Great groups but not great SD

[wyattreed2021]

Hello, I'm reloading for my 300prc, laupa brass, GM215M primers, Barnes LRX 208gr and H1000 powder. I'm getting great groups (under half inch 5 shot groups) for the most part some around 3/4 of an inch but all well under MOA. I've always looked for SD under 10 but I'm getting 15-17 fps for SD with this load and have never had a load shoot good but give poor SD. For the many people with more experience than me what would you do in this situation? I'm assuming this poor SD will begin to show itself out past 400 yards. Thank you for the help!

 

[gillettehunter]

Shoot it at distance. See how the groups hold up at 500-700 yds. Tweak from there.
Bruce

 

[mudflap621]

How many firings on the brass? I've always found virgin brass to give not so great SD numbers.

 

[Troutslayer2]

Dude your SD is less than 20, that's great. You can do the math and figure out how far away 17 FPS will cause a miss. It's going to be pretty far.

 

[Huntnful]

No it will not show up at 400 yards or probably even 800 yards.

Brush the necks of your brass with a nylon brush on a drill if you're not doing that already. Easy step to help with ES/SD

 

[DENRIK89]

17fps is already decent, especially with that big of a case. Anneal anfter every firing if you already aren't. When you load some rounds let them sit for a few weeks then check velocity. Loaded rounds that have time to sit, and let neck tension normalize, always have closer spreads

 

[RegionRat]

I have a minute to burn and this is easy if you run these numbers all the time.

Let's assume Barnes 208 LRX at typical speeds for the 300 PRC are roughly 2950 fps

If the SD is 17, then there will be a probability distribution that says at +/- 2 sigma the ES is 68 fps, and at +/- 3 sigma the ES is 102 fps
Now if we back calculate the trajectory at 400 and 800 yards we will see the following drops with a 200 yard zero.

2950 fps 400 yards 4.1 MOA, 800 yards 15.5 MOA
+/- 2 sigma gives 3018 fps 400 yards 3.9 MOA, 800 yards 14.7 MOA
+/- 3 sigma gives 3052 fps 400 yards 3.8 MOA, 800 yards 14.3 MOA

So, the worst case is when you look at it over +/- 3 sigma where the change from the average means the velocity ES is as high as 102 fps, so the elevation change is worth (4.1-3.8)x4= 1.2" at 400 yards, and (15.5 - 14.3)x8= 9.6" at 800 yards.

So, with the SD at 17, your vertical dispersion at 800 yards would be roughly 9.6" at 800 yards from the speed variations alone.

The math shows the round is still hitting very hard at 800 yards, but ethically the dispersion and flight delay doesn't leave you much room for error.

If I were to give you some advice it would be to be honest with yourself about your shooting ability, which includes shooting under pressure, the odds the animal won't move, the range error, and the wind error, and the DOPE error due to cold bore and temperature changes. I would go out and test yourself against a clock and a 10" diameter target and see what you can do, and then decide if you need more time at the loading bench, or the range, or both.

 

[Tiny Tim]

How much interference fit on your neck to bullet, and what is your case fill? Sometimes a little tighter neck or a light crimp will help SD. I have also found that getting 95+% case fill helps a lot.

Shoot it at distance and see what happens. Vast difference between shooting at 100 yards vs 500 or more.

 

[FireFlyFishing]

These guys say don't lose sleep over it.

 

[K.E.C. Darkside]

These guys say don't lose sleep over it.

I agree, some of my best shooting loads have what some think is terrible SD.

 

[buzby]

Edit: missed the above post, with same podcast link

Have a listen to this. By the numbers how much SD affects grouping at distance.

TLDR; missing a wind call by as little as 2mph will have more effect than chasing single digit SD.

 

[FireFlyFishing]

Have a listen to this. By the numbers how much SD affects grouping at distance.

TLDR; missing a wind call by as little as 2mph will have more effect than chasing single digit SD.

Refer to post 9.

 

[Justice1327]

Your Brass will need 2 to 3 firings to be fully formed to your chamber. What is your neck and inner diameter dimensions for your neck to bullet interference fit? Recommend using powdered graphite, size with a mandrel prior to seating a bullet. Is your brass neck wall thickness uniformed? What do you think your case capacity fill is? If they are available, you can try switching primers too.

 

[Mikecr]

I'm assuming this poor SD will begin to show itself out past 400 yards.

You can't assume this, nor can any sigma nonsense predict it.
Barrels shoot what they do partially independent of MV variances due to compensation. With this, it's possible your gun could shoot as well at 500yds (MOA-wise), as at 100yds. Post#2 suggests field distant testing,, I agree 100%.

To get lowest SD AND best accuracy, you need a barrel tuner.
With this, you can develop a best powder tune, and then tweak barrel tune separately (with a tuner).