Armory Sponsor
Posted: 10/3/2009 3:04:33 PM EDT
|
Today I shot 3 different loads and 2 were spot on at 100 yds but the 3rd was ~ 2 inches low.
Full details: 1) All experiments were 3 shots at 100 yds using the same gun –– DPMS with 20" heavy barrel. 2) Load 1 23.6 gr IMR 3031, 55 gr Speer spitzer, CCI-450 primer, OAL 2.490 - 2.540, length to ogilive +0.035 to 0.045 using Precision Mic 3) Load 2 24.5 gr H335 , 62 gr SS109, CCI-41 primer, OAL 2.250, length to ogilive -0.007, velocity 2825 fps 4) Load 3 25.0 gr H335 , 55 gr FMJ, CCI-450 primer, OAL 2.202, length to ogilive +0.045, velocity 2915 fps Loads 1 & 3 were spot on at 100 yds with a nice group. Load 2 had a nice group, but was 2" low. I repeated the experiment 4 times with the same results so something about load 2 is consistently low. Has anyone come across something like this? Because the SS109 bullets are very long, I had to seat them deeper than the 55 gr in order to fit the magazine. I guess I could load some SS109s longer than 2.260 and hand feed them just to see if the 2" drop changes, but that still would leave the magazine problem. Another thought is I could split the difference and move the scope up 1". However, I am reluctant to do this. TIA for any comments. |
|
You're shooting different bullets with different gunpowders or charges. Why would you expect them to impact the same point?
Usually when this problem crops up, it's caused by the additional recoil of the heavy bullet making the bullet strike higher than lighter bullets. In your case, you need to spend some time in a ballistics program looking at a predicted trajectory for those bullets and speeds. You will have to dig around on the internet to find the ballistic coefficient for the SS109 bullet. This one is easy to use: http://www.hornady.com/ballistics/ballistics_calculator.php |
|
Quoted:
Today I shot 3 different loads and 2 were spot on at 100 yds but the 3rd was ~ 2 inches low. Full details: 1) All experiments were 3 shots at 100 yds using the same gun –– DPMS with 20" heavy barrel. 2) Load 1 23.6 gr IMR 3031, 55 gr Speer spitzer, CCI-450 primer, OAL 2.490 - 2.540, length to ogilive +0.035 to 0.045 using Precision Mic 3) Load 2 24.5 gr H335 , 62 gr SS109, CCI-41 primer, OAL 2.250, length to ogilive -0.007, velocity 2825 fps 4) Load 3 25.0 gr H335 , 55 gr FMJ, CCI-450 primer, OAL 2.202, length to ogilive +0.045, velocity 2915 fps Loads 1 & 3 were spot on at 100 yds with a nice group. Load 2 had a nice group, but was 2" low. I repeated the experiment 4 times with the same results so something about load 2 is consistently low. Has anyone come across something like this? Because the SS109 bullets are very long, I had to seat them deeper than the 55 gr in order to fit the magazine. I guess I could load some SS109s longer than 2.260 and hand feed them just to see if the 2" drop changes, but that still would leave the magazine problem. Another thought is I could split the difference and move the scope up 1". However, I am reluctant to do this. TIA for any comments. Welcome to the wonderful world of reloading... try shooting the same weight bullet ,but different "brand" surplus ammo. I did it with my .308...the groups were all over the board. So in other words ...thats normal. |
|
100 yards is too short of a distance for exterior ballistics to explain bullet drop. This is mostly an interior ballistics issue and I would bet you are getting better, faster ignition from the increased engraving forces of the longer bullet. A bullet that is a full 12% heavier generating only 3% lower velocity with a half grain LESS powder? There is your answer. Acceleration of the 62 grain bullet is greater, resulting in shorter bore time. |
|
AeroE ––- The reason I expected the point of impact to be much closer is exactly what Keith_J said –– 100 yds is much too close for a different BC to account for -2 inches. I did the ballastics calculator exercise using .243 for the 55 gr FMJ and .304 for the SS109. The trajectories were virtually identical out to 200 yds where both showed 3.9 inch drop.
Keith_J –– I understand what you said about the 62 gr bullet accelerating faster with 12% more weight while having only 3% less velocity and 2% less powder, but I do not understand how less time in the barrel causes 2" of drop. I'm not saying you are wrong, just that I do not "get it". Will you explain more please? If I extend your point, will less powder behind the 62 bring the point of impact up at 100 yds? TIA |
|
Quoted: AeroE ––- The reason I expected the point of impact to be much closer is exactly what Keith_J said –– 100 yds is much too close for a different BC to account for -2 inches. I did the ballastics calculator exercise using .243 for the 55 gr FMJ and .304 for the SS109. The trajectories were virtually identical out to 200 yds where both showed 3.9 inch drop. Keith_J –– I understand what you said about the 62 gr bullet accelerating faster with 12% more weight while having only 3% less velocity and 2% less powder, but I do not understand how less time in the barrel causes 2" of drop. I'm not saying you are wrong, just that I do not "get it". Will you explain more please? If I extend your point, will less powder behind the 62 bring the point of impact up at 100 yds? TIA Yes. Less powder could make for higher impact. For every action there is an equal and opposite reaction, the rifle starts recoiling the instant the bullet starts moving. In the AR15, most of the recoil is straight back but depending on bench rest technique, you can still have slight muzzle climb. Of course the measured difference of impact would mean you have some very accurate loads. Next time, try position shooting. With the same load, see where the groups impact depending on the position the rifle is held. Prone and sitting/kneeling should be pretty close, depending on how well you shoot of course  .I know this well as most of my shooting is with a magnum spring-air rifle which DEMANDS consistency of hold since the instant the trigger breaks, a piston is shoved forward with a huge spring force. I can shoot this rifle much better from sitting than from ANY bench rest variation.  25 yards, sitting no sling. 5 rounds, yeah, I pulled the one at 12:00 . I would be lucky to get twice that size off the bench. |
|
When a bullet is shot thru your barrel, there can be different harmonics for different loads - and your loads/bullets certain are different (55 vs. 62 grain). Think of it as a garden hose that you grab at one end and shake up and down while simultaneously tilting it skyward (recoil). Bullets can leave the hose (barrel) at a different spot in the cycle and general arc due to harmonics, thus impacting in different places on the target as well as being a factor in accuracy. So don't worry about being 2 inches low - you are shooting so close that BC and drop really hasn't come into play in a major way and you can just adjust for that with the sights.
This is one reason that you just can't change loads willy-nilly without the need for sight adjustment. Ballistic programs can aid with both deciding on on a zero, and with bullet rise and drop at range for the load you finally choose. Having the actual velocity of the load + the bullet BC certainly puts you ahead with these programs - bur you need to treat each load's point-of-impact at 100 yards on an individual basis. Clear as mud????? 
|
|
Thanks for all the replies.
I have read about of the phenonmen of barrel harmonics, but clearly had no idea it would mean 2 MOA. This is the first time I have ever actually seen it in action. I will go play with changing the amount of powder behind the 62s to see if there is a "sweet spot". |
Armory Sponsor