Accuracy went to hell in my Tromix 458: 18" verticle strings at 100 yds.
Cleaned rifle- Sweets 7.62, Tetra solvent etc...- still stringing.
Since I've killed 2 scopes already I removed scope and tried it on my 5.56 AR- shot respectable groups.

I have been shooting Hornady 300gr over 35.5 gr Hodgen H110 which gave good accuracy until now. All of the rounds were loaded in the same session and I weighed all of the cases and bullets before starting. I weighed all of the loaded rounds afterwords- nothing amiss, primers from the same brick.

I'm now stuck on going back to the factory loading as that gave great accuracy with NIB Corbon ammo. I have several boxes of Barnes 300 gr. bullets.

 Does anyone know what powder Corbon uses?

CorBon uses a commercial powder that I have not been able to get a hold of.  I have gotten great results with Re7 and Win296.  To me, H110 is one of the lesser powders for the 458, the density to pressure ratio is less than ideal and it tends to give lots of muzzle flash.

Try lilgun. Everywere I used H110 I have switched to lilgun with good results. The only problem I had was with my 440corbon, the 30g I used was to hot for the 240g HP but was OK with the H110. Ron

Quoted: CorBon uses a commercial powder that I have not been able to get a hold of.  I have gotten great results with Re7 and Win296.  To me, H110 is one of the lesser powders for the 458, the density to pressure ratio is less than ideal and it tends to give lots of muzzle flash.

St. Marks makes powders for many companies.

A pariticular powder that goes to Winchester gets labeled W296... if the exact same truck is headed to Hodgdon the label becomes H110.

Spent 10 years in the Corps.  Even received the MOS of 8541.  BUT, I did not start this post with:
"Once upon a time" or "This ain't no shit".

In the words of Bernadette Peters, "It's trwoo, it's trwoo!".

Had to come back and edit:  A fellow shooter reminded me that he gets different velocities and pressure curves {I think he has one of themthere Oehler 43 thingies} with those two powders.  So, there ya go and here we are.  

Jerry,

Just to throw out this WAG, it could still be the scope.  As the cross hairs in most scopes are held in place by spring pressure, it could be that the erector assembly (inner tube) is getting moved by recoil, and possibly not settling back in the same place for subsequent shots.  Not sure of the scope that you are using, but any half decent scope will hold up to the 5.56.  Perhaps the tube is settled and staying put with the lighter recoil, but moving with heavy recoil.  Again, just a wild a__ guess.  If there is any way to check with iron sights, even at 50 yards.  I am assuming that you have tugged real hard on the mounts to make sure that they are not moving at all.

I have a Leupold scope (older, lesser model) that appeared to go to heck with my heavy loads.  I sent it to Leupold and they gave it a clean bill of health, no repair work.  Never put it back on the 458, and it works fine on lighter recoiling rigs.

Craig