Seating Depth - whats significant?

[Gollum]

I've done some pressure and accuracy testing on a load, starting with a 0.015" jamb. I would now like to fine tune it a little more, with seating depth.

Question is, how much do I vary it when coming out?

I'm figuring on -0.005" intervals, but is that enough? Should I be jumping in -0.010" intervals when testing? What makes a significant difference?

 

[tenring]

It is common to start with bullet just kissing lands, then increase or decrease by units of .005. Average three different seating depths before making a final decision. But last week, I lowered my 95 gr. Bibs by .003 and went from a .632 five shot group to .388. So you need to play around with it and fine tune until you are satisfied.

 

[Preacher]

Start at .005 and fine tune it from there after you decide what to use.
.010 at a time and you might miss that sweet spot.
Sometime, as stated you may find a few thousandths change will be a BIG improvement.
Run it a few times to get a good average before tweaking it a bit.

 

[Forum Boss]

I used to start "kissing the lands" but now I usually start .015" in the lands since that is where I do my fire-forming, and a lot of the match and varmint bullets perform best into the lands. I work out in .005" intervals, maybe to a max of .020" jump. But I've found with the short 6BR case, pressures can actually rise as you go pretty deep into the case since you are reducing effective case capacity.

I also think .005 in the lands to .005 out is sort of "no mans' land". Some bullets may be touching the rifling while others may be a little off. Better to be one way or the other. Of course, every barrel is different, YMMV etc.

 

[Aussie_bob]

Gollum,
Is this the 30BR you are talking about have you run it over a chronagraph to see that you have your velocity in the right area around the 3,000fps before you start playing with seating depth.

 

[Gollum]

yes, its the 30BR. I don't own a chronograph....yet. Its "on the list" though. My sweet spots do seem to correspond to what the 30BR Info Page loads indicate though.

I may have found an error in my resizing procedure that was causing some problems. It appears that I have been setting back the shoulder about 0.008 every time I resized. I've had a lot of groups with a lot of vertical, and thinking that my ignition isn't real consistent due to the excessive setback. Experimenting with that now. Lots to learn!

 

[Forum Boss]

yes, its the 30BR. I don't own a chronograph....yet. Its "on the list" though. My sweet spots do seem to correspond to what the 30BR Info Page loads indicate though.

I may have found an error in my resizing procedure that was causing some problems. It appears that I have been setting back the shoulder about 0.008 every time I resized. I've had a lot of groups with a lot of vertical, and thinking that my ignition isn't real consistent due to the excessive setback. Experimenting with that now. Lots to learn!

Here's a cheap tip. You can use a .45acp shell case to determine the amount of shoulder bump,setback) you are putting on your cases. Resize the .45 acp case,to make sure it's round), trim the casemouth square, and remove any burrs on the case mouth. Then slip the .45 acp case over your sized, unprimed 30BR round and put the two between the jaws of your calipers. Then compare this with a fired, but unsized 30 BR round,primer removed).

 

[Gollum]

Thanks for the tip! Easy and cheap.

But it brings up another question: How much should I be bumping the shoulder back each loading? Any?

 

[Forum Boss]

1 to 2 thousandths is what most people recommend. Just enough to allow the bolt to close comfortably. But you may need to do some trial and error,wiht a bumped case) in your chamber, cranking the die down a little bit at a time.

PS, if it wasn't clear from the above pix, you need to remove primers from both your 30BR case and the 45 acp case so they seat flat on the jaws of the calipers.

 

[Bullet94]

This is a good explaination of distance to the lands when full length resizing –

http://thefiringline.com/forums/showthread.php?t=211951