New bullet- Huge change in POI

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pacobillie

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Because of the ongoing component shortage, I have had to try a different bullet for my rifle (sporterized M96 chambered in 6.5X55) . I went from a 143 ELDX to a 139 Lapua Scenar. Everything else remained the same. Point of impact shifted down by 8 MOA.

This is highly unusual, as previous changes ( powder, load, bullet) yielded a change of about 1 MOA (downwards).

BTW, the action is bedded and the barrel is free floated. Why is this happening?

BTW, before anyone asks, the "old" loads still print in the usual location.
 
I agree that such a large shift of the POI is unusual, but definitely not unheard of.
It all boils down to barrel harmonics [where is that muzzle at the time the bullet
exits?] Most muzzles move in a figure 8 [or like the infinity symbol] when they are
fired. Different bullets not only create different harmonics, but also exit at different
spots on that "8", thus making for a new POI. Dave.
 
That drop sounds excessive. Suggest doing a ladder test of min to max load and chronograph the muzzle velocities, and compare to published load data.
I find that the 139 Scenar is hard to find published data for other than for Vihtavuori powders. I shoot 139 Scenars in my .260 Rem with Hodgdon and IMR powders and use their 140gr bullet data as a guide for min/max.
 
Your previous experience about downward movement might be throwing you a bit - but an 8 MOA is actually a lot. I have some rifles that go sideways, or at an angle - an up/down plus left/right as components are changed - and some do not seem to make much difference. So, a bit of a mystery to me how my Dad seemed to always get his deer with whatever brand he found in store the cheapest, that fall. When his stuff came to me after he passed, there was a whole collection of various brands (head stamps) and bullet weights in his "hunting stuff" - not sorted or identified - as if 150, 165 and 180 grain bullets all got stuffed into the mag of his M1917 when he went hunting - and he managed to make that work. I shot off most of his loose "stash" in his rifle, from sandbags at 100 yard target - something like 16 bullet holes that I could cover with palm of my hand - apparently was "good enough" for him.
 
jimbo14 said:
Have you chronographed the two different loads?
Click to expand...

Only the 143 gr. ELD-X load (47 grains of H4831, 3.11 COAL) which yielded 2650 FPS or thereabout. I used the same exact powder charge with the 139 gr. Scenar, but have not chronographed it. I know that the variation in POI is due to barrel harmonics, but all other loads/bullets that I tried before shot more or less to the same POI, plus or minus 1 MOA. Somehow, this one is totally out of whack.
 
That’s why I’m really curious about chronoing these loads on the same day. Five of each or something. Maybe the width of the bullets is different enough to create pressure/velocity differences? Or distance to the lands? How far away is each bullet from touching the lands?
 
A change in p.o.i. with a different bullet is not that unusual in my experience. I'm pleasantly surprised when I do get two different bullets that shoot to the same place.
 
The high group is the bullet exiting as the barrel is whipping up. The low group is the bullet a bit faster or slower and existing as the barrel whips down.

Change the velocity of the low group a bit, and it will shift up.
 
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