FPS Standard Deviation

JoKa

JoKa

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Hey All,

First time ever I decided to Chrono my .308 Handloads.

I had a High of 2626FPS and a Low of 2590FPS, AVG 2608 and a Standard Deviation of 12.5

I have NO clue what is typically considered an acceptable SD when it comes to your bullet speed for shots intended for 700+ yards

Thoughts?
 
JoKa said:
Nice,
Then I too, shall aim for single digits.
Thanks for the guidance
Click to expand...


Single digit deviations are extremely difficult, if not impossible to achieve with an off the shelf rifle, with a standard commercial chamber. You need to talk to or better yet, go to a Bench Rest or F class shoot and find out what goes on to make it possible.

It can be caused by primers, lots of powder etc.

When you are getting averages in the low 12s, that is very good in a commercial sporting rifle. You might bring it down a bit, using BR rated primers.

Brass can play a big part in this as well.

One old friend, now passed on, Used to mark his brass for his HBR rifle for deviation. He would only use brass marked for deviation below 9. I used to scoop up his rejects. Our rifles had chambers cut with the same reamer.

I'm just not a good enough shot to be able to tell the difference.

My old friend, swore his attention to detail was the reason he was consistently in the winners circle. The extent he went to for consistency, would take all of the fun out of the game for me.
 
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Deviation is usually counted as ES, extreme spread, so you had an ES of 36. If that is for five shots, it is probably a bit larger than it could be, but certainly not bad in a sporting hunting rifle, or in hunting ammunition.
 
It was over the course of 11 shots. (16 but I was focused on shooting and missed recording a couple)

Rifle is a R700 5R Milspec. Re-bedded by ATRS.
It puts them all in the same hole at 100.
Ammo is 175gr SMK HPBT with 42.0gr Varget w/lupua brass
(Rifle twist on that one is RH1:11)


**So some of y'all are thinking chamber tolerance has more of a factor here then the ammo loading itself?
 
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Bump your charge up ! 42gn varget is a pretty light load. I bet your getting black necks ? That would be by the cartridge nt expanding enough for a complete seal.

An incomplete seal will cause a bigger ES
 
OK, I guess next batch I will play around with 43 to 44 then :)
Mind you.... hodgdons reloading data is saying 45gr max at 2690FPS

If I go to 43 or 44 and go higher then 2690 is that not a bad thing?


PS: I dont recall my necks being black no.....
(using competition dies with a bushing that is sized for my rifle though.... so it is not a generic size)
 
Brand is "Shooting Chrony"
Model is "F-1" i think..... it is a very common basic one. You should have no problem finding one..... I think they are roughly $100-ish
 
JoKa said:
OK, I guess next batch I will play around with 43 to 44 then :)
Mind you.... hodgdons reloading data is saying 45gr max at 2690FPS

If I go to 43 or 44 and go higher then 2690 is that not a bad thing?
Click to expand...

Nope not a bad thing at all, reloading data is more or less just a safe guide line . Every rifle is different and should be treated as such, work up slowly watching for pressure signs . Reloading data is more "this load on this day with this lot# of components was deemed safe in our particular rifle ". While modern powders /primers etc are very consistent, there can still be some variation. But as long as you pay attention to what the rifle and case are telling you, it's perfectly safe .

Iv had rifles that showed pressure signs at 2.5-3 grains below published max, and a rifle that with my choice of powder , I literally couldn't get enough powder in the case and seat a bullet and it still didn't show any pressure signs
 
Make sure your chrony is dead level even a slight angle will give you a faster reading. I bubble level mine.

A recent load I tested 5 shots ES 12 SD 6, 5 shot below .4moa i put 4 through one hole at 100m and pulled one shot high. However tests at 600m showed not so great results and fliers. Neck turned, pocket sqrd, lapua brass for f class using N150. Trying to get as little ES as i can single digit is possible with the right components.
 
Whether the chrony is at an angle or not, will make no difference in spreads, as long as they are all fired over the same angle at the same time.

Other things that will give your spreads wider extremes are fluctuating light conditions.

The reason I mentioned chambers, is because match grade chambers are reamed to much tighter tolerances than off the shelf rifles.

Are you full length resizing your brass or neck sizing only??? This will often but not always make a difference.

I have heard people getting extremely anal about a tenth of a grain of powder difference from one side of mean to the other. In my experience it makes absolutely no difference one way or the other. Some of the F class people insist it does. It hasn't been proved to me yet.

It sounds like you are wringing every bit of accuracy you already can out of that rifle. Why play with success???
 
Good to know 12 is good. Just got back from the range testing Nosler 180 grain Partition bullets in 308 Win. My best SD was 10.79.

Lapua Brass (once fired)
CCI BR2 primers
2.830" COAL
42.4 grains Varget
 
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