Left and right POI change with different powder loads?

[rickster]

Greetings all, this is my first post and I am new to precision shooting.

I have been shooting rifle since the age of 6 (mostly plinking and hunting), but now at the age of 51 I decided to build my first precision oriented rifle. I settled on the Remington 700 in .308 as that seemed to be the most recommended starter rifle. I purchased a new 700P 5r 24" barreled action and fitted it to a Choate V-block stock. It also came with the older 40x trigger. This last Sunday I took it out for it's second round of range time. I had worked up some new loads for ladder testing as follows:

Fire formed brass from my first range trip (barrel break in procedure)
168gr SMK
2.850" COAL (this is still .125" off the lands given the R700's long free bore length)
Varget powder loaded to:
41gr
41.5gr
42gr
42.5gr
43.5gr
44gr

I noticed something that I had never heard discussed before that I would like to run by you? I fully expected to see slight elevation changes as the velocity changed through the different powder charges, what I didn't expect to see where POI changes left and right between them. I suspect it has to do with the barrel harmonic changes at different power levels. As I understand it, the barrel vibrates slightly differently as the velocity changes? This got me to thinking about shooting at ranges other than what the rifle is zeroed for. If my selected loading with the tightest group shoots 1/2 MOA to the right, I would suspect that this is because the barrel harmonics shifted (whipped) the barrel slightly to the right as the shot was leaving the barrel. If you extrapolate this out to 500yds this would be a 2.5 MOA shift to the right. If I just reset my zero to account for this wouldn't the rifle still be shooting right as the round leaves the barrel, while my scope is still pointing in a slightly different plane to the the actual bullet trajectory? Wouldn't this require me to have to change windage settings (assuming zero actual wind) in addition to elevation settings for different ranges?

I fired 5 shots of practice ammo prior to starting the ladder testing to try to avoid starting with a cold barrel.

I think I have settled on the 42gr loading as it is the middle of the groups that did best, but all 3 have slightly different lateral POI shifts.

Thanks a million, and I am having a ball with my new hobby.

Rick Johnston
Cedar Ridge, CA

 

[HoustonB77]

Zero your rifle on your chosen load and role on. Wind will have a lot more effect on horizontal movement than anything else. Happy shooting

 

[rickster]

Thanks B77! Appreciate the input

 

[Terry H]

I'm learning all of this as well. Tons of reading and research out there. I've read until I'm just about stir crazy but slowly getting it. It's not uncommon to see some horizontal shifts with incremental charge weight changes. Especially with a sizable grain spread that you tested. What you are looking for here is the one with the least vertical dispersion or more accurately the ones with the least vertical. That will indicate a node. The horizontal is almost a non factor. Here's a recent load development I did on mine. You can see both vertical and horizontal shifts in these as well even though they only spread 1 grain. You can see how 43.0 and 43.2 have almost no vertical. Those 2 charge weights also show virtually no velocity difference. That is the node. All the rest have climbing velocities.



Once the OCW is obtained then you can move on to seating depth testing. Here's mine. Pretty amazing how much .003 difference is seating depth can make.



From your testing I'd probably go back to 42.5 and load up and down from there in .2 increments and see how it acts. Did you chrono the loads?

 

[rickster]

Thanks Terry!
Unfortunately without replacing the barrel, I am stuck on setting the jump distance. The R700P I have has a .125" jump because of the long free bore that Remington uses. I measured this the other day in preparation for making some testing rounds after I got my load dialed in. I was sure that I had some how measured this wrong and then started reading... Apparently this is normal for my model rifle. At some point in the future I will likely upgrade the 5R barrel. I will load another set of rounds ranging from 42gr to 43gr in .2 increments.

No chrono this last time, but a friend has told me I can borrow his next time I shoot.

I'll post my results in a few weeks

Thanks again!

 

[Tuxedo007]

The Remingtons and their loooooong jump are infamous and notorious. That being said, it should be noted that most Rem owners have experimented & tweaked their loads to an acceptable level of accuracy! Others either rebarrel or swap rifles!