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Posted: 8/17/2011 7:52:53 PM EDT
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Ok guys, I need some help from the hive. I have a 16" Bushmaster ORC .308, an Eotech, and South African surplus ammo. I have zeroed this rifle at 50y [all the range I have right now] but my buddy just shot the chrono that I was going to borrow, so I am kinda stuck. What I need to do is figure out what my hold-overs/unders are from 100 out to 4-500 yards. I am fairly good at range estimation, and most of my shots are going to be quasi-known distance [that one is about 400y]...
So, #1 has anyone put SA .308 over a chrono out of a 16" bbl and have a guess at what the Velocity should be? #2 once I have a swag at the velocity, I can plug it into a ballistic calc program to get drop info. #3 then I need to take the drop info and convert it to sight picture estimations for my eotech. I swear after this season of 3 gun I am going back to irons... In the meantime, help? |
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Dude, just shoot on the targets and measure ups when you retrieve the target. I have found that chronos will only get you close.
Too many variables. When I true my long range stuff I use an estimate fps reading like 2600 for .308. I then run that through atrag and get my up in mils for like 700 yards. Then I go shoot that hold at 700 to see where it actually hits. It is always within a mil. I then use the actual drop to find my fps and build a range card. |
| I agree that actually shooting at range is the best option, but I am going to a 3 gun match in a different state with targets out to 4-500 yards, and I don't have the time or access to a long enough range to do that. The only rang I have access to that goes past 100y is 90 miles away and there is an organized event there on the day that I have off to go shoot between now and the match. Practically, I have access to 100y outdoors, and 50y indoors, that's it, so I need to make that work. I am zeroed 5 " high at 100 right now... |
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OK, using TaylorWSO's information, and handloads.com's ballistic calc AND JBM ballistics
I came up with this... [terribly sorry for the formatting, I combined the 2 tables]. Velocity Bullet Wt. Sight in at Ball Coef. Sight Ht. Intervals Mx Range 2508 ...147... 28... 0.41... 2.5... 25 ....500 DropDrop Range….Velocity….Impact….Drop….(in)….(MOA) 25….2454….-0.25….0.24….-0.2….-0.9 50….2405….1.64….0.84….1.6….3.1 75….2357….3.17….1.81….3.2….4 100….2309….4.29….3.18….4.3….4.1 125….2262….5.01….4.95….5….3.8 150….2216….5.29….7.16….5.3….3.4 175….2169….5.15….9.8….5.1….2.8 200….2124….4.53….12.91….4.5….2.1 225….2078….3.42….16.51….3.4….1.4 250….2034….1.81….20.61….1.7….0.7 275….1990….-0.31….25.23….-0.4….-0.1 300….1947….-3….30.41….-3.1….-1 325….1904….-6.26….36.16….-6.5….-1.9 350….1862….-10.12….42.51….-10.4….-2.8 375….1820….-14.61….49.49….-15….-3.8 400….1780….-19.75….57.13….-20.2….-4.8 425….1740….-25.58….65.45….-26.2….-5.9 450….1700….-32.13….74.49….-32.9….-7 475….1662….-39.43….84.28….-40.4….-8.1 500….1624….-47.51….94.86….-48.8….-9.3 So, it looks like if I zero my eotech at 30 yards or so, I should be able to hold 6 O'Clock out to 300, then over out to 500, but the MOA numbers have me confused, AND the last time I tried this I had disasterous results... I tried this for CMMG and ended up having to re-zero to about 100 after totally blowing the first 2 stages... but my problem of only having a limited range to work with remains... |
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What did you set the zero range at on the Calculator? That range card looks like you plugged in a weird zero range? anyway MOA on it does look correct at 500 yards. It does depend what your elevation adjustment is in clicks though. 1/2 inch clicks at 100 yards=1/2moa,1/4 inch clicks at 100yards=1/4moa. TO explain the MOA at 500 it means with 1/2moa clicks with you would need 18 clicks to get to 9 moa not exactly 9.3moa as on your chart but close. Its your choice if you decide to dail 9.5moa and shoot low or 9moa and shoot a little high. This is elevation only doesn't include wind.
http://www.youtube.com/watch?v=VA2PZBD5Tjg this video will help you fully understand MOA. It isn't has hard as you think. I would suggest simple though and practice. Also i think the VORTEX OPTIC's calculator is one of the better ones out there and it is free with a lots of adjustments. |
| According to the ARMALITE FAQ referred to by TaylorWSO above, I started with 28 yards which with a 20" bbl works out to something like 285, like on a .223 AR zeroing at 25 gets you 300, or 50 gets you 250 on an M4. The problem with clicks, is I am running an EOtech XPS, so all I have to work with is the dot and the circle. Since I know it's around 30MOA between the dot and the edge of the circle, My plan is to guess. If I need -10MOA, then I am going to estimate a third of the way from the dot to the circle, and so on past the zero range, and hold 6 o"clock for everything up to the zero range. The problem I am having is the last time I tried this, my 25y zero was not effective at 100+ yards, so somewhere I went horribly wrong, so now, I am starting over... |
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