Question for the .308 gurus.

yukoncoyote

yukoncoyote

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Hey fellas, I am in the middle of some load workup these days. I went out to try my new chrony yesterday, and, well, am a little confused with the results. Here is my load and rifle combo:
Kreiger 24" 1 in 10 barrel
Lapua brass
175gr SMK
Fed 210M primers
44.5grs of Varget

I am wondering if my muzzle velocity results are realistic as they seem a little fast to me.
I fired 10 shots over the chrony, one didn't read so I have 9 results, and they all average out to 2818fps! Is this a viable speed for this load? Any info is appreciated, thanks fellas!
 
Might be a little quick, but not unreasonable - I can get (using Lake City brass, so a bit lower cap/higher pressure) 2825 or so with 45gn Varget and 168 SMK's in my Savage 10FCP and CCI BR-2 with just a hair of extractor marking on the case... maybe the Krieger barrel has a good enough finish that you're getting better efficiency?

I'm not sure why you're up there with 175 MK's but if you know the BC of your bullet (look it up for the 175 in the Sierra data) you should be able to back-calculate if you can measure your dead-hold drop at 300yds or so; if it fits the ballistic model for that velocity then I'd say it's doubly confirmed.

Just shoot a zeroed group at 100yds, measure your sight height to the scope's centerline, then shoot a center-hold group (with no adjustment) at 300 and measure the drop between point of aim and point of impact. Compare it to a table (back of your reloading manual), and voilà!

-M
 
Doc M said:
Might be a little quick, but not unreasonable - I can get (using Lake City brass, so a bit lower cap/higher pressure) 2825 or so with 45gn Varget and 168 SMK's in my Savage 10FCP and CCI BR-2 with just a hair of extractor marking on the case... maybe the Krieger barrel has a good enough finish that you're getting better efficiency?

I'm not sure why you're up there with 175 MK's but if you know the BC of your bullet (look it up for the 175 in the Sierra data) you should be able to back-calculate if you can measure your dead-hold drop at 300yds or so; if it fits the ballistic model for that velocity then I'd say it's doubly confirmed.

Just shoot a zeroed group at 100yds, measure your sight height to the scope's centerline, then shoot a center-hold group (with no adjustment) at 300 and measure the drop between point of aim and point of impact. Compare it to a table (back of your reloading manual), and voilà!

-M
Click to expand...

Sounds like a damn fine plan Doc! I am after an accurate muzzle velocity so I can build a drop chart for my rounds. I have been using Brian Litz's book and program for all of this, alot of good pointers in there. Thanks.
 
yukoncoyote said:
Sounds like a damn fine plan Doc! I am after an accurate muzzle velocity so I can build a drop chart for my rounds. I have been using Brian Litz's book and program for all of this, alot of good pointers in there. Thanks.
Click to expand...

Anytime!

I figure that if you have a chrony reading that fits the unchanged nature of gravity within the error inherent in a ballistic coefficient model, you can probably say that your reading is accurate enough! :D

In fact, sometimes I dope my loads this way if I don't want to lug out the chrony and tripod. Works like a charm - nothing like real-world results to back up instrumentation.

-M
 
i have worked up a load using 155gr j4 w/45.5gr of varget, using LC match brass, i wonder what my velocity will be out of the 26'' barrel?? 3000fps maybe???
 
155 Sierras (old ones, #2155), Lapua brass, 45.5 Varget, 2.80" COAL get 2975-3000fps from my 30" barrel target rifle. (46.2 Varget and Winchester brass gives 3015-ish).

Your 2818 might be real, it might be a slight over-reading by your Chrony (e.g. if it wasn't completely parallel).

44.5 Varget / Lapua brass / 175SMK is probably OK/safe in your rifle (you are responsible to verify this though), but it is also probably a max load or close to it. There's also a very good chance that this will be quite accurate and quite consistent.

Confirm that your primer pockets have not loosened. If they have, then you are definitely too hot.

You said your average was 2818fps, what was your slowest and fastest shot? (i.e. what is your extreme spread?)

Is your Krieger barrel a .308" bore, or is it a .3070" or a .3065"? (the tighter bores can nudge up pressures and velocities).
 
To confirm what the Chrony is telling you, sight the rifle in for 100, then shoot a group at 300 and see if the drop is similar to what the value is from a ballsitic calculator like JBM Calculations.

Edited to add,
OOPs, I see Doc M beat me to it.
 
Boomer said:
To confirm what the Chrony is telling you, sight the rifle in for 100, then shoot a group at 300 and see if the drop is similar to what the value is from a ballsitic calculator like JBM Calculations.
Click to expand...

Actually this is pretty insensitive to muzzle velocity. Run the numbers and see how much drop you get, and then run it again with muzzle velocity of 200fps more and see what you get.

For example, my .308 target rifle at 2800 fps and 3000fps is 17.3" vs. 14.5" drop at 300m relative to 100m. This is a difference of 2.8".

If I shoot a 0.75" group at 100m and then zero my sights so that point of impact equals point of aim (perhaps to 1/" accuracy?), then shoot a 2" group at 300m whose centre is 15.25" below my point of aim, how do I interpret this? How accurately do I really know where its group centre is?
 
rnbra-shooter said:
Actually this is pretty insensitive to muzzle velocity. Run the numbers and see how much drop you get, and then run it again with muzzle velocity of 200fps more and see what you get.

For example, my .308 target rifle at 2800 fps and 3000fps is 17.3" vs. 14.5" drop at 300m relative to 100m. This is a difference of 2.8".

If I shoot a 0.75" group at 100m and then zero my sights so that point of impact equals point of aim (perhaps to 1/" accuracy?), then shoot a 2" group at 300m whose centre is 15.25" below my point of aim, how do I interpret this? How accurately do I really know where its group centre is?
Click to expand...

Fair enough. I ran 3 velocities (2850, 2750, & 2650) on the program with a 2.5" sight height, 40 degrees F, at sea level and found that the drop of 1.3" was consistent for each 100 fps decrease in velocity between 2850 to 2650. A 1 MOA rifle will produce 3" groups at 300 yards, provided that human error is removed from the equation and that atmospheric conditions are optimum, and allowing that this fellow's rifle might be a little more accurate or a little less once he reaches out that far. The center of the group though does not change with group size, as the center is always the center. Regardless whether the rifle shoots in a half minute or in 4 minutes, and there will be a measurable difference in drop with a 100 fps variation at 300 yards from group center to group center. Therefore, if the drop of his group center at 300 is at odds by more than 1.3" with the program calculation for the drop at 300 (provided he has entered all of the pertinent data correctly) his chrony data is suspect.
 
Boomer, now you are getting into the nitty-gritty of stats, a subject I freely admit to exceed my mathematical pay-grade (if you'll pardon my Obamism).

I do know is that the group centre fluctuates a bit as shots are added to the group. But I think that I also understand that the location of the group centre is known to a greater certainty than the group size (i.e. it converges pretty quickly).

So even though a 300m group might measure 2" or 3" in diameter, the centre a 5-shot group will actually indicate the actual group centre to a higher level of certainty. You have to start asking a real stats person though whether you know it to within a 1/4", 1/2", and inch, etc - they'll mumble things about sigmas, and confidence intervals, and 95% something, etc... ;-)
 
Well, I'm gonna go out today and fire a few more over the Chrony and make sure everything is set up just right. I am very curious to see what happens. As far as my barrel goes, I am not sure of the exact bore size, but I will find out.

The velocity results from the nine shots were
2869
2873
2848
2871
2734
2760
2745
2830
2838

I think the weather conditions may have been a factor as well, low scudding clouds with some sunny patches, also extremely windy.
Will post results of sencond chrony test this aft. Thanks all!
 
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