SD increase at range?

[D-4297]

It is another piece of referential data that will establish , or show a consistency in your load velocity . To create a "curve" if you will . In my mind ; it can be used along with LR information , comparing shot at the muzzle , to shot on target . As long as you have a understanding that there are extraneous variables involved , as mentioned above . Put all of the data together , and it creates a curve of information . How it's interpreted is solely in the hands of the user .

 

[JimSC]

Yeah. My calculator tells me that, for my load, ±20 fps is less than 2" at 600 yds.

bit more to it than just a ballistics issue. Barrel harmonics seems to play in there also. Here is a study I did a few months back. Velocities and points of impact were recorded using different case preps. For that matter sometimes the numbers do not make any sense even with rounds loaded in the same session and shot 20 minutes apart.The group shot first and the group shot third were from the same box of reloads, all were prepped and loaded at the same time and shot within 20 minutes of each other. Drastically different ES and SD numbers

Case Prep, Velocities and Point of Impact

A couple of years back I experimented with annealing, various neck lubes, and checking concentricity but saw little if any improvement in my match scores so I stopped and began doing minimal prep work. At the time my scores were in the upper 180's and low 190's. Over the past couple of years the...

forum.accurateshooter.com

 

[Webster]

I noticed something yesterday and I don't know if this is real or an artifact of the Shot Marker electronic target system. When I tested my .308 loads through my CED M2 chronograph, I got a SD of 9.2 at the muzzle. At 600 yards yesterday the Shot Marker electronic target system gave SDs for 20+ round strings of 12.7, 17.1 & 18.6.

This got me thinking about the possible causes for the different SDs:
1. Since they were separate sample sets, loaded on different days and shot under different conditions, the SDs were actually different.
2. The Shot Marker system isn't really accurate for measuring SD and those numbers should only be used for entertainment value.
3. There are really things that happen between the muzzle and the target that act to increase the SD of rounds at 600 yards.

I was just wondering if anyone had studied this subject, for instance, by comparing statistics of bullets as they leave the muzzle and as they enter the target? Has anyone compared the velocity statistics provided by an electronic target system with those provided by a chronograph at the target? Just thinking out loud.

Just a guess. If the bullets have slightly different point closures could they slow up at different rates? Is there vertical stringing. Do you metplate?

 

[DRNewcomb]

Just a guess. If the bullets have slightly different point closures could they slow up at different rates? Is there vertical stringing. Do you metplate?

In this case, I was shooting plastic tipped Hornady ELDs. In a prior match I tested them against Sierra MKs and TMKs and found they shoot as well out of my rifle as the TMKs, both of which shoot better than the MKs.

 

[RegionRat]

Some nodes are forgiving and durable, other tuning nodes might be narrow. As you gain experience you will learn to decide how much of a trade off it is to chase a very good narrow node, versus a good enough wide node.

The whole chasing the lands versus jumping debate comes to mind. As you find yourself shooting matches, remember that throat wear can change the tune or you may find the stars aligned and the tune hangs in there. I wish I could tell you how to predict this, but you really can’t. You just live and learn.

Personally, I have learned how to predict my Service RIfle because I have patterned them. The experience tells me it is better to adopt the wide forgiving node, even if it isn’t as tight. This way I don’t have to “chase lands” or buy into troublesome logistics on multi-day matches. I would rather have a node that lasts nearly the whole life of the bbl, than one I must adjust after every few matches.

In so many words I am telling you that unless you test, it is hard to know if you have worn the throat and gone out of tune, or if your score was just part of learning. Good vertical is more than just a velocity stat, but a gross ES/SD at the muzzle is never a good sign for me. If the average speed identifies the node, I try hard to keep my ES/SD well inside of it. YMMV