Mike, maybe the 350's will be OK, but here's what I'd do if they do not print well.

Other than leaning on good ole' Kentucky Windage, I would be honored to extend the Hillbillie Hand of friendship in the sincere hope that the skeletons of my past; sordid, tasteless, traditionless cobble-jobbing to make preposterous projectiles might be of service in this endeavour! Truly I hope these can be of help in your emergency situation and at least help you make it till the real stuff arrives!

Try these:

Option 1:

Check zero with one or the other barrels and use the closest falling barrel as a single shot till your 300 grain stuff gets there.

Option 2:

Pull a bullet. Grind it till it weighs 300 grains. Scribe a line where you need to remove material on 6 or 8 LOADED bullets and grind to that line. Take to the range and see how close they print. You may find them spot on or at least gud'nuff'fer'gubmn't'werk! If they work well, gnd some more so you have enough to hunt with.

I've done Option 1 with a buckshot shotgun and Option 2 for various bullet tests {not with doubles but I don't see why it might not work well for regulating}. Anyway, it takes no time at all to do.

Give it a whirl and get that gun on the line!!!!!

WISHING YOU all the BEST!!!

And please report. You have a spectacular gun, a really fascinating wonder that almost none of us will ever get to experience. THANK YOU for including all of us in this saga.