Home Forums Hunting & Shooting Ask The Gunwriters chronograph reading diagnosis- 308 loads

I think I was happier not using a chronograph

Last year I got a Mod 70 featherweight 308.
Loaded up 44 grns Varget with 150 grn interlock below max for deer load for niece. She shot a nice doe at 125 yds.

Shoulda left it at that, maybe I still will but I am curious

Last week I chronographed the load
4 shots 2565, 2590,2572

Little slower than I thought, but a 300 savage sounds good

But thought I would try Mule Deer's 308 load of 46 grns Varget

So first 45 grns
2 shots 2682, 2687

then 46 grns
2 shots 2676,2680

Now I am REALLY courious. I know some barrels are slow, chamber dimensions and other things, thats not the issue for me.
But 46 grains shooting slower than 45 is odd to me.

Obviously I need to reshoot the test and with more shots to get a better data sample if I want a more accurate picture.

My question is:
Can this be a sign of erratic pressures, and if it is, is it safe to reshoot 45 and 46 grains before looking for a cause?

Your velocities are not quite at a maximum load for the 20� barrel 307 Winchester. Not bad performance. I have had more one shot, dropped in its tracks kills on white tail deer with the 307 Winchester than with any other cartridge, and my wife and I have tried a few.
There is a bit of magic in a conventional 150-grain .30 caliber bullet driven to 2,550 fps or so. I prefer heavier bullets but it is simply a preference.

There are many circumstances which can cause a load to chronograph slower with an increase in powder charge, particularily when you are not at a mximum load level.
Your velocites look pretty good to me but you might consider these thoughts.
For uniformity I often tie a string between the chronograph stand and the front sling swivel to ensure a uniform distance from the start screen between shots. I generally find this of most use with 22 lr cartridges but you might consider doing something to ensure you have your muzzle the same distance from the start screen for each shot and each group fired. This will help to uniform some of your day to day differences when choreographing the same load from the same rifle.
If it helps, all of my barrels appear to be slow barrels.

EDIT] I should have said hat 44.0 grains of Hodgdon Varget will drie the 150-grain bulk remington JRN bullets between 2,525 and 2,550 fps in the 20" 307 Winchester, depends on the day. Reloder 15 gives higher velocities with not quite as good accuracy.

That does seem odd, and unusually slow. I'd try to shoot something of known velocity through it to confirm it's accurate. My 308 shoots 165 ABs at 2820 with 46gn of Varget> It's always been on the fast side though (26" barrel). ...or, can you shoot at long range and measure drops to get an idea if it's accurate?

Just a guess PRM but if you cut your 26" tube to 22" like the OP's what do you guess your 2820 loads would be running?

I'd guess you'd drop 80-100 fps or so?

Dober

He's shooting 150s, not 165s though. It would be in the ballpark if they were 165s.

True that he is

Thx
Dober

Re-shoot them. Two shots with each load is not statistically significant and the velocities may reverse.

For all intents and purposes those two loads are identical. That the one with more powder is reading a blink slower means nothing.

Were it me, I would split the difference (45.5 grains) and bet money on an accurate load that had low SD.

You're shooting a .308 Win by definition you can't be worried about getting max speed out of it or you'd be shooting something else that would give you that speed. It looks to me like you are shooting a load and rifle with a lot of potential for accuracy and probably are getting it.

Chronographs spoil lots of loads especially when we move from Internet or loading manual expectations to shorter hunting length barrels,and different components which vary from that used in the tests.

I don't see anything unusual going on here.

My rifles tend to be a bit slower than Mule Deer's.

I figure as long as I stay behind his position on the pressure curve, I probably won't blow anything up.

ruger438,

What was the light like when you did the chronographing? One of my chronographs is 4-5% faster under a high sun than when shooting under clouds. This was confirmed one partly cloudy day when shooting a 10-shot string as a few clouds periodically passed in front of the sun.

Hornady Interlocks usually result in lower pressures (and hence velocities) than other bullets of the same diameter and weight.

I did not know that, and you know I'm a Hornady fan...why is that??? jacket alloy or some such?

Short bearing surface, especially on lighter-for-caliber bullets.

OK...dat makes sense...

According to a ballistic lab tech I know, the only bullets that create less pressure are the lighter-for-caliber Triple Shocks. He often has to switch to a faster powder to get up to speed with light TSX's.

I've seen the same thing.

What kind of chronograph are you using? Do you have a known load you can test your chronograph with? Like MD said light conditions etc. can give false readings. As you can see I'd question the chronograph first. OTOH some rifle can be hard to understand. Some years ago I had a factory M700 7RM that was just plain lazy and there was nothing I could do about it. With factory 150gr Rem loads it just barely broke 2900fps. With reloads I had trouble reaching 3000fps with 150gr bullets. What I had was a loud, hard kicking .270 so it went down the road.

I don't think he's pushing too hard yet. My 308's like a bit over 47 gr. of Varget with flat base 150 Interlocks in Lapua brass, lit by a CCI 200 or BR2.

Yeah.....don't work to fire wall with a 338-225 Hornady, then switch to a 225 Nosler P without a substantial load reduction....ask me how I know...

Ooops! Gee...what's that funny smoke?..... where'd that primer go?

[quote=Mule Deer]ruger438,

What was the light like when you did the chronographing? One of my chronographs is 4-5% faster under a high sun than when shooting under clouds. This was confirmed one partly cloudy day when shooting a 10-shot string as a few clouds periodically passed in front of the sun.

Mule Deer,
Any idea which condition gave the more accurate reading?