Problems with my Dope

Rabid

Rabid

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HI guys, I'm having some problems with the dope I get out of the calculators VS my real world findings. Heres the deal, I'm Shooting a 300 WM with 200G Accubonds. My average speeds are 2967 fps. I've tried 4 different ballistic calculators and they all come up with the same numbers for for 500 (which is the longest I can shoot here) which is right around 35" low at 500. When I shoot I'm at 40" low. I know five inches isn't a miss at that range but I know it will be further out of wack at longer ranges. I have to bring my speeds down to under 2800 to get everything to match. I know the crony is possably misleading me but it's not the only gun I shoot over it and I don't have this problem with any of the other ones matching up with the calculators. Anybody else with this kind of set up out there?? Thanks
R
 
Rabid said:
HI guys, I'm having some problems with the dope I get out of the calculators VS my real world findings. Heres the deal, I'm Shooting a 300 WM with 200G Accubonds. My average speeds are 2967 fps. I've tried 4 different ballistic calculators and they all come up with the same numbers for for 500 (which is the longest I can shoot here) which is right around 35" low at 500. When I shoot I'm at 40" low. I know five inches isn't a miss at that range but I know it will be further out of wack at longer ranges. I have to bring my speeds down to under 2800 to get everything to match. I know the crony is possably misleading me but it's not the only gun I shoot over it and I don't have this problem with any of the other ones matching up with the calculators. Anybody else with this kind of set up out there?? Thanks
R
Click to expand...

What BC are you using? Quite often the advertised BC of bullets is over inflated. I suspect thats your problem. I have found the data that Brian Litz developed in his book "applied balictics for long range shooting" to be very acurate and creat good dope.
I'm sorry I don't have the book handy so I cant look up your BC as I am away from home. Mabey someone else on here has the book handy and can help you out with a accurate BC.
 
I suspected the inflated BCs. The wind was at my three oclock and light. I just wanted to check my drop so I held on target and got 40" drop to the center of my group.
 
I'm not trying to be an ass here but my question would be why does it matter since you already know how much lower it hits? Accept that it hits lower and make a note. Then next time you shoot 500m just dial it in or hold over for it.
 
You can do one of two things? Figure out why the calculations are wrong, which might be a nightmare, or accept things for what they are and adjust accordingly.

Obviously is something missing in the math, or an assumption is incorrect, or a variable is not being accounted for. It may not be worth your time or effort to figure it out.

Go with what your actual results are.
 
Also why not use ballistic software that lets you correct your MV with your real world data? I haven't even used a chrony for my .300WM load but after inputting my actual drop data into Shooter and recalibrating, it says my MV should be 2735 at the muzzle. The second time we went out after recalibrating my drop, I managed a 1000yd cold bore hit on the target. YMMV
 
Wouldn't you have to true your dope?.....using whatever Scope ht., BC, velocity, and the actual come up measurement your scope is., ie(maybe.135"vs the .125"(1/8) it says on turrets.

In other words get you closest calculator info to agree with your actual dope range increments @ 300-400 500 600 etc. by adjusting the parameters?
 
Plus 5" is a whole moa, I'd consider that a miss at the range, no? I'm not a big fan of the hold over, I believe your point of aim should always be the same, just make the adjustments.
 
@Rabid, what are the conditions you input to the ballistics program (air temp, altitude, barometric pressure, ballistic coefficient)? Did you use standard conditions, or conditions that were present at the place you were shooting?

At what distance was the rifle sighted in? You say it was shot at "500", is that yards or metres? How did you measure the distance?

Did you observe 40" drop at 500 (with your scope set for say a 100m zero), or did you dial some elevation in? Oftentimes the amount that the scope reticle moves is not exactly what it should be (one "sight minute" can be different from a true minute; on one of my scopes, a sight minute is only about 0.8 minutes of adjustment in reality)
 
Thanks for all the input guys. I dont have a problem with just setting the numbers in my calculators to match my drop it just bugs me that they're out. Heres a little more info for some of you that were asking. The gun is sighted in to be 2" high at 100. This is a hunting rifle not a range gun and the reason I wanted to check the numbers was because of an upcoming elk hunt in Colorado that may present shots in the 500 yard range. I do this check on all my hunting rigs to make my range card that goes with the gun. I did take the atmospheric conditions into account when entering the data. The target was 500 YARDS. I know this is just me being anal but I havnt had the calc data be this far off before. I suspect the BC of the bullet isn't as advertised and perhaps my crony is a little on the high side.
R
 
(BTW I like the airplane in your avatar..! I never got a chance to fly one with big wheels like that, they always looked like a lot of fun)

Using correct data, you should be able to match your results to about a quarter of a MOA (i.e. well under 2" at 500 yards), out to 600 yards. At longer distances (800y, 1000y) you can usually match to a bit better than one MOA.

Here are a few possibilities that would explain your rifle having more drop than predicted:

- your Chrony is reporting too high a speed. If set up properly (level and straight), and fired using decent lighting conditions, this isn't terribly likely.

- your bullet has a worse BC than published. Unlikely though.... JBM's library indicates a b.c. of 0.588(G1), which predicts 34.4" drop at 500y (and sea level atmospherics). If I run the same calc using a BC of 0.52(G1), then a 35.7" drop at 500y is predicted. This is not nearly big enough of an effect.

- How do you know the target you shot at was 500 yards? If you taped it, or lasered it, then I would say that it is; but if you didn't personally establish the distance, this would be my prime suspect. If the target was actually at 525 yards, the bullet drop is calculated to be 40.0".

- How accurately was your rifle zeroed at 100? (and, was it 100 yards or 100 metres?). How many shots did you fire at 100 in order to establish that your rifle was sighted in 2 inches high? If you didn't fire enough shots in order to accurately locate the centre of your group at 100, it is possible that your rifle was actually sighted in 1.25" high (in which case, it is sighted in 3/4 MOA lower than you expect it to be... and so the bullet at 500 yards would be 3/4 MOA lower, which is 3.75" lower than you would expect)

- How many shots did you fire at 500 yards? How big was that group? Unless you've fired a number of shots to prove otherwise, it is quite conceivable that your rifle is shooting a 2 MOA group at 500 yards (BTW a perfectly respectable and usable accuracy figure for a long range big game hunting rifle, 90% of Internet chat notwithstanding). If you fired only a single shot at 500 yard, it is quite possible that it might have been the lowest or highest shot in your group - in other words, your group's centre could have been 5" higher than the location of that one shot. If you fired a 3 shot group, the centre of your group is *much* better established than with just a single shot, but it is still quite possible that the actual group centre is several inches away from the centre of your 3-shot group.
 
I use 200gr accubound in my 300wm and G7 B.C. in JBM calculator and I am dead on at 400 yards
correct with turrets or reticule, makes no difference
did not find time to try farther yet
speed in mine is 2895 fps

if you have zeroed with a warm barrel, your first cold shot long range might be litle higher
 
Yep the plane is a blast to fly, the sandbar hopping is as good as it gets in a plane IMO. I'll try to answer your questions. My target was lasered to be 500 yards on the dot. I've done lots of work on my hundred yard bench and it is infact 2" high at 100 yards. Its a custom Giallard 700 and with my handloads its at worst an MOA gun. I shot two groups of three at 500 yards and evin with the gusty little wind that we had my groups were still under 4" (under 2" vertical). I should just forget about it and set my cards up for my new found reality I guess. Thanks again guys.
R
 
Did you take into account the correct height of the scope above the bore?

If shooting in the direction opposite to the rotation of the earth, coreolis drift can have a small downward POI effect.

Was there any down draft at all over the course of fire?

If all of those factors were accounted for, then I would re-shoot for velocity over the Chrono to make sure some weird lighting wasn't messing up your results. If velocity is consistent, then I would suspect the published BC, which can change a little in individual guns, and in different shooting conditions. After accounting for everything else, I would modify the BC in your ballisitic program to match your actual trajectory.
 
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