Accuracy Differences Between Loads?

[Hosted]

I've been reloading for my Savage 223 for a few months and have only been using 68gr Hornady BTHP with RL15 for punching paper. I recently loaded up some of them along with some 50gr VMax's with Benchmark for groundhogs. I got a new Burris scope and rings so I used the 50's to sight in with and do some testing. Everything was shot at 100 yards. Came back out a couple hours later and shot some of the 68's and they were all around 6" low. Found some PMC Bronze in a bag and it was also around 6" low. Everything grouped fairly well considering the 20mph wind at my 4 o'clock and shooting off a tailgate. So I went in and loaded about 20 more Vmaxs and they were right on again. Like I said, I'm new to this but wanted to know if this was normal or if something else is going on? Thanks

 

[BoydAllen]

If the same loads repeated, then you have an extreme difference in trajectory, that would seem to be load related rather than rifle. Have you chronographed them?

 

[Hosted]

Unfortunately I don't have a chronograph or know anyone who has one. I may have to break down and buy one sometime this summer.

 

[lrgoodger]

I had the exact same question in this post back in '09.

http://forum.accurateshooter.com/index.php?topic=3737679.msg35817711#msg35817711

Brian Litz confirmed my suspicions. It is barrel harmonics. The different weights (and shapes) of bullets exit the barrel at different times in the muzzle vibrational pattern, resulting in differnt POI for different loads.

 

[dryhumor]

Was the scope adjusted for the 50s during sight in, and not re adjusted for drop for the 68s?

Could be something simple.

 

[Hosted]

I did not adjust the scope for the 68's, I was just surprised at the difference at 100 yards.

Good info lrgoodger, I never thought about barrel harmonics. The part the makes me scratch my head though is the 55gr FMJ shot close to the 68's.

I'm loading up some more now and am going to load some 55gr ballistic tips and see what happens tomorrow.

 

[LIUNA]

Keep in mind that when you switch powders with out prior cleaning you can have different effect, some powders are compatible others are not.