I realize numbers in the computer and bullet holes on paper can be way off. I am shooting a Remington sendero 300 rum 6.5 x 20 LR Leupold with a gentry brake. I am loading 100.5gr retumbo w/ 180 gr accubond. I have chronographed the load at 3350fps avg. I am shooting 2.8 inches high at 100 for a 300 yard zero. Here is the problem, when I shoot the 300 yd target, I am actually 3.5 in low. My question is how do I dial up to 400 yards or further if the ballistics info is that inaccurate at close range.
I am using the shooting chrony's ballistic program. Any suggestions other than shooting and documenting the poi. I am limited to a 300 yard shooting range. I am new to trying to strech the range but not to shooting out past 300 yad. I just have never had the optics to do it.
Thanks for the responses,
TXGS

The last post I was at had a 300 yard range, and the paper ballistics never matched the actual. Some dropped less at 300 and some more. Just a matter of shooting at that distance and learning where they hit. As a matter of fact, the sight in so much high for a 200 yard zero, rarely worked as advertised. Just one of those things.

Adjust to the zero range you want and go from there........that's really all that's important.

MM

Yes, do the zeroing at the range. I would never trust some black box calculations unless they were time tested. 1Minute

Pretty typical experience...you are not alone.

I'd also shoot for a 300 yard zero and proof 100, 200, 400 for sure and if so inclined, 500 on out if you so desire just for grins.

Just a thought....altitudes and temperature input into these programs are as important to the outcome of your task as any other required ballistic input. Also, most BC's are inflated by a bunch and under many conditions can be much different than what the book says.

I've run into the same problem as you and perhaps a thousand others. When I get these discrepancies, I often start to reduce the BC's in the program until it fits my drops at 100, 200 and 300 yards. In many instances, a stated .450 BC gives me much more drop than the program states it should. When I reduce the BC down, methodically, and keep inputting these reduced BC #'s, more often than not I find that all the drops and distances seem to line up at around, let's say, a .375 BC instead of the stated .450 BC.

If I'm doing my job right at the bench, all entries into the ballistics program are right as well and I know where I'm impacting at 100, 200 and 300 yards.........changing a BC to match your REAL results can prove a effective way to determine what your drops will actually be. It would stand to reason that a manipulated BC where the ballistics program agrees with your impact points at 100, 200 and 300 yards should carry on out until your bullet nose dives back to earth.

This is an approach I've found necessary to use on occasion and it has more than sufficed.

YMMV....good luck.

Tx: Couple of questions. Is the barrel on your rifle fully floated, or does it have some forend presure, AND are you shooting from the same position and type rest at 100 as you are at 300?

I ask this because sometimes I have noticed that,with rifles having forend pressure, in shooting from a firm sand bag rest to establish zero at 100, they tend to shoot lower at 300 when I switch to a "softer" field position at 300 yards; not always, but I have seen this often enough to pay attention.A change in rests, position, etc can result in a different POI at longer ranges.

What makes me suspicious is that with that load and velocity, you should be POA at 300 yards with impact 2.8-3" high at 100. I say this based on having shot a pile of 180 bullets driven 3100-3200 fps from various 300 mags.

Had this happen to a friend with KS Mountain rifle in 300 Win Mag, bedded forend. We zeroed off the sandbags at 100 (+3"@100); load was 165 Partition at 3100+. Went to 400 yards where we fired field prone. He was 2 ft low, which was not right. He floated the barrel and at the next range session he was now about a foot low at 400, which is more like it. What happened is the forend bouncing off the firm bags was causing higher impact at 100; when he went prone at 400, his rest was softer and the rifle no longer hit high.Free floated barrels are not so sensitive to different positions, IME.

If the Sendero IS free-floated, I have no idea why this is happening.....

Where is the Center of you group at 300 yards

Have you confirmed your velocity reading your Chrony is giving you.
X2 on the erronious BC's.
Also what sort of optical issues you might have with your scope. ie; paralax.

Real world practice from the types of positions you will shoot in the field is what you need. I dont recall seeing sand bags and rear rest out in the field too much.
Use the bench for evaluation of the load and rifle. Use the positions you shoot out hunting to determine your zeros, and come ups.

TxGunslinger �

There is no way a 180g AccuBond launched at 3350fps and 2.8� high at 100 yards should be 3� low at 300 yards. The AccuBond has a published BC of .507. Even if you use .400 instead, zeroing 2.8� high at 100 should put you dead on at 300 yards.

Either your chronograph is seriously wrong or you are changing something when switching between 100 yards and 400 yards. Or you are zeroed about 1.8� high at 100 instead of 2.8�.

you may find that the BC is not as stated by the bullet maker. Drop your BC a few points at a time and see if the program give you the numbers that match the real world results.....

I would think there are skewed numbers filled in such as sight height, altitude, and temprature.
These make big differences from where I live at 350 Ft above sea level, and the 6000 where I hunt antelope.
Just a possibility.
One program I use has a default site height of 1/2" that would make things way off. That particular rifle is 1.7" sight height.

OK here are the values, 90degrees 86% rel humid. 15ft elevation.
I shoot off the same rest every time. The chrony numbers are as follows;3358, 3348, 3355, 3352, 3364. Chrony is a gama master at 10 ft. It rained here today so unable to shoot. I have run thru everything that I do and cannot see anything I am doing different. All the brass is preped the same, same oal length, same bullet AND same lot and bottle of powder. I am shooting .304 rough measured 3 shot group average at 100 yds with this load, shooting 2.8" + at 100. Could it paralex? I am shooting with the scope at 20x. When I get back to the range, I am going to shoot at 300 and bring it up as close to 0 as possible. Once I shoot 3 , if it is consistant, I'll go back and shoot at 100 and 200 and see where my poi after the change. If I do adjust it 2 clicks at 300, it should bring it up to about 3.25+ at 100. There I go thinking again.
Guys, I appreciate you input , keep it coming.
Shawn

Hell....15 foot elevation around here is just an average high tide!!! You could drown before figurin' this thing out!!

Sight height (center of barrel to center of scope) is important in the program input department. Many people assume that the standard 1.5 inches high is always appropriate.....not so. It's generally close, but somethin' about horse shoes fits here as well. Be as close as possible on this required measurement. Heck....I've got scopes on my AR's that have a sight height of 2.75....sure it's a different animal, but some may use defaults on their AR's at the 1.5 sight height setting without actually having done the work.

Zero at 300, shoot 100 and 200 and see where they impact the target, take 'em all home, have one beer (no more!) and measure them all as precisely as possible......THEN keep droppin' your "stated" BC until it fits your real world impact heights. They may not all come out exactly perfect for a myriad of reasons, but they've often come out real close if not spot on for me using that method.

If nothing still makes any sense............punt.

Good luck.

I know Berger bullets either changed or is in the process of changing there BC as they were too high. From what I have seen a few others maybe a little on the high side as well.

Is your chrony giving you accurate velocities?
I have seen some Chronies give inaccurate readings, especially in changing light conditions. I'm tending to beleive this is your biggest culprit.
Get somsone with another Chrony and run them nose to tail and see how much different your Chrony reads compared to theirs.

I have a feeling that 100 plus gr of Rerumbo with an accubond will yield at least 3300 fps. I found that with the pressures the 180 AB generates, that 98 gr or so of Retumbo will get you there, let alone 100.5 gr. I do not think speed is the problem.

BTW 100.5 gr of Retumbo at 3300 fps (according to Hodgdon's website) is a max load using the 180 Speer...not the longer AB which will generate more pressure.

In response to POP, the only pressure signs I see is a flattened primer, no buldging of primers or tight extractions. The same signs as I get at 99.0 graninf. The 180 gr accubond shot simular if not exact groups at 99.0 grains at 3284 fps. Maybe I should back off to 99.0, save the barrel and maybe myself, then try shooting it like I said in my last post. I will rechronograph as well just to check. The chrony is new so I kind of doubt thats the problem but I am going to check to see if I get simular results.
Thanks Again
Shawn

Aside from the argument +2.8 at 100 should be flatter than what you're seeing at 300 yds, I'm not at all surprised that computer figures are off ~ 3" at 100 yds. That's actually pretty close in my experience. I never rely on those but shoot out to what I determine my max distance will be and note drop at each 100 yd increment and then do it all over again with the dogzapper reticle at the same increments and to the same total distance.

Obviously, I'm not a a turret-turning guy.
Gdv

Well I just found the old problem. The 100.5 gr shoots good at 100 but out at 300 very unstable shooting irratic probally the excessive pressure as POP spoke of. When I changed to the 99.0 gr load, The first shot was dead 0 and 2.0"r windage at 300, the second shot was at the 2.25"r windage but 1.5 high, I pulled the third way low guess I flintched. I let it sit for about 15 minutes and shot two other guns. When I shot the 4th shot, It was 1.5" high at 1.5r windage. So the center of my group is +1" ^ and 2" right.I going to shoot again tomorrow or Saturday waiting 10 minutes between shots so I can get somewhat of a cold barrel shot each time and recheck my results.

I sight my rifles in to my preferred zero- in my case, it is usually 300 yards. Then, I shoot the same load at 100 yards, to see where it prints for future reference.

If the numbers are close to published ballistics, fine. Otherwise, I simply don't worry about it- I know the load prints dead on at 300, will be high at midrange, etc.

Knowing how that 300-yard zero prints at 100 is useful, especially if you need to check it at times and locations where a 300 yard distance is not available.

Awesome! Man, that happened to me so many times!

I found the culprit but read on for the results.
Just came home from the range,, loaded 10 rounds. First out of a clean bore was erratic at 3.5 high and 2 L windage 2nd, 3.25^ and .75< 3rd, 2.25^ / 1.25< They were all spread out. The 4th shot settled at 1.75^ and .75< followed by the 5th shot in the same hole!! I made my scope adjustment and the 6th and 7th shot hit 3.25 elev and 0 wind in the same hole!!. I then adjusted 2 clicks down and 8th shot 2.69 elev 0 wind. Well I shot the 200 yd mark and hit the target at 2.75 elv. 0 wind. I then shot the 300 mark .30 low 0 wind. What did I do to the gun to get it to preform? Nothing at all. It was not me either. I had been shooting my brass for the second time and I suspected something was wrong so I loaded new unfired brass .. I sized it all and loaded it. It shot like a dream. I guess when I was working up loads the first time, some of the brass stressed . I hate to discard the once fired brass but my gun just don't like it! Thanks For all your input.