Comments on seating depth test?

[tcoz]

168gr SMK over 42.5gr IMR4064, CCI LRP, FGMM once fired brass in Ruger Precision .308.
All four seating depths tested are five shot. Number refers to bullet jump in my rifle. .038 (FGMM) means it's the same seating depth (jump) used in FGMM ammo.

I'm not used to testing and evaluating different seating depths so I'm not sure how to interpret the results. Can anybody add comments? I don't get to the range often so I want to choose from these four rather than running any further tests.

Thanks in advance.

 

[243winxb]

FGGM .038" off.

If you did the test over, groups would be different.

An overlay of 038 & 040 would seem to produce a nice 10 shot group.

 

[tcoz]

243winxb, I'm not following what you mean by FGGM .038" off.

 

[243winxb]

Number refers to bullet jump in my rifle. .038 (FGMM) means it's the same seating depth (jump) used in FGMM ammo.

Off of rifling.

FGGM .038" off

Best.

 

[Tony k]

This is how range buddy calls em

 

[243winxb]

range buddy

Interesting that the 2 bottom groups are almost the same. But the oal is very close to being exactly the same. I know i cant load a .002" difference in oal.

The .030" group is the best overall for sure, but i am thinking a fluke. Is it repeatable?
I would find out by loading 20 rounds of each to compare .030 and .038"

A good excuse to shoot more.

 

[tcoz]

I'll follow your suggestion and load 20 rounds of each before my next range trip which will be at least a week away. I felt the same about the .030 group.

To get the .038, I just backed off the mic seating die two tics from the .040 and called it good. I had measured a box of FGMM and they were almost all .038 with a few that varied .001 one way or the other.
It makes sense to me that .040 and .038 should give very very similar if not identical groups which they did. It helped me to confirm that I did my part, at least on those two groups.

 

[Andrew Leigh]

tcoz,

what is the purpose of the test? If I was to guess, it is to establish if jump has any effect on accuracy?

If so then you may have some issues. As you well know any change in seating depth will be met with a change in pressure and consequently speed, if you keep the same charge weight that is. So that means you are now simultaneously altering two variables and expecting to measure one.

In tests I have done specifically on a .308 the pressure has a marked increase from 0.04" as you approach the lands. So what you are seeing in my opinion is more the effect of the change in pressure more than the physical proximity to the lands. Now depending on what you are trying to achieve this may not be a bad thing.

So if you are trying to establish a good load then no problems. But if you specifically are trying to establish the effect of the jump then you would need to do things a little differently.

I would concur with the others in that the 0.03" jump is looking best.

When you do load up may I suggest that you shoot in a round robin fashion. Shot 1 target 1, shot 2 target 2 until shot 4. then allow the barrel to cool for 10 minutes and repeat. This will stop any laddering effect due to barrel temp and any temperature effect will simply show up on each group as they marginally change POI.

good luck

 

[tcoz]

Thanks Andrew. I'm just trying to establish a good accurate load. I'm not sophisticated enough of a reloader nor do I have the time to really play around with the variables, changing them one at a time.
Because of health issues with my wife, my time at the range is limited to once every 10 days to two weeks and then for only a few hours each time so my object is to come up with the best, most accurate load in the least amount of time. I have lots of time to spend at the bench but unfortunately that doesn't do me much good if I can't get out to shoot and test.
Thanks for your input.