I've been playing  with a Ruger #3 with a Ron Smith barrel in 256 mag. With H110 and a Barnett 120g  bullet with a .262 base tapered to 255 my groups are all pretty much under an inch and the good ones sneaking under half that a 100 yd. I had loaded some in fixed ammo sized. 258 and had pretty decent accuracy. I decided to try the same bullet breach seated. And what I did was turn my barrel into a lead mine. Repeated the experiment this morning in fixed ammo they do no they do not lead but breach seated they lead like crazy.

Premod70 wrote on Jun 10^th, 2022 at 3:25pm:


I played with the smaller diameter bullet seating and forward in the throat but it didn't seem to help.

It's a Weber seater. 

  Quite obvious the bullet is not sealing properly when breach seated. In fixed ammo it is surprising to me that it did not lead.

calledflyer wrote on Jun 10^th, 2022 at 7:40pm:


That's what I am using but being that it shot reasonably well in  fixed ammunition I thought it would give it a try. It surprised me that a gas cut severely breech seated but jumping out of a case into the tapered throat with fixed ammunition it did not.

This is what it's been doing (good groups)with that Barnett bullet breach seated mind you there's a bit of wind I didn't quite catch but it's definitely showing promise at a 100 yd. I've shot a little bit at 200 and right now a bit under 1.5" is where I'm at for the best groups.
Now that I've got it narrowed down I will start changing small variables and shoot 10 shot groups. It is a number 3 in 256 win mag so it's not like it's cutting edge.

When breech seating you have to inserts the bullet so the base of the bullet is fully seated and sealed.

At least 1/2 of the base band needs to have land grooves in it to achieve the proper distance of inserting the bullet.

Failure to do so will result in gas cutting around the side of the bullet and result in leading.

That is odd.  The fixed is sized .258" but the BS bullet is unsized at 0.262"?   If so is it possible that he base band is too large and the bullet is getting shaved when being breech seated? 

Chris.

Gas cutting may be occurring from using too narrow of a bullet being used.
Confirm your groove-to-groove diameter.
I always breech seat bullets .002 " wider than the  groove-to-groove diameter.
Also if bullet alloy is too hard then gas cutting happens faster.

JLouis wrote on Jun 11^th, 2022 at 2:04pm:


20-1

Myself I have been down this road before. 
Not trying to take away from anyone's advice by any means.
But what I did was to fully engrave the entire length of the bullet and just beyond its base.
But I would also recommend that you do the bullet gas sealing test that I had mentioned in a previous post before you even try to take it out and to shoot it.
If it does not seal and this is just myself I would not spend anymore time with trying to breech seat that bullet.