Trying a new powder with same bullet...need to revisit seating depth?

[Archis]

I have developed a load for my Ruger Precision Rifle in .243 with 115gn DTACs and VV165. The load shoots pretty good, good enough for my purposes (.5moa - .7moa) but my local sources for VV165 have dried up. I know I can find it online but would like to try a powder that is easy to find locally. I'm hoping to workup a load with H100V or H1000 as I've read that a few people have had good luck with it. Now to the main question: do I need to revisit bullet seating depth when working up a new load with the same bullet?

At the moment I've found good luck jumping the DTACs .015". Is seating depth a mechanical interface issue where regardless of powder, in my specific rifle, .015" jump should consistently be the most accurate seating depth? Or is seating depth more a function of fine tuning barrel time to get best accuracy?

I've read and have experienced that some bullets prefer to be jumped while others shoot best jammed. This suggests to me that there is some mechanical relationship between the shape of the bullet and chamber throat / leade that affects accuracy.

I've also seen people say that they use seating depth to tune barrel time which suggests the opposite.

Just curious as to the collective thoughts on the matter. I'd like to avoid another tedious seating depth workup if it isn't necessary. Thanks all.

 

[normmatzen]

I started long range BR with a 243 WIN and 105/107 gr bullets. I ended up using H1000 as I read an article claiming barrel life was slightly extended with it. I used 46gr of H1000 which was a full load and gets slightly compressed. I don't remember what the throat length was, but I used 0.025 jump. If you are tuning for load, the old jump should be just fine unless you change bullets.

 

[Lapua40X]

If it's safe, shoot it.
The target will answer your question.

 

[dmoran]

@Archis
My own experience is: optimal seating depth stays the same fairly well from one powder to another, and from one primer to another. Switching bullets on the other hand sends me looking for a new optimal seating depth, but can often be found near the same placement !.!.!
Donovan

 

[Bart B.]

I also see changing powders can use the same bullet jump. Some loads are best with bullets soft seated long so they set back several thousandths when rounds are loaded. Jump's zero.

A 243 Win chamber throat erodes down the barrel .001" for every 20 or so shots. Bullet jump distance to the lands increases at that rate of fire for a given bullet seating position.

Does this matter?

 

[Homerange]

From what I have read and learnt you need to match the velocity of your old powder to attempt to reproduce the same barrel harmonics of the old load.............if a different powder will let you and still remain within acceptable pressure limits. I would leave everything the same and workup the powder load only.
Good luck and report back.

 

[fredo]

When switching powders, I'd 'jam' that same bullet to determine pressure threshold of new powder.

A 'jammed' bullet will show pressure sooner than a bullet jumped a couple/10 thou. Therefore, finding the pressure 'ceiling' @ 'jam' with a new powder will allow another degree of safety when testing later at a (jumped) OAL...

Once a max. charge limit has been determined @ 'jam' with the new powder, go back & seat back to your prior/proven 'jump' depth for that bullet THEN, run your charge weight ladder to find a sweet spot...

If you always start @ 'jam' the bullet to pressure test a new powder, you'll establish a firm baseline. Therefore, you only have one way to go with your ladder...down! Which in turn, helps insure your ladder remains safe, pressure-wise...

Remember, we're making little bombs that explode next to our faces! Considering that, I choose to establish a pressure limit @ 'jam' because it allows another margin of error for subsequent seating a few thou. off the lands.

Have fun & be safe!

 

[Bart B.]

From what I have read and learnt you need to match the velocity of your old powder to attempt to reproduce the same barrel harmonics of the old load......

Barrel harmonics for a given barrel are always the same for every load. Barrels have the same metal makeup and dimensions for all shots. That makes their resonant and eight or more different harmonic frequencies the same. Only the amplitude the barrel whips and wiggles at changes. Guitar strings of a given size and played length vibrate at the same resonant frequency regardless of how hard or direction they're stroked, don't they?

This assumes barrels are totally free floated and don't bounce off stock fore ends; a certain way to change where the muzzle points from shot to shot while the bullet goes through the barrel and finally leaves.

Matching velocities with earlier loads does not produce the same pressure curve shapes. No two powder lots produce the same pressure curves with equal weight charges. A 1% to 2% spread is normal. Different pressure curve shapes makes bullet barrel times to the muzzle vary; even if they all leave at the same speed. Therefore, bullets equalling chrono velocities will depart the bore with different angles in the muzzle axis whip.

 

[Northridge]

yes and no, if you start a new work up with seating in a given location from previous work up you can get 95% tuned on that seating location through a powder ladder but likely you will find or verify through a seating depth ladder that the best location has moved slightly because powder start up is slightly different exit time is altered etc. even though velocity may be the same with two different powders. even though its effort you may hit it or not but verification will leave all guessing out. in the finer aspects of seating tuning I find that there will be two definite locations a rifle will want to shoot when fresh. there will be a spot slightly off and one spot slightly in where the results will be very similar but as throat gets some mileage on it the spot slightly off will start to diminish. depending on the shooting venue one is a part of this may or may not be the different but some venues are more demanding than others. happy tuning... also IMO If you pick a spot anywhere .050ths off .100ths off you can find a solid tune through powder ladders and slight seating depth ladders.

Shawn Williams