DITTO what rcmodel and Steve4102 are telling you.
I'm doing something very similar to what you are doing.
I'm loading for a .257wbymag, and forming my brass from OFB 7mmRemMag brass.
After forming the "ogee" or double radius shoulder, I have to fireform them to the chamber, and in some instances a slight ring is formed just behind the case shoulder on the case body. The formed cases are sized just sufficiently to easily close to bolt on, but still require the "filling out" of the shoulder similar to what you are experiencing. The ring behind the neck will only "dissappear" when loads are within 7-5% of Max. or roughly equivalent to factory loads.
(yes the cases are ~0.050" short, but it's not an issue. When brass is so long it has to be trimmed, it's time to throw it away !!!!!!)
I'm using mostly WC-860 powder that is from pulled down .50BMG brass. It has a small (less than 1 ppt -part per thousand- of something simular to H50BMG) the WC-860 has a burning rate essentially that of H870 and is somewhat slower burning than 7828. If you can find some of this, you should get it, as it's perfect for that "flame-thrower" you have !!!
(bought mine for $5.00/lb from Widners back in August. Bought a lot of it !! I also have a .300RUM thats it the best and most accurate powder I've used in it.)
I'm using 75gr (temp under 70deg F.) and 74.0deg over 70deg.F with a 115-117gr bullet. I found this out by developing my loads when temps were over 100deg this past summer. I rechrono'd the loads and found that they had lost ~150fps, so I had to "bump" them up a little for hunting season. Especially with bullets 110gr and lighter. Very similar to what you are experiencing.
I Saw a very similar pattern to what I'm seeing with your loads when using the aforementioned loads and 100gr bullets. I'm now back to using H4350 for the 100gr bullets as it's very accurate and gets essentially factory ballistics and velocity as well. I started with the starting load for the 115-120gr bullets and worked upward, till I reached the predicted max load of 80.0gr. The case will only hold 82.0g, and bullets can only be seated to depth over 80.0 before bullet deformation occurs, hence 80 is the max as that's all the cases will hold......... Velocity was decent at ~3,400fps and accuracy was excellent. However, drop the temps to ~65deg and velocities dropped to 3,180-3,220fps. Not bad, but not good for 80gr of powder, when my .257Robt's will get 3,100fps with 45-46gr of H4350 !!!!
So, the 100gr bullets are "out" with anything slower than RL-25 or Retumbo, which I have, but haven't had time to "play with" meaning, work up the loads. But with the 1,000 Hornady 100gr "Blems" I got from MidwayUSA, who knows ????? anyway, whats wrong with a 100gr bullet a 3,500fps anyhow ??
Back to the "problem".
Note that the powder charge of H4350 for my somewhat smaller case (approx.15%) is close to what you're having problems with in a significantly larger case and slower burning powder.
I'll repeat the question that Steve4102 asked, as he's in the right direction. Some "pressure signs" can also be signs of "low pressure" and with Newly formed brass being fireformed, also indicate excessive headspace (which in a way it is..) and "excessive pressure" which is actually low-pressure with an accelerated case "impacting" the bolt face causing burnishing and flattened primers. A clear sign that it's LOW pressure is that the primer is flattened, but not cratered. Cratered firing pin holes can be excessive pressure, or an undersized firing pin/over sized pin hole. (got a tack driving .223Rem that does that with ANY load........). Also, the blow-back is a sure sign of low pressures. I see that a LOT with the .45Colt in my 24"bbl Winny M94 and "Cowboy" loads.
Be more specific, or better yet more detailed pictures so we can help..........
Are the dents you're showing in the picture from sizing/forming the cases? If so, they're from excessive lube in the die. If from firing, it's the gas backing up under the case before the pressure rises sufficiently to obturate the case to the chamber. And as others have stated, if the case gets enough backpressure under the case, it can cause case failure and a lot of HOT GAS in your face, and everywhere else it vents out from the action. In an extreame situation it can case a pressure excursion from the bullet accelerating and then stopping in the bore, and then being essentially an obstructed bore causing the "alledged" detonation, which as Steve4102 has been shown not to be detonation of the powder column but the reaction of the firearm from a second pressure "spike" or WAVE as it appears on an ocillograph or computer printout. This then results in the firearm "exploding" due to a lack of a sufficient vent.... ie: the open bore from an accelerating bullet and bullet exiting the bore. After all, the firearm is only a single stroke heat engine, with the piston leaving the bore. Ever experienced a crankshaft failure in an internal combustion engine ???? I have; a crankshaft siezed on a Continental O300D aircraft engine I was operating. Not a pretty sight! and a LOT more expensive than a $5,000.00 rifle letting go.
Especially if you're at 5,000' and in the clouds as I was.......
BTDT, but fortunately no scars to prove it !!!!!
Do be carefull out there !!!
WAR EAGLE !!!!! 17-10, 6 in a row and counting.....
AU ALUMNI- CLASS OF '78
