I have a 300Winchester in a Tikka/Tac. I use 76.5 grs of H-1000. My ES is ~50fps average velocity is 2833fps with a 208gr A-Max. I have read that a 'soft' primer is one of the methods to reduce ES , I'm using a Winchester LRM now. Ant experience with this is much appreciated.

I think I would try some of the Federal 215s if you can find them. This is the hottest primer on the market if I understand correctly, and you are using a large charge of H1000, which has a pretty good graphite coating to slow the burn rate. I think the 215 might help you...

I use Winchester primers almost exclusively and have not had trouble with ES. I believe the ignition burn rate of primers have more to do with velocity deviations than the cup hardness/softness from my experience. Others may have differing opinions though. There are always at least two opes to every topic.

My understanding is similar to Doubless's. The Fed 215, I've read, was developed for the Roy cartridges. (I like to do that, call Weatherby cartridges Roys. Makes me feel like one of the true inner circle guys.)

I read an article years ago in Precision Shooting (RIP)on primers. There were photos of flame patterns, and the Feds were super hot compared to all of the others.

I use Feds because they hold ALL of the BR records. One exception for me: Winchester primers give me better groups in 375 H & H in my M70.

I look to brass (TIR), bullets, powder, powder charge, seating depth in about that order. Primer selection would be way down the list on accuracy testing.

Lots of guys acknowledge that ES is often not related to poor accuracy. Just as some really rough looking barrels shoot well. I thought I was just going to be a real pro when I bought a boresope. Guess what? On a Montana PD hunt with several guys, and more rifles, Senior Borescope could not reliably predict the accuracy winners.

Like weighing brass, de-burring flash holes, etc. A couple of things I will tell you with great confidence: Accuracy testing off a truck hood with a sleeping bag is a total waste of ammo, and can ruin a windshield, too. Guys who put their test loads in baggies and do not have well organized loading records......... I will stop there. Too negative for me.
Brice

I think the theory is that the weakest primers that reliably ignite your charge is best because it give the least contribution to pressure and velocity.

You control pressure and velocity with charge not primer. Velocity increase from hotter primer increase pressure more than similar velocity increase from charge.

This is from the internet but sounds believable.

I see that you guys are all well read and properly schooled on the latest (if conflicting) theories. Perhaps if you had owned a chronograph for the last 40 years you would understand that powder has much more to do with velocity deviation than do primers. Primers may, particularly in small cases, contribute some of the velocity deviation from round to round, but the deviation caused by the type and amount of powder is almost always much greater.

Bottom line: Switching primers around in search of a lower SD isn't usually effective. And even if you do get some marginal improvement in SD with a different primer that improvement doesn't usually come with a corresponding improvement in accuracy.

I agree that switching around with primers is mostly for benchrest an long range target shooting. But I have experienced that using the wrong primer can be bad for velocity variations. In my case the primer probably was to weak, non magnum vs magnum, and in line with the theory.

I'm sorry I didnt get the chronograph 40 years ago.

Although primers have an effect, I think that to get your extreme spread down, focus on powder charge and bullet seating. I used 83 gr H1000(when it first hit the market), Sierra 180gr SBT, Win brass. CCI BR2 primers to get 3062 fps averaged velocity. The extreme spread was a mere 7 fps. It grouped 3/8" ten shot groups every time I sighted it in for hunting every fall. That load now exceeds max in the new loading manual.

My way to get the best load with the lowest extreme spread and best group is to start off at the starting load with my bullet .050 off the rifling and increase the load 1 grain at a time till I have found the charge with the lowest extreme spread. I then move the bullet closer to the rifling .005 and back off the powder charge 2 grains under charge that gave me the best extreme spread, then by 1 grain increase till I find the next charge that gives the lowest extreme spread which normally is a grain or two higher than the previous charge that gave the lowest extreme spread and stop there once the group starts to open up. Repeat the procedure seating the bullet forward in .005 increments at a time as you find the lowest extreme spread for that bullet seating depth till your bullet is seated .005 off the rifling. Select the load with the best group. A much more time consuming twist to this is to tune the action of your rifle as you do this procedure, but the reward can be significant.

Although not the only factor, velocity spreads tend to be lower the greater the loading density of the powder, that is, the more room in the case the powder takes up the lower the velocity spreads. This is the most significant single factor in velocity spreads I have found in 40 years of chronographing handloads. Slightly compressed loads tend (again, generally speaking and on average) to give the lowest velocity spreads.

It was Weatherby that tested various primers and determined that the Fed 215M was the only primer to reliably ignite the then being tested 378 Weatherby Mag. I do not know if they switched all heavies to Fed 215M or not after that.

Weatherby used to be very open about their components and recipes if you called and asked. I know they told me years ago that their 375 H&H load was Fed 215M, Norma case (Wby headtamp is all), Hornady 300 gn RN(old) and whatever charge of Norma 202 or 203(my mind doesn't remember that. But, this was back in the 1980's or early 90's when they loaded 375 H&H; it was about the only non Weatherby caliber they loaded/sold ammunition wise.