Help lapping a seating stem

[RmeJu]

After switching bullets, it was having runout problems (~.001-.0015" runout average went to ~.003-.005" runout average). It was recommended to me that I chuck the seating stem in a drill, put some lapping compound on one of my new bullets, and grind away.

Has anyone done this? And if so, what grit lapping compound did you use (and did it help? hurt?). I tried some 1050 grit diamond paste, but wondering if that's maybe too fine, since it didn't seem to do much, although maybe I just need to run it a bit longer.

Seater is a Forster ultra mic

Any help appreciated

 

[memilanuk]

Just use JB Bore Paste or Iosso. Not sure what that works out to in microns, but 1k grit diamond paste is probably okay... maybe even a fuzz on the coarse side.

Alternately, call Forster... I think you can send in the die and a few sample bullets and have them hone it appropriately for you.

 

[PinesAndProjectiles]

I don't think it's your die. If you switched bullets and now the runout increased, but everything else is the same, it's probably the bullets.

 

[memilanuk]

Ah. I missed the part about wanting to do it for runout. Usually people do the whole lapping/honing the seating stem because of a mis-match between the angle of the seater cone and the bullet ogive that is causing a 'ring' or other mark left on the bullet itself. Not sure that it'd do anything for runout.

 

[RmeJu]

I don't think it's your die. If you switched bullets and now the runout increased, but everything else is the same, it's probably the bullets.

They're Bergers... I'm open to the idea, but bad bullets wasn't high on my list for this brand...

 

[RTH1800]

It’s not in seating stem. It’s in your setup.

 

[RmeJu]

It’s not in seating stem. It’s in your setup.

Can you elaborate? I was getting great numbers until I switched bullets...

 

[RTH1800]

Hard to say. Doubting Berger bullet is the issue. Seating stem has never been an issue for me. Just don’t see how it could be.

 

[spife7980]

1050 grit is too fine. Even with my 600 grit it takes forever, try 300-400 or so. Or coarse still and finish with a fine polish.

Check to see if its bottoming out. If it is then polishing isnt the answer, drilling that tip out so it doesnt contact prematurely. It should sit down there with even contact all around the circumference, not wobbly back and forth on the tip. Being forster Im inclined to believe that this is not the problem but an easy check to tell for sure.

Just polishing fixes this

 

[RmeJu]

1050 grit is too fine. Even with my 600 grit it takes forever, try 300-400 or so. Or coarse still and finish with a fine polish.

Check to see if its bottoming out.

Thanks. I'm not bottoming out (as you point out, the Forster goes pretty deep) , so I'll try some 300 or 400 grit to start. How long do you think it'd need in the drill? Can you overdo it?

Btw, did you have luck improving your runout when you did it?

 

[spife7980]

Thanks. I'm not bottoming out (as you point out, the Forster goes pretty deep) , so I'll try some 300 or 400 grit to start. How long do you think it'd need in the drill? Can you overdo it?

Btw, did you have luck improving your runout when you did it?

No idea. I dont fuss with runout.

If the die was good before then a new bullet couldnt suddenly make it bad now. The difference is in what changed, not what stayed the same.