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At Bisely over the weekend I did some Chrono measurements of some of my reloads for 308 which used GGG brass (so this is from the 147gr GGG ready made ammunition which I have kept and cleaned and swaged to remove the crimp). Sharing in case it's of interest to anyone.

Rifle: Steyr SBS tactical, 20" barrel, 1:12 twist
Chronograph: Magnetospeed V3, all speeds in ft/s
Reloads: Once fired GGG brass, swaged (Dillon 600), 2156c Sierras, N140, S&B large rifle primers, 2.790" OAL

Factory 7.62 GGG 147gr
2595, 2575, 2589, 2606, 2595, 2603, 2592
Average 2594+/-10 ft/s, spread 31 ft/s

41gr N140
2410, 2419, 2427, 2415, 2424, 2393, 2396, 2412, 2409, 2412
Average 2412+/-11 ft/s, spread 34 ft/s

41.5gr N140
2457, 2449, 2446, 2441, 2428, 2431, 2428, 2427, 2437, 2438
Average 2438+/-10 ft/s, spread 30 ft/s

42gr N140
2469, 2486, 2483, 2483, 2483, 2497, 2476, 2462, 2464, 2459
Average 2476+/-12 ft/s, spread 38 ft/s

42.5gr N140
2520, 2505, 2533, 2499, 2531
Average 2518+/-15 ft/s, spread 34 ft/s

Didn't check for accuracy on these, was just shooting into the sand as the targets were busy (NRA club day) and I was just after getting some speed data on them all.

Not sure whether this is good or bad, but it was a first attempt at reloading for 308 using the GGG brass, and I was very much on the safe side for amounts of powder being used (and why I only made 5 of the 42.5gr ones), so I now know and can take it higher if I want. However most of my shooting is at 100yrds, so I may well stick to 41.5gr in future.

I would say your running a little slow
Could do with getting to 2800-2900fps with the 2156 bullets,
Consistent neck tension and consistent powder load, throw and trickle up to desired amount, don't just throw will help with lower ES

Given that it is a 20" barrel I would say they look OK

I'm getting 2600fps out if 20" barrel using 165 SGK
Not saying it's bad just saying would probable be better nearer the 2800 FPS
Test them for accuracy will be the deciding factor

Thanks for the feedback. So if I look at muzzle energy based on these values I get 2978 J for the factory round, and 2959 J for my 42.5gr load (slightly greater mass but lower speed). So fairly similar overall energies for my hottest load compared to the factory rounds. Even the 41.5gr load gave 2774 J.

I have a newbie question then. Why would I need to push the 2156 out to 2800fps, if I'm shooting relatively short distances where it wont be falling from supersonic down through transonic? Do they stabilise better at higher speeds, above and beyond any degree of stabilisation I would see around 2500-2600fps?

Of course you could ask why have I got 2156 if I'm not worried about shooting long range at high speed which would be a valid question. I got some on recommendation to try is as valid as my answer gets on that one. And yes I fully accept that at this stage these are just speeds, and I have indication of how they are grouping.

I use 155gr Lapua Scenar's in my 20" Tikka and I get around 2,600fps with them. There's not much point in trying to squeeze more velocity than that from a 20" barrel, you just get excessive muzzle flash, extra recoil and not much extra velocity for the additional powder used.

Ok
If your not looking to shoot at long distance then it all come down to how accurate you wish to be at the distances you do shoot in which case velocity is less important as you say, so just see what groups the best from your home loads and go with that

Out of interest jmc did you use a Lee factory crimp die on your loads? If not have you thought about trying the same loads using the fcd to see just how much more consistency if any you would get.

Excellent, that's good to know. Yes I did use a factory crimp die, but only a light crimp. Case prep was fairly minimal to be honest, clean, deprime, swage, size, trim to length (using the manual Lee case length gauge and trimmer), chamfer inside and out. I use the Lee 308 dies, and as mentioned a light crimp. Powder load was checked using a Lyman scales, but was not checked for each load once the powder thrower was set up. I don't sort brass for weight, or bullets. So I'm guessing my routine is a fairly basic one.

I'd be interested to know what type of spread people get in their home loads (and what type of routine they have to follow to get the spreads they see)?

I made a chart and summarised the data, feel free to save for your use. If you're not familiar with the boxplot, the vertical lines show you the minimum (0th percentile) and maximum (100th percetile) velocities. The box outline represents the 25th and 75th percentile. The horizontal line between these percentiles represents the 50th percentile, better known as the median value. Percentile basically means what percentage of the data is below that data point. The maximum (100th percentile) would mean that all the data would obviously lie below it.

Although your extreme spread looks pretty consistent, the standard deviation and Inter-Quartile Range (IQR) increases with higher powder charge. The extreme spread represents the difference between the highest and lowest velocity, obviously the smaller this number the better. However, the IQR and standard deviation are good indicators of general spread of your velocity. Your ES values are mostly consistent but the standard deviation and IQR are higher with the 42.5gr load compared with the 41.5gr load. You can have a large extreme spread by one round having a wrong powder charge, you will have a high standard deviation/IQR if you cannot throw consistent charges repeatably. Both of these can be improved by measuring each individual charge. You didn't mention which powder thrower you use, but in my experience powder throwers like the RCBS uniflow can struggle with stick type powders and sometimes I feel the thrower bind up, which will give a different powder charge compared to the rest. These type of throwers work very nicely with fine ball powders such as Ramshot Tac, Hodgdon H-335 etc.

You could deliberately throw a slightly lower charge with the hopper and then trickle up on the scales using a powder trickler. For stick powders, I invested in a Lyman Gen6 electronic powder dispenser which has made a world of difference! Using this device I keep the Extreme Spread for my 6,5x47 down to 14fps and on one occasion I even managed to attain an ES of 3fps for my 8mm Mauser over a ten round string!
Also, the rate of increase in velocity increases with powder charge. If you wish to reduce your extreme spread the easiest thing you could do is to measure each charge.

I would repeat the experiment and look at grouping at 100 yards. The lowest extreme spread does not always produce the smallest group.

EDIT: Further explanation of IQR and StdDev.