Question on Load Development

[griffin ordway]

Technical Particulars:
300 PRC, 22" Proof Barrel, Loupy MK5 5-25 Scope
Lapua Brass, 215 Berger Hybrids, 80.8 gr. RS Magnum
Avg. 2800 fps, ES=36, SD=13
First group (3 days ago)= 3/16"
Second group (this morning)= 1/2", but shot 1" high with no change in scope settings.
First session was a different time of day (later), and was warmer. Second session, this morning, was colder by about 15-20 degrees (60'ish vs. 40'ish).

Question: What could cause such a large impact elevation change? Environmental, i.e. temp change? I know that the numbers (ES/SD) are not the absolute best, so maybe this?
I really don't want to "chase the numbers" when it's grouping this well, consistently. I also don't want to chase the elevation/zero on my scope, or be unsure of the scope zero every time I shoot.

Thanks in advance for any input and enlightenment.

 

[BRADMOTO]

More than likely a positional change with different clothes and connection of gun to body.

 

[nksmfamjp]

Well, some of that is velocity variation due to temp stability of the powder.

To me the key questions are….how did you establish your group size? Is 3/16" the results of a 3 shot group or 30 shot group? Same question for your Sd and your zero.

Those 2 different groups are likely the result of some temp stability and not really establishing group size or zero.

In all fairness, I tend to develop/test/zero with 10 shot groups and prove out with 20 shot groups.

This is fine for the 600yd capability I'm trying to establish, but I'd have to bump those numbers to be confident at 1000 or further.

If that Sd is based on 20 shots, it is fine. If is is based on 3-5 shots, id argue that we are not looking at your load's population Sd, or even close. When you are close, 6*Sd ~= ES

 

[med358-boise]

I assume you are talking about a distance of 100 yards. Even at only 100 yards, you are talking about infinitesimal changes that would move POI 1". There are dozens of things that could cause that. I would not look at your load. I would repeat the process on 5 different days to see if you have an actual problem. As Bradmoto says, likely a change in the shooter mechanics to use a general term.

 

[griffin ordway]

Thanks so much for the input/responses. Yes, I do admit that I do not have enough rounds fired to establish a consistent average with any numbers. This is because I am very early in load development. It was the 1" elevation change that stumped me, because every thing was the exact same from session to session...also, I've never had this happen before, with any load I've developed.
I will take the advice, and shoot larger strings for more data points, and shoot those strings over a number of days with differing temps.

 

[jseandsje]

Your position on your forearm, if the forearm is flimsy will change poi.

A 1" change is significant.

If you have a legit 1/4" grouping gun, get off the bench and shoot from hunting positions.

That's how you find the true impact point.

 

[griffin ordway]

Also, I didn't think about positional changes or clothing, etc. I am a fanatic about eliminating variables, yet I did not think about this one. I don't detect any parallax, and adjust for it every session, but even a tiny bit could make a difference.

 

[griffin ordway]

Photo one is this morning, photo two was 3 days ago...so you all can see it and not just go by my words. Great groupings, but poi changed pretty dramatically.

 

[griffin ordway]

OK, my bad...I realized that I misstated the the original measurement. First grouping was 3/8, not 3/16. Wow, wish I had a 3/16" gun, if one even exists. My appologies.

 

[nksmfamjp]

I think both groups are great, but I'm not sure we know the capability or the zero very well. Just my opinion.

Could be a bad scope, but I doubt it.

 

[Phil Rizzo]

Is the scope slipping in the rings, by chance?

 

[Koda_]

Shoot a 3 shot group and note the POI.
turn your elevation turret way up, then turn your windage turret way over. Then reset them back to zero and shoot another group same aimpoint.

 

[nksmfamjp]

Is the scope slipping in the rings, by chance?

Why would you think that? His 5 shot groups are outstanding!

Shoot a 3 shot group and note the POI.
turn your elevation turret way up, then turn your windage turret way over. Then reset them back to zero and shoot another group same aimpoint.

Why would that help?

 

[Phil Rizzo]

Because the poi can shift between groups. How do I know?
Believe me when I tell you, I was that lucky. Groups were good, POI shifted BETWEEN them. Hard to believe?
Probably would've been for me prior to 2-3 weeks ago.

Got it handled though… the target groups were dialed differently after I found the issue.