JBM Ballistics website help! (drop chart)

[esheato]

I need some help using my scope properly...I shoot to 500 on a regular basis, but I use Leupolds cheater reticles (B&C and Varmint Hunters). I'm trying to wrap my head around the MOA system of developing a drop chart and shooting to longer distances. There is a 1k range fairly close to me and I'd like to start taking advantage of it.

Goal: get to 1k yards or more.

Gun: Cooper Mod 22, 243 AI, Nightforce 5.5-22 NP-R1 (MOA reticle, MOA knobs, .250 MOA clicks), Nightforce rings, Warne bases (no built in elevation, USO makes a 20 MOA base though...I might upgrade). Scope has 100 MOA of elevation travel.

Load: Lapua brass, Fed Gold Medal Match primers, 44.5 grains of H4831SC, Berger 70 grain Match, OAL 3.245"....chronographed average velocity: 3082 fps (high 3101, low 3058 fps, SD of .44%)

Zeroed at 100y.

Conditions: Elevation of 2000 ASL, 60 F, variable wind (this is West TX, it never stops)


Looking at the JBM Ballistics website, there are multiple charts available. I've been tinkering with the trajectory, trajectory cards, trajectory ucards and trajectory ucards-constant drop.

My plan is to dial elevation and hold off for wind...which chart would you use and why? I like the trajectory card, but don't know what the data in the center column is. I'm assuming one side is inches of drop while the other is come-ups for that distance. See the graphic below.



The trajectory ucards-constant drop makes sense....it shows click values that match my knob, but only runs out to 750 yards with the 100 MOA turret limit. Would that necessitate the USO base with built in elevation of 20 MOA? If I put the USO base on, can I still have a 100y zero or does it have to be farther? See the graphic below.



When I run any of the first three charts (trajectory, trajectory cards, trajectory ucards), I end up with click values that don't match my knobs....ie, 7.6 or 12.8...how do I dial that on a Nightforce? Looking at the pic below, that's how my knob looks. Quarter MOA per click....I can't physically dial 15.3!



Also, do you recommend intervals of 25 or 50 yards once you get out past 600? Using 25 yards would make for a long chart.

Sorry for the simple questions, but using these reticles is new to me and I don't want to look the fool at the range (I'd rather look the fool to you guys...lol)

Thanks!

Ed

 

[taliv]

nice setup!
post some pics


http://www.jbmballistics.com/cgi-bin/jbmtraj_drift-5.0.cgi

pick your bullet, length, velocity, etc
pick your 100 yrd range

near the bottom, in "column 1" select MOA. in "column 2" select inches, then you should see a value of 0 MOA at 100 yrds. beyond that is what you dial onto the scope to hit at a given range. the 'inches' column is just how much it drops

if you are going for unknown dist, you'll want 25 yrd.
if you're going for known dist, i'd just scratch the 600, 800, 900, 1000 dope on a napkin and wait til you get to a range and try it before making a perm card or writing it into your book

 

[esheato]

Posted JBM pics while you responded....I'll have to snap some gun pics.

taliv,

Using the link you provided, here is my data:



Looking at the highlighted area, how do I dial 111.7 for 800 yards? Can I round it to 112? Seems silly to round when everything else is so precise. Would I also be able to divide 111 by 4 to get the MOA required versus the clicks? 111.7/4=27.925 Could I round that to 28 MOA? In other words, I could dial 112 clicks (I would think this would take to long) or 28 MOA and pull the trigger?

Maybe I'm missing something....

Ed

 

[taliv]

so when you select 'column 1' change the units from .25 to 1. that will make the output in MOA instead of 1/4 MOA

you'll definitely be rounding. to put that into perspective, using 1IPHY or SMOA, 1 MOA = 8" at 800 yrds, so each 1/4 MOA = 2". since you can't dial in that .7, you either round up or down. In this case, up, and in theory your bullet will impact about 2/3 of an inch below your POA. Now, if you think after 800 yrds, shooter error, mirage, wind, etc... 2/3" is going to be a an issue, then you'll just need a scope that does 1/10th or 1/8th MOA instead of 1/4.

i've no idea what that middle column above is about, or why you'd want to use the 2nd table you posted

 

[esheato]

Alright, alright...

Put the output in whole MOAs, and of course my knob remains at .250 MOA....that way I just dial what I need. I was trying to get the chart to output the same as my scope knob markings.

Now, if you think after 800 yrds, shooter error, mirage, wind, etc... 2/3" is going to be a an issue,

Nope...not even worried about it. The rounding just seemed counter-intuitive considering the precision of everything else.

Do you think I need an elevated base? How far should I be able to get out to?

Last thing...what should my zero be? I'd like to keep the standard 100y zero if possible.

Let me try this once...here is the adjusted chart:



Looking at the drop for 600 yards, it is 15.2...of course I could round that to an even 15 but wouldn't it be easier to dial 15.25.....15 MOA and one click extra?

Rounding makes sense for everything but:
15.2=15.25
15.5=15.50
15.7=15.75

Ed

 

[taliv]

i keep a 200 yrd zero on all my rifles except my HP Service Rifle,which has NM style iron sights, which gets a 100 yrd zero.

right, i'd round stuff exactly like you have it. 15.2 is 15 1/4. 6.8 is 6 3/4.

btw, are you sure you got your BC right on that projectile? i thought pretty much any 243 round would still be supersonic at 1000. and especially an AI. granted, my 6mm shoots 107g bullets, but it's supersonic a little past 1200 and starts out much much slower than yours

 

[esheato]

I wondered the same thing and checked it before I posted the original message. Considering the bullet is a flat based varmint bullet, the low BC doesn't surprise me. Unless I change out the barrel, there's not much I can do as the Coopers are designed for the light weight stuff. Current velocity is 3082 (over 10 shots), if I push it to 3400 (nearing max pressure), transonic is only pushed out another 100 yards. Still 100 shy of the thousand yard mark.

BC is whatever JBM has on record...I selected the bullet and it input all the projectile info. Oddly enough, Bergers site says the BC should be .277.

Ed

taliv, I really appreciate you helping me with this. Sure cleared up a lot of questions I had. If we ever meet, beers are on me.