please explain this

[lwink]

The other day was out testing loads for a 308 and something occurred that surprised me, hopefully somebody out there can give a reason for what I though was strange. One five shot group with 42.0 Varget was about .75" above the bullseye -- the following group was 42.5 grains Varget and that one was perfectly centered in the bullseye. I thought it weird the group with more powder would be lower on the target -- now I am 99.99% sure that I didn't just shoot the wrong targets with my loads and everything else about the loads was the same -- bullet, case, primer, jump to lands, seat depth, ... the only variable there was .5 grains of Varget. Why would the hotter load shoot lower on the target?

[Elkbane]

Harmonic vibration of the barrel. Based on the velocity, each load has a different "dwell time" in the barrel, so leaves the muzzle at a different point in it's vibration cycle. A "tuned" load leaves at the same point each time and it happen to be a point where the barrel is "stable". It's an "accuracy node" where minor variation in powder charges don't print differently on target.

That's the short layman's description.
Elkbane

[1Shot]

...Since everybody is in the bbl. whip theory..The faster bullet left the bbl. on the down stroke..lol.. ;D


...Elkbane types faster than I do..lol..

[steveinwv]

The faster the bullet leaves the barrel, the less the muzzle has risen, thus the lower impact point.

[lwink]

I see... makes perfect sense now. So what will happen when I shoot some with 43 grains of Varget

[Uncle Jack]

Effen ya'all don't know which way your barrel whips, there could be a serious problem here.

uj

[pdog06]

Could it be the first group was on a clean barrel and maybe the fouling shots were just higher. Some of my barrels tend to shoot a bit high for the first couple rounds after cleaning.

[Bad Water Bill]

3/4 hi 3/4 low who cares. That is why they have adjustments on the scope. Your biggest concern should be which is the best group

[lwink]

Could it be the first group was on a clean barrel and maybe the fouling shots were just higher. Some of my barrels tend to shoot a bit high for the first couple rounds after cleaning.

good info to keep in mind pdog -- but the first group was about 20 shots in and was the higher of the two groups. Both were shot one after the other, my 21st-30th shots of the day respectively.

3/4 hi 3/4 low who cares. That is why they have adjustments on the scope. Your biggest concern should be which is the best group

I'm not worried about where my poi lies, I was just curious about why a load with more powder would shoot lower than one with less when everything else is constant. I guess the barrel whip theory makes the most sense to me... hope to get out Monday and try again with these two and one other load behind the SMK's.

[trappst]

Load with more powder is going faster.....thus the bullet gets to the target faster, having less time in flight leads to less of an arc (trajectory). Scope height over bore vs. actual bullet path and all that jazz.

I think! ;D

[Uncle Jack]

If you really want to "FUZZ" your mind, read this:

http://www.vni.com/successes/threerivers.php

In the 30's and 40's the Germans did a lot of research in barrel vibrations and that is why you can still find some WWII surplus Mauser rifles with stepped barrels instead of tapered barrels. The steps in the barrel were to coincide with the nodal points and thereby dampen the effects of "barrel whip".

uj