AR Sponsor
Posted: 1/25/2004 10:56:44 AM EDT
|
A while back Bartholomew_Roberts put up a terrific post re; the various types of, and applications of, the ACOG BDC's. He laid out some really good info derived from his ballistics program and some data gained (I don't want to know how) from Trijicon. Since I also have a ballistics program I have learned to have great faith in (Pejsa) and since I have no life because the chicks don't dig me, I thought I'd do a little numbers noodling of my own. I came up with conclusions that are essentially identical to BR's when the assumptions about atmospheric conditions are in the statistically normal range. I was working with the assumption that the actual rifle used would be a 20" flat top running M193. When the atmospheric specifications were: 76 deg F, 0 ft msl altitude, pressure 29.94 in. both the 55 gr and 62 gr ACOG BDC's would seem to be plenty close enough from 100 - 500 meters for any practical applications. HOWEVER, when you change the atmospheric specs to something I'm more likely to shoot in: 100 deg F, altitude 1,500 - 7,000 ft msl, pressure 29.50 in. things go to worms in a hurry. At 400 hundred meters you'll find the actual POI to be 9" - 14" high using either of the ACOG BDC reticles. At 500 meters that POI moves 16" - 27" above POA. That ain't "plenty close enough". Upon reflection this really isn't surprising. A 40 degree temperature drop will cause the 210 gr Bergers (which have the ballistic coefficient of a freight train) I shoot through my 300 Win Mag to droop 14" @ 1,000 yards. Relatively speaking, the M193 projectile has the BC of a ping-pong ball and therefore its flight path is radically affected by moving through very thin air. Do I have faith in this computer program's projections? Yes, I do. I've used the Pejsa program for years and have found its predictive abilities to be be freighteningly close when applied to the range. In any event, it appears to me that I would be just as well off, given the conditions that I shoot in, ( I used to live where it's "normal" and it sucked) to leave the Aimpoint on the gun and use the 4 minute dot for "Kentucky Elevation" or to try to locate an ACOG without the BDC stuff cluttering up the reticle. I humbly await to endure the flames of the ACOGophiles. SD |
|
Look, it's really simple. One, the ACOG if for killing people >.< not for target shooting. If someone is exposing 2" of themselves at 1200 feet you'll never see them and two never shoot them. Now give me 8" of face out to 500 or 600 yards and I may be able to make the shot. Two, mount the bitch on your rifle and shoot the thing at targets from 50 feet out to 1000 feet. Learn your weapon and how it shoots your ammo. If you're really concerned about accuracy spend hours and days ranging people from 250 yards out to 600 or so. Closer than that is a waste as if you're zero'ed at 200 yards the bullet is flat enough from the barrel out to 240 yards to not worry about it. You'll rarely find a concrete bench and known distance targets in combat. Beyond 600 yards you ought to trade that .223 in for something with more balls like a .308 as after shooting the guy with the 62 grain bullet coasting out there at 800 yards you'll piss him off and he'll be likely to return fire with a real 800 yard weapon. Range finding is important as if that 8" plate is at 550 yards and not 450 yards you'll miss it. Understand what accuracy is good enough for a combat kill and what isn't. I don't know anyone that uses a .223 to engage people at 3000 feet. Wearing hunter orange standing in a plowed cornfield covered in bright white snow ... you might beable to see me - now finding that handy concrete bench, laser range finder (good for 1000 yards) is going to be difficult. Sounds like you're trying to use a crude ACOG for benchrest work but as you noticed there is no range finding provision on the scope and the BDC is only approximate - if it's good enough for combat is your opinion. When it's all over the only thing that counts is hits the weapon and its scope are the least problem. |
|
I came up with conclusions that are essentially identical to BR's when the assumptions about atmospheric conditions are in the statistically normal range. Yes, the BDC calculations all assume a statistically normal condition. However, if anybody would like to expand on any of the loads posted with practical results on a real range or calculated results for different temps, pressure and humidity, I am sure we would all welcome them. [:)] |
|
[b]Paul:[/b] Since I agree with most everything you say and you have taken exception to what I had to say, it would appear that I did a poor job of explaining myself in the first place. This was an academic exercise to show that BDC's using pre-determined drop tables; be they etched in glass, gear-driven, or knob markings, are only applicable within a fairly narrow range of atmospheric conditions when used with a bullet of low BC. Your point that a .223 caliber bullet becomes an irritant rather than a fight-stopper beyond a couple of hundred yards is well taken. I've pulled too many .223 bullets out of pine target frames with my bare fingers at the 600 yard line to believe that the round represents a threat to anything other than paper at this distance. If someone is engaged in unacceptable behavior at a distance of more than two hundred yards, it would seem that my options are: A: Crawl closer. B: Run home and bring back my SR-25. C: If they're really out there; bring back the 300 Win Mag. If I am reading you correctly we are in agreement that (given standard sight line height) zero’ing an M193-launching gun 1” high at 100 yards will put you well within the money (10 ring, A zone, take your pick) at any distance within the cartridge’s relevant range. And I believe that we are in agreement that the functions of optical magnification within the context of these calibers is target acquisition, clarification, and discrimination. You may or may not agree with my belief that BDC’s for light caliber anti-personnel rifles may well be irrelevant as the distances at which BDC’s matter are distances at which the bullet is not an effective behavior-stopper. I don’t know how closely you read my post but, if you’re claiming that an 18” deviation between POA and POI is close enough for combat purposes, that is something we do not agree upon. Sorry for any misunderstanding and hope this clarifies my position. SD |
|
[b]So would it be better to spend more time at the range practicing, then it would sitting around crunching numbers?[/b] Depends on what your goals are for that range trip. To wit; When I was looking for an 800 zero for my 300 win Mag I first crunched the numbers on my handy-dandy Pejsa program. Upon arriving at the range, dialed in the predicted elevation, fired three rounds and measured the target, dilaed in an additonal 0.5 min elevation, and landed smack on the X. Thereby freeing the rest of the day for [i]productive[/i] trigger time. Read [i]productive [/i] as practice during which I'm getting accurate feedback as to the results of my actions; not to be confused with blowing holes in the air and hoping I hit something. Best to all. SD |
|
Quoted: [b]So would it be better to spend more time at the range practicing, then it would sitting around crunching numbers?[/b] Depends on what your goals are for that range trip. To wit; When I was looking for an 800 zero for my 300 win Mag I first crunched the numbers on my handy-dandy Pejsa program. Upon arriving at the range, dialed in the predicted elevation, fired three rounds and measured the target, dilaed in an additonal 0.5 min elevation, and landed smack on the X. Thereby freeing the rest of the day for [i]productive[/i] trigger time. Read [i]productive [/i] as practice during which I'm getting accurate feedback as to the results of my actions; not to be confused with blowing holes in the air and hoping I hit something. Best to all. SD Damn, that almost takes all the fun out of zeroing your rifle in. [:D] |
|
[b]However, if anybody would like to expand on any of the loads posted with practical results on a real range or calculated results for different temps, pressure and humidity, I am sure we would all welcome them.[/b] Splendid! Just to have a starting baseline, let's see if we agree on the drop tables ACOG is presumably using. Using the data you got from Trijicon (and how you got it is none of my business), I used my handy-dandy Pejsa program to reverse-engineer what I believe the drop tables (in inches) are represented by the two ACOG reticles. The data I used is: "M193" reticle: 3,260 fps, BC=.243, bore line-to-sight line: 3.5" "M855" reticle: 2,970 fps, BC=.304, BL-SL: 2.5" In both cases I speecified atmospheric conditions as 76 feg F, Sea level, pressure=29.94 inches of Mercury. What I came up with for drop tables is: "M193": 200M 1.86" 300M 11.1" 400M 30.8" 500M 65.7" 600M 123.7" "M855": 200M 3.6" 300M 15.2" 400M 37.3" 500M 73.5" 600M 129.3" Is this reasonably close to what you came up with? Best, SD |
|
Quoted: Using the data you got from Trijicon (and how you got it is none of my business) Trijicon gave me no data other than what was on their website regarding the various ACOGs. I used know data for M193 and M855 in various barrels and measured the height over bore to the ACOG. The data I used is: "M193" reticle: 3,260 fps, BC=.243, bore line-to-sight line: 3.5" Same, except I used 3,225fps as the velocity. No real difference. "M855" reticle: 2,970 fps, BC=.304, BL-SL: 2.5" Exactly the same. In both cases I speecified atmospheric conditions as 76 feg F, Sea level, pressure=29.94 inches of Mercury. The program I used was an online Java applet located here: [url]http://www.norma.cc/htm_files/javapagee.htm[/url] It uses standard atmospheric conditions and doesn't allow you to tweak them except for wind direction and velocity (both of which were left at 0 degrees and 0 mph) for the chart. Is this reasonably close to what you came up with? It also doesn't display drop in that way. It only displays drop from zero range. I used 100yds as the zero range. |
|
OK, sorry I didn't read that more closely... Looking at my results, mine were (these are in yards, not meters) 55gr 200: -0.8 300: -7.3 400: -22 500: -47.9 600: -89.9 62gr 200: -2.4 300: -11.0 400: -27.9 500: -55.3 600: -96.8 So it looks like we are seeing some significant differences - probably in what the Java applet program defines as "normal" ranges. |
|
[b]So it looks like we are seeing some significant differences - probably in what the Java applet program defines as "normal" ranges. [/b] Nope. When I slow the 55 gr down to 3,225 fps and use "standard" (by USWS and FAA standards) conditions 76 deg F, sea level, 29.94 in. pressure, and increment in yards, I get results so close to yours as to be essentially identical. Can your program use 110 yard increments to simulate 100 meter increments? My understanding is that ACOG BDC reticles are calibrated in meters and I was trying to coordinate the drops to the numbers you would see alongside the verticle crosshair leg while operating the gun. The real eye-opener occurs when you vary the atmosheric conditions. A light weight bullet like the 5.56 is extremely susceptible to variations in air density. (Duh!) If you were to zero a rifle using M193 at 400 meters in Wisconsin in January; and then took that rifle to a carbine course at Gunsite in July (5,400 ft msl, 95 deg F) your attempt at a 400 meter chest shot would put your round two feet over the head of the target. That ain't close enough even for gubmint work. If you really enjoy this stuff, you might want to invest the measly sum of the Pejsa ballistics program. It's an old DOS program developed by a stark, raving genius named (surprise) Dr. Pejsa. He developed the algorhythms that are still driving the Army's Paladin batteries, and the Abrahm's shoot-on-the-move gun. If you feed the program good data, it'll produce actual range results that are down right eerie. For instance; I zeroed my new Nightforce scope on my 300 Win Mag at 100 yards. Then I backed up to 800 yards, dialed in the elevation Pejsa called for at that distance, and cranked off three rounds. From there, it took 2 clicks (0.50 MOA) to put me in the X. Put another way, had I only been using the program's output and a known 100 yard zero, and had I been trying to put a round in the center of an opponent's chest at 800 yards, I would have put the round through his liver. For all I know, that 0.50 MOA correction could have been due to a difference between the actual temperaure shot and the assumed temperature I plugged into the program the night before; or a micro-anamoly in the scope's internal gearing. I don't know about you but, that comes pretty close to my definition of voodoo. Luck, SD |
|
I wish we had a range where I could shoot at ANYTHING more than 200 yards out. I would be pretty confident to use my 16" Bushy AR out to say 300 yards for a good clean kill. I'm using an EOTech on it, it's free floated.... Beyond 300 yards I'm really wanting an AR-10 with a 20" barrel, maybe even a 24", free floated, scoped with something like a US Optics, a Knightforce, or at least a Leuopold Tactical model. I have to do some MAJOR studying in ballistics. After spending almost 3 hours on the net this evening looking at the websites of US Optics, Knightforce, Schmit Bender etc. studying all the reticle options, front or rear focal plane reticles, parralax adjustments, MOA click adjustment options, 30 mm tube vs. 1" tube etc. I realized I don't know Jack about shooting long distance. In fact I know less than Jack. Can anybody recommend any good books on the subject that are easier to read. I don't really want to wade through 300 pages of "watching paint dry" to learn more about ballistic properties and how scopes relate. Thanks for listening. |
|
[b]BVM:[/b] As a good starting point (but not a culmination of knowledge) I'd suggest John Plaster's most recent addition of [i]The Ultimate Sniper[/i]. It's a good read to develop a framework for long distance and contains a superb bigliography to extend your knowledge. My own, admittadly limited, observations: The German stuff (Schmidt & Bender, et al.)exhibit stupendous optical clarity but lack adjustment range. Germans are just not long range shooters. Nightforce scopes are built by glazed-eyed fanatics who are utterly psychotic about optical clarity, adjustment range, and durability. They are very expensive and very heavy. I own three and would give up one of my sons before I would give up one of those scopes. If you are thinking about an AR10 I would give some thought to a Shepherd scope. I've got one on a Knight SR-25 and find it to be an excellent compliment to an auto-loader. It's not an X-ring scope but if you are facing multiple targets at multiple distances, nothing else can stay with it. Be a little cautious with Leupold. The Leupold I grew up with and worshipped as a boy is not the current Leupold Co. The current company is being run by a bunch of Docker-wearing, Volvo-driving MBA's. There is still some good stuff in their inventory but, they are also putting out some real ka-ka. If you are really considering joing the world of out-past-500-yards, you are embarking on a journey frought with bewilderment, frustration, and joyous accomplishment. Luck, SD |
|
Quoted: Can your program use 110 yard increments to simulate 100 meter increments? My understanding is that ACOG BDC reticles are calibrated in meters and I was trying to coordinate the drops to the numbers you would see alongside the verticle crosshair leg while operating the gun. It can out to 550M (since it maxes out at 600yds). I had done the initial calculations in yards because I simply hadn't paid attention to the Trijicon literature. Later when I discovered my error, I redid a few in meters but it didn't make any real difference to the impact points since you were basically matching the same parabola regardless - just choosing different points along the trajectory to compare measurement. Since it wasn't having a major effect and it was a lot of work to recalculate everything, I didn't redo the others that had already been calculated (so all of my notes still show yards). The real eye-opener occurs when you vary the atmosheric conditions. A light weight bullet like the 5.56 is extremely susceptible to variations in air density. (Duh!) If you were to zero a rifle using M193 at 400 meters in Wisconsin in January; and then took that rifle to a carbine course at Gunsite in July (5,400 ft msl, 95 deg F) your attempt at a 400 meter chest shot would put your round two feet over the head of the target. That ain't close enough even for gubmint work. Looks like a good idea to confirm zero after long travel. You should tag the note about atmospheric conditions to the BDC chart so that people who have only a passing glance at ballistics understand that a zero in one location isn't always a zero in another. |
|
Quoted: BR calculations are very acceptable with TA-31...hits @ 0 ft to 400yds using the BDC he calculated with M193 M855 77gr OTM , M4 14.5" ......it has worked for me archad: What altitude/temp wre you shooting at? Thanks for the input. By the way BR; I just got that "fun fact" you put in the original post describing how, when you lower the sight line one inch on the gun, the POI converts to (YTT - 1)" low on the target. (Where YTT = yards to target expressed in 100 yard increments.)That's a very handy thing to know for those of us who have, or plan to have, a 55 gr reticle ACOG on a flat top. It also explains why I had such a bitch of a time developing a .308 load that would track the Shepherd scope BDC reticle for my SR-25 (which sits about 1.75" higher on my gun than on the bolt guns it was designed for). With Mr. Pejsa's help I finally got there but, it would have been a lot easier process had I examined the data as closely as you did. SD |
AR Sponsor