My 30-378 did the same thing every time. factory or hand loads. I brought it to two gun smiths, the last being Bob hart, and after putting a better brake on it and lapping the barrel, truing the action, trigger work, cryoing(spelling?) the barrel, and bedding the action securely, the rifle began to shoot properly as I consider it. It now will shoot 5/8 all day. probably the bedding and cryo treatment had the most to do with it? In my humble opinion the weatherby rifle is not worth the money, only custom for me now. Just ask any of the reputable gun smiths on the site and they will probably give you a list a mile long why they don't like them. Sorry weatherby guys, but this comes from someone who used to love weatherby's. That is until I tried something else! Don't get me wrong I still use, and have my longest kill with a weatherby, I just think the gun should not have required all that work when it cost $1900 when I bought it! No one can deny that most factory rem's can run with any factory weatherby for half the cost. Still, if you have the gun I guess you have to make it work like I did. take it to a reliable gunsmith and let the man do his thing!
I know most weatherby's have about .50 of free bore, That said the only way to load closer to the lands is to have a custom barrel installed.Weatherby rifles in most calibers have extremely long chamber throats. This freeboring, as it is called, allows the use of powder charges that often exceed the maximum one should use in a rifle of the same caliber but with a more standard chamber throat length. I am going to try a different powder first then try a different bullet.
The first 30/378 I had did about the same thing as your,s did
but no flyers with about a 1" group and factory loads.
So I decided to rebarrel and hold the headspace to .001 or less
so that freebore was not a problem.
I used a #7 tapered barrel 27.3/8" long and was very pleased .
The two best group,s were with a 120grs of H 50 BMG and a 180gr
blistic tip at 3431 ft/sec 5 shot group of .134 .
And 114grs of H 50 BMG and a 200gr noz partition at 3273ft/sec
and a 5 shot group of .092.
Over a period of a couple of years I came across a good weatherby
donor rifle for a future biuld and still having the factory barrel and
stock I decided to try and make another run at making the accumark
So I put it all together and head spaced it to less than .001" and went
to the range , well the headspace helped to reduce group size to .654"
(not bad for a light barrel) but I wanted more so i tried an old trick for
heavy magnums with light barrels by installing a adjustable tip pressure
screw in the front sling swivel an "did it work" I was able to adjust the
group size down to .259" this using the factory barrel and chamber.
So before giving up you might try these things in this order.
Try H 50 BMG ( none of the loads with H 1000 or Retumbo worked for me).
Try different primers.
Try 200 gr bullets.
Try tip perssure (make sure barrel is floated) I know it sounds funny to
float the barrel and then apply tip pressure but this allow,s the barrel
to expand but will dampen the harmonics of the light barrel.
And dont shoot 5 shots back to back (this thing heats up after 2 or 3 shots
shoot,dry patch and wait 5 min then shoot and repeat process.
I hope this helps you save the rifle.
Please let me know how it work's
My Accumarks all shoot and have yet to find one that won't go sub moa. H1000, Retumbo and 50BMG are all to slow for a 26" barrel and that case. IMR7828 with Fed 215 primers. 200 grain Accubonds seated as long as will feed from the magazine reliably. The Accubrake is a fine brake but does kick up a bunch of dust if shot prone. I would expect velocity around 3200 to 3250 for this set up.
To really get it to sing I have found that bedding the action and recoil lug tight will get rid of the fliers. Make sure you are not trying to use windage adjustable scope rings on it. They shift all over on this application. Double dovetails work well. I also prefer my Weatherby triggers adjusted down to around 1 pound. The action must be torqued down with a torque wrench. Hand tight won't cut it. Unbedded they need 65 inch pounds. Bedded I am getting away with 55 inch pounds.
Make sure you are not resting the front of the rifle where the sling stud is getting into the rest on recoil. A firm but steady hold really helps the groups. I have found it is very sensitive to hold and rest consistancy. I have grabbed up more Weatherby's from people that can't get them to shoot only to run a sub moa group for them right there when they were shooting 3" groups and bad mouthing the brand. 3 shot groups are plenty for evaluation. I never shoot foulers as my cold clean first shots always go right in the groups.
I use Wipe Out to clean my barrels and do it every time I take it out. My Accumarks all seem to work best with clean barrels.
__________________ Some kids want to be a fireman or a doctor to help people. I wanted to be a gunsmith.
I also found 200 gr Accubonds able to tolerate the freebore the best of any in my Accumark. Seated so a live round just clears the cutout. Never used the mag so I don't remember if they fit.
Ditto on the tight bedding/ bedding action Alex and Hired gun mentioned.
Aluminum bedding block does nothing for it. That was the only change required to fix flyers in my rifle.
Aside from the deep throat, chamber was nearly perfect.
Accubrake is a good one.
Half MOA attainable, factory barrel
Limit to 3 or 4 shot groups good idea if you want it to last.
My factory barrel died due to heat and RL25.
Preferred Retumbo and Fed215GM, but never tried 7828.