 210 Berger or 210 JLK. After seeing how many MOA higher they impacted at LR, coming back to 100 yards to see how many MOA they impacted high or low compared to the JLK would tell the story. BUT, in the same amount of rounds, I could have measured the exact BC with two chrono's... and I would know how much flatter POI would be as well. 15-20 bullets and less than an hour at the range and the actual BC would be settled in my mind.
I've heard that the Wildcat bullt have had large swings in base to meplat length, but what worries me it that I've heard the same thing concerning bearing surface length... very inconsistant as well. If there is, the BC I'm thinking will prove very inconsistant as well, possibly MV and vertical group size too.
I'd been waiting to hear that Richard had resolved these issues before messing with them, as the JLK is so uniform why go backwards... I wonder who makes Richard's dies and if they are of the quality that Jimmy Knox, Bob Cauterucio and others use. As many different type, caliber, weight bullets Richard manufactures, I wonder if he uses the same real expensive, high quality Nemi dies as the others do? Anyone know where he gets his dies?
The Sierra bullets are not near as consistant in bearing length as the JLK, but still they are far better than what I have heard Richard's have been in the past year or two. This has always been my main concern. Even though the BC's do seem pretty high and might seem a bit optimistic to many, I knew at some point I'd answer that one for myself. It is good to hear they have been shooting so well for many folks, and that Richard is highly spoken of too.
