Shall I worry more for a seating depth or for a speed?

bigHUN

bigHUN

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This will be a max 300 meter rings setup with my old stock - several boxes of Sierra 2155 Match bullet 155 gn.
I have also one box of Sierra 2156 (only), but saving it for a longer 500+ range when the opportunity arrives.

Varget 45.8 grain, 155 gn Sierra 2155 in a Lapua brass.

Through my loading development research I got to a point where the Varget 45.8 grain gave me a tightest groups @ 300 meters with bullet seated incidentally at 0.032" from land (not calculated, it just happened that I started from that ballpark).

Ones I got the powder to speed ratio, I starting moving from there...

First I started searching for a node point going with longer (from land), started from 0.032" and got until 0.050" in 0.006" increments.
Along the way I have seen the speed gradually dropping from 3000 almost to 2850, but also the pressure started rising until I noticed a bolt footprint on the brass.

Spoke about this in my club to one "older" shooter, and I wanted to try out what he was saying going towards shorter depths.

Second I started from 0.029" with a same volume of powder, in 0.003" increments down to 0.008" and the speed is rising.

I am fully aware of that the speed kills the accuracy, and especially for shorter 2-300 meters no need to overkill.

Shall I now reduce the powder volume to lower the speed, or just stay in the middle of the seating depth about 0.030" and look for a sweet spot up-down from there?
 
You're seating deeper, right?
That would decrease the effective volume of the case. Whether or not it would be a significant factor, or the only factor affecting MV, I don't know.
 
OldDude said:
You're seating deeper, right?
That would decrease the effective volume of the case. Whether or not it would be a significant factor, or the only factor affecting MV, I don't know.
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Well, it would certainly increase pressure due to the smaller effective case volume. I would have expected higher velocities for a short bit before falling off though. - dan
 
The highest pressure you will see it when your bullet is jammed (touching) the rifling. Seating the bullet deep into the case will reduce pressure. Sierra 155's and 2156's are pretty intolerant to bullet jump, but I have not found a barrel yet that won't shoot 155's seated .020" off the lands.
Depending on your barrel, not all .308 barrels are the same. Since getting away from issued ammo (IVI) when tighter barrel were required i.e .299"-307" the trend now is .300"-.308". A tighter barrel will produce higher pressure even if we are only talking .001" difference in bore diameter.
Having shot out a few .308 barrels over the years, I have seen some barrels that will shoot 155's at 2950 fps out of a 30 inch barrel. Same rifle, different barrel, same load, same seating depth will produce 3000+ fps.
 
maynard said:
The highest pressure you will see it when your bullet is jammed (touching) the rifling. Seating the bullet deep into the case will reduce pressure. Sierra 155's and 2156's are pretty intolerant to bullet jump, but I have not found a barrel yet that won't shoot 155's seated .020" off the lands.
Depending on your barrel, not all .308 barrels are the same. Since getting away from issued ammo (IVI) when tighter barrel were required i.e .299"-307" the trend now is .300"-.308". A tighter barrel will produce higher pressure even if we are only talking .001" difference in bore diameter.
Having shot out a few .308 barrels over the years, I have seen some barrels that will shoot 155's at 2950 fps out of a 30 inch barrel. Same rifle, different barrel, same load, same seating depth will produce 3000+ fps.
Click to expand...
Try taking two cases and using the same max load try one jammed into the lands and the other shoved deep into the case. Both increase pressure, just for different reasons. Same amount of propellant burned in a smaller volume will of course increase pressure. - dan
 
There’s a couple different factors at play. Decreased case volume wants to increase pressure, but a longer jump to lands wants to reduce it. With the tugowar going on one side might win, or it can cancel out. With a straight wall handgun cartridge the decreased volume is a bigger deal than a rifle.

20 thou difference between 30 off and 50 off shouldn’t be making that much difference but if the 20 thou was the difference between jammed and not jammed it starts to make some sense. I’m wondering about the OPs method of finding the lands.
 
Dogleg said:
.............I’m wondering about the OPs method of finding the lands.
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I put some red loctite in the dummy brass neck, install the (one Sierra 2155 and one Sierra 2156) bullets, and lock the bolt for couple hours until the loctite cured.
This way I have two samples, the 2155 and 2156, and these for a hair different doe to the ogive shape.
I zeroed my Wilson inline seating die to 2155 for now, this is a zero distance to rifling touch = zero jump.
And I am calculating everything seating depth from that zero.
minus 0.020" and so on...

Yes, I have the Hornady Head Space Comparator, but I have no value from it any longer.... money wasted ones I got the Wilson die.
 
OP, no matter what the distance is you're shooting, seating depth has a lot to do with the types of bullets you're shooting.

The next thing is how much freebore your chamber has. It may not make any difference how deep you seat that bullet of any type, simply because you cant get close enough to the leade to make any difference.

I find old school cup and core bullets mostly shoot best when they're loaded as close to the lead as possible in most commercial, off the shelf rifles.

In custom rifles or some commercial offerings with very tight throats, even with lots of freebore, seating depth has little to do with anything.

That's where loading to suit the "harmonics" of the individual barrel comes to the fore, and it's why a load used in one rifle will shoot like a lazer but, give wide scattergun type groups in another.

A lot of factory chambers are cut on the large side, about the best thing you can do in such situations is fireform the cases, then partial neck resize only. This helps to keep the bullet centered to the axis of the bore until the bullet makes contact with the leades.

I was having issues getting monometal bullets to shoot. Someone on CGN mentioned they needed to be seated further back off the leade for best accuracy. This went against everything I had been used to, but it was spot on. Seating depth made a huge difference in accuracy.

I tried three different depths before a significant shrinkage of groups showed up. There was a sweet spot though, and if I went to deep, groups opened up again, even if I reduced or added powder.
 
m1008 said:
Nodes don’t exist
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