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Posted: 3/21/2003 1:20:56 PM EDT
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Hi all, I need some expert ballistic info from you AR gurus. I have a Colt AR-15-A2 Sportster II. I have mounted a small 4x20 Scope on the rifle. I have been shooting at 50yds. The rounds are landing 1 inch above the bullseye in a very tight group. Will this mean that I'll be getting a bullseye dead on at 100ydrs? I'm using 55gr fmj rounds. Thanks in advance |
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No telling. If you need to know (and if this is a hunting gun, you DEFINITELY DO) then shoot targets at 50M, 100M, 150M and 200M, and further if you are a good shot to see for yourself your maximum range, to get a better idea of the ballistics. Several variables, such as the scope's height over the boreline, barrel condition, bullet velocity, ect all play a role, and without those numbers to crunch, well, who can tell? You will probably get a 200M zero pretty close by zeroing at 25M, but some AR's do not conform to this rule of thumb. Regards, FastVFR800 |
| Thanks fastvfr800. Absolutely right. There are a lot of variables. i was hoping for an approximation. Well the scope is mounted on the handle of the AR. The barrel is in excellent shape , 20" long and it's 1:7 twist. So zeriong at 25M and not yards ..should set the scope pretty close to 200M. Correct me if I'm wrong. As far as the ballistic attributes to the 223 round how flat is the trajectory in an AR rifle?? Any ideas?? |
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with the line of sight line of bore issue involved ....my opinion is. the bore is about 2 inches below line of sight and bullet rises to line of sight for a zero at a given range. it will then rise above line of sight then drop as velocity drops and gravity gains hold it will then drop back through line of sight and continue its drop. by sighting in 1 inch high at 50 yds the bullet is still rising in its trajectory. i figure your going to be about 4 inches or so high at 100 yds. you may need to sight in 1 inch low at 50 yds to be dead on at 100 yds (just speculating as these are approximates). |
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oh one more thing. as far as trajectory goes. with a 55 grain bullet at 3280 fps from my 20 inch barrel i get as follows at 50 yds im about 1/2 inch low. only tried it once but i believe thats where it was. 1)dead on at 100 yds 2)approx 2 3/4 low at 200 3)approx 10 1/2 low at 300 4)approx 25 1/2 low at 400 5)approx 51 inches low at 500 this will all vary depending on hight of scope above bore (on your a2 it will be different than on a flat top). this was all done on my varmit rifle and am in process of figuring it out for my a2. for the ar i dont think id ever mount a scope on the carry handle for distance work because the hight above line of sight would create a scenario in which you would have to start from scratch like i did because of the rainbow trajectory the bore/sight relationship created |
| No, your rifle is zeroed for 200 meters. At 100 meters your bullet is 1.6" high and still climbing. Max. height is 1.7", and it's back down to the line of sight at 200. I wouldn't change a thing - this is a very useable set-up, and my personal favorite. For any shot out to 200, you just put the reticle on the spot and fire. After 200, it's starting to run out of steam, so the drop accelerates. You'll have to find a ballistics computer on the net somewhere for that. |
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Quoted: So zeriong at 25M and not yards ..should set the scope pretty close to 200M. Correct me if I'm wrong. Nope - that is very wrong. Generally zeroing at 25M gives you a 375M far zero. At 25 yards its in the 400M range. And if your Scope is higher than 2.6" over the bore - it pushes the zero even further out. To zero your Scope/Rifle combo set the BDC on your scope to '4' then zero at 25M (or yards) - this gets you 'on paper'. You can then go to 50 yards (set the BDC to '2') for that 200M zero [also my favorite]. Or you can set the BDC to '1' and fine tune the zero at 100 yards. We have lots of info on Zero and Trajectories at the Maryland AR-15 Shooters Site ([url]www.MD-AR15.com[/url]) under the 'Zeroing Methods' section. As far as the ballistic attributes to the 223 round how flat is the trajectory in an AR rifle?? Any ideas?? For M193/M855 type ammo you're looking at a flat trajectory (within 2" of the LOS) out to 225M with a 50 yard zero. If you go to the site mentioned above I've got some charts showing bullet trajectory for a variety of zeros (25y, 36y, 42M, 50y) |
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Thanks again Forest, Yes indeed I have a BDC on the scope, correct me if I'm wrong so if I adjust at 50yards with the 200M setting on the BDC and I hit bullseyes, then it should be automatically adjusted for all the other ranges. PS: I noticed when I set the BDC to 200M, the crosshairs went higher. Is that my perception? |
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