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Mate tested his 308 load today and had this happen.
First 5 shots consecutively and last 3 were fired 10 minutes later. What would explain the drop in speed? First 4 being pretty close together obviously.
Components are norma brass, 2206h, 168eldms and fed LR primers.
Any help would be greatly appreciated
Attachment 192295
Attachment 192301 100m (first 5)
Attachment 192302 200m (last 3)
First ones shot while the sun was out
Last three were shot under cloudy conditions
you have 136 ftps extreme spread....what is the issue???? unless your shooting for gnats nuts at 600 yards you do not have an issue.
do they group ok??? yes= no problem
ducking as dannyb reaches for his jandles again.
the last 3 are 94 fps apart...again where is issue???
What sort of chrony was used?
so 8" drop at 200 yards....either group is very dead deer= what problem??/
He is worried about the spread in speed. 16" bushpig with the chances of shooting out to 400m when he hunts south island.
Re chrono the drop for the last 3 works out to the speed on the chrono but could that cause an issue shooting out towards 350-400 if his speed is a bit crook?
hold the line caller...will consult manual and reply.....
bear with.... have data for 165grn ballistic tip so in ball park. his will be slipperier so slightly better,
2400fps 2.8 high hundy-31.6 at 400 -64.1 at 500
2300fps 3.1 high at hundy-35.1at 400 -70.3 at 500
bboth sighted for 200yard zero,
so 4" difference at 400 and 6" at 500 yards.....so if aiming for fat part of shoulder,it would be dead. at worst slightly low of high but still dead.
speeds given are 100fps apart same as his load data from chronicgraph...
Ok so I should tell him to relax basically.
168grn nosler solid base
2500fps at muzzle, at 400 yards-29.4
2400fps at muzzle ,at 400 yards-32.5
2300fpsat muzzle, at 400 yards-36.0
7" with a 200fps difference in speed
yes and sight the friggin thing in couple of inches high at hundy.....makes heap more sense.
Greetings,
That is a pretty wimpy load, about 3 grains under max by my estimate. I note that the drop works out by estimate so it is possibly not a chrono error but I wouldn't rule it out. I would recomend working up closer to max and re testing. Sometimes light loads are not as good as those closer to max even with AR2206H. It would be worth sitting down and making a list of what changed from 100 to 200 metres. I don't thik I have ever seen that much velocity difference in AR2206H loads other than with very light loads.
Regards Grandpamac.
PS I assume that 40.5 grains was the charge. If not what was it?
Regards GPM.
Leaving bullets sitting in the sun for 10 min. would do it
The barrel cooled down. Test it: Shoot three rounds, one every ten minutes so the barrel cools down in between, and I am betting they will be the similiar speed as the last three.
Make it go faster. You're letting down .308's everywhere. My .30/06 is laughing his cake hole off.
I don't think it liked spicy loads from memory
Greetings again,
Looking at those velocities again the 136 fps loss of velocity is HUGE. It is equivalent to dropping the charge by two and a half grains. It is really important to find out what the problem, and there is a problem, is and correct it if the rifle is to be used at longer range. So please more information is needed starting with the powder charge and how it was measured and with what. What was different between the start and end. What was the chronograph and how was it set up. In no particular order I would suspect:
Chronograph error due to bright sunlight or varying set up.
Inaccurate powder measurement.
Loose suppressor.
Cartridges left in sun.
And many, many more.
Regards Grandpamac.
I would count it only as around 80 fps reduction overall. (That last shot I would consider a chorny anomoly until I retested it and found different)
Like 7mmsaum alluded to, some chronies will give varying results depending on whether its clear skies or overcast. Assuming his reloading process was good to go and the data shown was all for same charge weight then I'd validate at range using an MV of around 2480fps (close enough to the average of first group which from the data is what I'd interpret as the most reliable). Sing out if you want me to send you data for what your drop should be at desired range. Cheers
Got him to measure the drop at 200m and it was 18cm so that worked out to about 2380fps~ which is closer to the last shots. Only a pro chrono so not very trusting
What is the shot order of those last 3 shots?
bottom line here...its a short barrel and a slow load....400 meters is a long way away...he will NEED to be using rangefinder with somewhere around 30" of drop..dialing will probably be needed too..... he might just have to stalk closer..300 is much more achievable.
its your chronograph, thats why i switched to a magnetospeed. bit not worth it if you not shooting long range.
My Prochrono used to drive me mad as the light changed, there are tricks you can use to make it better but easiest is work out approx where you are at 1350fps down range and shoot to test the drop, then backcalculate the speed.
Starting to look as though it is a chrony problem in the bright light which I have seen before. The clincher would be the load. Hodgdon has velocities for a 15 inch barrel which should be close enough for 16 inches. It would seem that it is a light load that is most accurate which might indicate a bedding problem. Something to look into.Quote:
Originally Posted by superdiver
Regards Grandpamac.
it a couple of steps above a 30/30.....lets be completely honest....not that much faster...so is it really a 400yard rifle???
Correct me if I'm wrong but the difference between 2480fps and 2380fps at 200m is only about 1/4 MOA (weather and atmospherics aside). Looking at the group in the first picture it doesn't look to be holding anywhere near that mate so measuring drop at 200m and making a velocity assumption based on that (where the rifle is not grouping well enough to give a "yes" or "no" between your two potential velocity ranges) may not be the most accurate. I'd be looking to do so at his max intended hunting range and adjust accordingly.Quote:
Originally Posted by superdiver
Thats what I thought that first group looks like 1.75 inches or close to it, the error is too great to be able to make a calculation about velocityQuote:
Originally Posted by Link
Maybe just tell him to leave it at home and get something suitable for South Island mountain country. Can't go past a .270 Win if he expects to knock down big stags at 400. Shorty .308? yeah/nahh.Quote:
Originally Posted by superdiver
For the record, I only hunt mountain country, and having used a shorty .308 to knock off a few animals at distance, quite frankly my .243 gives me more decisive killing. Everybody has their own take, and ethics.
Old rule of thumb for projectiles going at between 2400 and 2600 fps 3 inches high at 100 equals dead on at 200 and 3 inches low at 230 yards, so basically point and shoot to 250.Quote:
Originally Posted by Micky Duck
If it was my rifle I'd be concerned too. I'd follow GPM's advice. Aside from that the chrono could be the issue, certainly if the overhead conditions changed. MD has a good point as well. Zero 2 to 3 inch's high at 100. For me the issue is having confidence in the rifle and load more than anything else. Cheers and good luck.
That’s a massive issue. Hi don’t have even spread of more than 30 fps. That’s over 3 hunting rifles. If the speeds not consistent what’s the point ?Quote:
Originally Posted by Micky Duck
