Seating depth as a function of harmonic tuning

[conditionzero]

I did some testing this morning to try and shrink my groups. I will prefice this with, I don't think my groups at 100y are bad, they are 1/2" MOA or better. But... I like to see where the limit is. Anyways, seating depth is one way your can tune barrel harmonics if your ES/SD (combustion) is good, but your groups are "bad". Another way is a barrel tuner, as some here have done. I thought it would be cool to show this on "paper (targets)" as a way to visualize it.

Here is my control group, this is a known good load. I have confirmed this at 1000y.

COAL	CBTO	Powder Charge	Projectile
2.8750"	2.2040"	42.3gn StaBall 6.5	Berger VLD 140gn

Here is the next depth I tested, 2.20" CBTO. I did this on accident, I was wanting to go to 2.2010". This had my barrel SWANGIN'.

COAL	CBTO	Powder Charge	Projectile
2.8710"	2.20"	42.3gn StaBall 6.5	Berger VLD 140gn

Next I tested up to 2.2020". Not as bad, but you can see it starting to tighten up as I get back to the known good load depth.

COAL	CBTO	Powder Charge	Projectile
2.8730"	2.2020"	42.3gn StaBall 6.5	Berger VLD 140gn

Finally I hucked a few at 2.2030". I had a pretty large drop in FPS on that 5th shot. I am learning that StaBall is... not very... stable at low temps. It was about 30F when we were shooting. If I bring that shot up 10 FPS in to the average range things get nice on the SD side (8.1 if my excel math is right) and if I omit it from the group the size gets real nice! I know... CHEATER.

COAL	CBTO	Powder Charge	Projectile
2.8740"	2.2030"	42.3gn StaBall 6.5	Berger VLD 140gn

Anyways I had some fun today, I am going to switch my main recipe to 2.2030" inches for now. I am maxed out on mag fed length. I am going to try some AI mags hopefully soon and see if I can push them a little more. So if you have great ES/SD and are struggling with group size, and do not / can't use a barrel tuner, start to play with your seating depth.

 

[Kingfish]

Seriously interesting info

 

[Cowcatcher]

What velocity are you running there?

 

[conditionzero]

What velocity are you running there?

Averaging 2670.

 

[Kwaynem]

What rifle?

 

[conditionzero]

What rifle?

Aero SOLUS.

 

[ClintC]

great job on setting depth. I was doing some research on setting depth tuning. weather conditions plays a lot with pressures in are ammo we load, along with barrel temp. So would that mean setting depth would change to keep the same group size? I would think yes. If you could get your load to an average group size then tune off that. another rabbit hole I know. But what do you do when your setting in training all day doing the same training you do every year.

 

[conditionzero]

great job on setting depth. I was doing some research on setting depth tuning. weather conditions plays a lot with pressures in are ammo we load, along with barrel temp. So would that mean setting depth would change to keep the same group size? I would think yes. If you could get your load to an average group size then tune off that. another rabbit hole I know. But what do you do when your setting in training all day doing the same training you do every year.

Yep, I shoot that "control group" load a lot so that is what I am going off of. I am maxed out on length so I will have to order some AICS magazines without binder plates to stretch it out a bit more to see what happens.

 

[danc46]

Bullet Jacket material on the barrel can change your barrel pressures and the harmionics on the barrel. Simply put some bullets go down the bore easier than other makes. Berger rep told me to really clean the barrel - copper free - after shooting other make bullets. He said the copper from the other bullets weren't as as hard as the Bergers and caused more friction when shooting the Bergers.
I give some credence to that statement. I once cleaned the heck out of a Savage 308 using Kroil, Wipe out and Accelerator, bore polish, etc to get moly out. Then heated the barrel up snd applied Tetra gun oil to the hot barrel. I’ve used that same method on muzzle loader barrels. They clean up super quick and much easier after you have done that. The group on that gun had a really nice reaction in both 3 shot, 5 shot, and 10 shot groups.
My son has probably dropped 20+ whitetails most all DRT since I did that over a dozen years ago. The only load he shoots is the 150 Ballistuc tip over IMR 4895 and Federal primers. He got pissed at me last year because I held him back from making a 500 yd shot. It wasn’t the deer, he just wanted to brag on the shot. He knows his rifle is exceptionally accurate eith that load. He also does that Tetra treatment ever year.
ETA there was an article some years ago about bsrrel and chamber pressures on 308 with bullet touching the grooves, then moved back 010 and measured again, and again, they had at least 5 Seating depths. Thy had the digital strain gage probes taped the chamber and barrels.
If I remember right the less the bullet jump the higher the barrel pressures tI’ll it hit a certain seating depth where the pressure no longer changed much.
Accuracy was best at the depth. It was the “ sweet spot”.