What is causing this?????

This was a brand new un-fired piece of brass. The load was 2 grains less than the max recommended load according to Hogden. The load was

300 win Mag
200 gr Nosler Partition
70 gr H4831
Winchester large rifle magnum primer

The bullet is seated .04 off of the lands.

According to the Hogden Manual, the max load is 72 gr H4831 using a winchester large rifle magnum primer.
According to Nosler the max load is 71.5 gr H4831 with a Fed 215 primer. The Nosler guide groups Accubonds and partitions together. The Hogden guide seperates them and says the max load for the accubond is 71.5 gr.

I am wondering if I should back up from a magnum primer to a large rifle primer. This is making a mess of my bolt face.

My Nosler book says max is 70.5 gr. Pic is blurry so I can't really tell what the case looks like.

Too much pressure?

Typically you should start 10% below max. Numerous things can cause high pressure in a load/rifle combination. I would stick with a magnum primer for a magnum load and start at 67ish grains of powder

It seems like your camera was either out of focus or shaking when the picture was taken.

Nosler groups partitions and accubonds together. Hogden seperates them. I also had the same thing happen to some cases which had been fired 3 or 4 times which were loaded with 68.5 grains. The only load that I did not have it happen was 67.5 grains. The picture really just shows where the flame came out around the primer. You can also see a hole at about 4 oclock. The primer is also flattened.

I weighed the bullets to make sure nosler did not mislabel the box. They were around 200 grains. Most were around 199.4 grains. The variation in weight is over a grain. They weighed anywhere from 199 grains to 200.2 grains, I have some 225 gr accubonds in 338 that I weighed and the variation was about .4 grains. The majority were right at 225 gr with some as light as 224.8 and some being as heavy as 225.2.

How old is the powder?

I bought it a couple of weeks ago

Primer leak, primer pocket is to large. Is that nickel brass it's hard to tell. But regardless that's a leak.

What velocity u getting with that load .?

The brass is new unfired brass and I have no idea what velocity it is. Think I may need to buy some commercial ammo and see how it does just to make sure its not the rifle.

I went to the range today and bought some remington corelokts. I also brought some target loads which I have shot hundreds of in the past with no issues. I shot every coreloct without a single issue. I shot 5 of the target loads and the 5th one blew the primer. There were 2 commonalities between the target loads and the hunting loads. The first was the primer (winchester large rifle magnum primers)and the 2nd was the temperature. I have been to the range 3 times now where the primers blew and all 3 times the temperature was in the 20's. Could the temperature be causing a spike in pressure when using the winchester primers? I bought some CCI 250's which I will try next.

How about casing trim length? If the casing is not trimmed and is too long it can cause dangerous starting pressures.

Well, yes, cold weather can have a marginal impact but what you are seeing is telling me the load is near max, or over max forget the weather.

I do not use Winchester primers but that is not because of any bad experiences and I have not heard of any Winchester primer problems particularly as regards quality.

Blowing primers is what happens just before all hell breaks loose. Either your Brass is junk or something else is wrong.

It sure looks like the brass is bad but I would recheck everything as your fingers and other parts of your anatomy are kind of important.

I blew primers with 68.5 grains of h4831. I purchased unfired brass last weekend which the primers blew in as well as some brass that had been fired maybe 3 times before the primer blew. The target load that blew a primer was a nosler 190gr custom competition which was loaded with 70.6gr of RL-22, using a winchester large rifle magnum primer. I have fired over 200 of those target rounds before this happened. For R-22, nosler says the min is 69.5 and the max is 73.5.

All brass was measured and trimmed to spec where needed.

Looks like a combination of excessive pressure and soft/thin primer cups. I would try different primers with starting loads and see what happens. Years ago I had a batch of Remington large rifle primers that blew and gas cut two different bolt faces using a long time load.

I heard that about the remington primers but I have not with the winchester. I have some cci 250's that I am going to try next.

not to sound insulting but have you checked your scales?

Also try checking the water capacity of the new casings and see if they may possibly be greatly reduced in their capacity due to the walls being too thick or something...

I have had good luck with H-1000 in my 300 win mag., by looking at the pics you are getting to much pressure.