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Posted: 8/13/2008 1:05:16 PM EDT
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Just got back from sighting in my new MSAR STG-556 (16", 1:8) and everything went smoothly until I compared groups from the 62gr Silver Bear I use for practice & my Prvi Partizan 55gr. The two different ammo types have POI's that are exactly 3" apart horizontally at 100yds! IE: When sighted in to be dead-on w/the 55gr PP I find the 62gr SB groups exactly 3" to the left. Now, vertical differences in POI I understand and have experienced. But this is the first time in the 40-odd years I've been shooting that I've seen *horizontal* differences in POI w/different ammo types. Has anyone else experienced this before? Any idea as to the cause? Thx!... Tomac |
It's not canting and not heat-stringing. Once I noticed the difference I began carefully shooting groups of each ammo type, alternating one after the other. The Silver Bear always grouped 3" to the left of the Prvi Partizan. Tomac |
It wasn't larger horizontal groups, it was horizontally separated POI's for two different ammo types. Tomac |
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Try working a gun counter on November 10th, and trying to explain to the beer breathed customer on the other side that "No, any old 180 grain 30/06 won't do...you need to use the same stuff you sighted your rifle (always a Rem.742 with aluminum see thru rings) in for...." It's like pissing into gale force winds...they took what I love, and killed it. 
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The different bullet weights could explain different POIs, but as stated in the OP (which I actually thoroughly read just now  ), having the difference be horizontal is odd.
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I feel your pain. While I certainly didn't expect two different ammo types of differing bullet weights to have the same POI, I'm just surprised that the difference is so horizontal!  Tomac |
| A while back I had my 14.5" Colt sighted in at 50yds, most normal 55grn ammo would print to POA in a close group. I then fired some IMI M855 62grn ammo and the POI was just about 1.5 inches low and 1.5 inches to the right. I thought the group being different horizontally was strange also, but I guess its not. |
| I would say it is bullet harmonics. I have mentioned this before and most of the guys didn't agree but I have always felt that the bullet resonates like a bell (OK, not "just" like a bell since lead is pretty dead) or like a kicked football, you know there are shockwaves moving around inside that ball from the kick. I think the cheaper the bullet the more likely these is something like this having an effect on it's path. Before learning the more cool words of the industry I called it crawling, when loads like AE were low and to the right of better ammo. |
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Very common in my expereince. In my rifle I have 4 MOA pure horizontal dispersion between 75 grain 5.56 TAP and 62 grain Federal Tactical SP. Its a combination of barrel harmonics and the amount kick out or yaw the bullet has leaving the bore causing some altered shift (bullet design and interaction with twist on bore exit). You wont notice ths with ammo that is more "similar" like slight BTHP FMJ, 55 grain bullets comapred to another brand of the same type and weight of bullet. When you use vastly differing bullet types and weights the more pronounced shift horizontal shows up more often. |
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