308Win Load Question - POI Shift and Velocity

[mjphawk]

Anyone have any idea why groupings would shift lower as the charge is increased?

308Win | 175SMK | Varget | Lapua Brass | CCI BR-2 | 0.020" from hard jam in lands.

100 rounds loaded in one sitting, 10 per charge weight, split into two days of shooting. All 5-shot groups. Breakdown of fundamentals on the last two groups of the second target caused some crappy groups after dropping 3 onto the same hole.

Based on SDs and target velocity (~2,750fps) I'm thinking I need to play some more in the 43.7-44.3gr range.

Posted on another forum and many were stating that 2,750fps was likely too fast for a 308Win. This is my second barrel where I get approx that speed with a 44.3gr charge and no pressure signs.

Best explanation I can come up with is recoil control.

Shot from stock Savage action, Shilen 24" Match 1:10 barrel, HS Precision stock (bedded), Vortex PST

 

[spife7980]

Yep, I would say 44.0 is it. The groups one charge step above and below it impact the same location relatively so you won't have wild swings with minor variances in the load. A tenth of a grain here, a thousandth of depth there won't plunge you off course. Maybe do two tenth grain steps above and below to see if it can be improved upon and then maybe a seating depth if you care to but it looks pretty solid as is.

As for why it shifts down, I would say barrel harmonics and it would go up again if you could slow it down enough or speed it up. It was repetitive over multiple days and I doubt you changed your zero between sessions so it sways away from shooter error but it could still be with your bag control differences raising and lowering your position like that. .

 

[pell1203]

^^^^^
ditto!

 

[supercorndogs]

It is perfectly normal for POI to move around as changes change. It is what we use to decide where the accuracy nodes are when running OCW tests. Considering your nodes are showing' and they are right where they are for most everyone shooting that combo. I would say all normal carry on with load development.

My experience with the high node for that combination 44g varget 175 smk in Lapua brass. Is that the primer pockets dont hold out. The low node I see is from 42.5 to 43.1. This will put you in the 2620fps area. If your primer pockets hold out, use whichever you like.

 

[Lowlight]

Faster = Less Drop

Settle on a load, re-zero, drive on and don't sweat the small stuff

 

[GhengisAhn175]

That's a great velocity for a 308 load.

i have a 26" barrel and with 44.8 grains getting 2770 average.
You got 2" less and .5 grains less and getting the same with a dialed in load around 44.0

tune it and update this post

 

[gigamortis]

I would go with the 44.0gr load. It is normal for barrel harmonics to do weird things with POI while creeping up the load chart. I have seen plenty of times where a faster load impacts lower on the target. A good sign that you are in the neighborhood of an OCW load is to pay attention to the vertical POI across different charge weights. Look for a range of charge weights that have a POI at the same vertical level on the target - high or low. Your lower charge weights favored high, while your stiffer charge weights migrated lower on the targets and pretty much stayed there. Once you have found your OCW load, a loading error of +/- 0.1gr of powder won't change vertical impact on your target at distance much at all.

 

[mjphawk]

Thanks for the response. What was throwing me was that I would have expected the increased speed to result in a flatter trajectory and therefore an increase in the POI. Either way I guess it doesn't matter because it seems to be shooting well enough.

I remeasured the gun today and realized that the bullet was being seated into the lands which likely is resulting in a higher starting pressure and the reason I am getting the velocities I am.

My last barrel liked 44.3 of Varget and saw similar velocities but I wore out the primer pockets in 4 firings until they were so loose that I don't want to mess with them.

I loaded up 50 more rounds at 44.0gr today with CBTO length at 0.000", 0.005", 0.010", 0.015", and 0.020" off the lands. I expect to see the velocity drop off as soon as I back off the lands (per Quickload).

I may have to develop the one load loaded into the lands for accuracy/speed as shown above and another with some clearance for any matches I might partake in.

Again, thanks to all that took the time to respond.