338 Norma Magnum Pressure Signs

[JW74]

I just had a 338 NM built and have started putting some preliminary loads together based off of both the data shooters have posted here as well as QuickLoad. I decided to start out with 300gr Bergers, on top of 85.0 grains of H1000 and Federal 215M primers. The common load with 300 grain bullets and H1000 seems to be 88-89 grains. I figured my load @ 85.0 grains would be a safe starting point. What surprised me is that my first 3 rounds @ 85.0 grains showed signs of pressure already. I had a stiff bolt and flat primers. I stopped there and didn't bother trying the rest of my loads that went up in 0.5 grain increments. I verified my chamber dimensions and it confirmed my loads had a 0.020" jump. I will definitely back down 2-3 grains and try again. I didn't even have my Magnetospeed chrono hooked up yet as the first three shots I assumed would be really tame loads and I was going to put it on for the remaining load work up so unfortunately no velocities on those first three rounds fired.

I am wondering if I have a tight barrel and that is causing the pressure? I have used that same lot of powder in my 280 AI and everything was normal there with no signs of pressure at my regular load volumes. I know every chamber and barrel is different but this pressure seemed to come on early. Any thoughts here from the folks that may have had this happen to them?

Thanks,
Joe

 

[cperazza]

@JW74
Do you have pictures of the cases?
What is the specs of your rifle?
- Action/brand
- Barrel contour/length/twist/brand

I shoot mine at 88.5-89.5. Recommend starting at 10% less, and have you Magneto Speed IOT get some feedback.

 

[JW74]

@JW74
Do you have pictures of the cases?
What is the specs of your rifle?
- Action/brand
- Barrel contour/length/twist/brand

I shoot mine at 88.5-89.5. Recommend starting at 10% less, and have you Magneto Speed IOT get some feedback.

@cperazza

The barrel is a Bartlein 9.3 twist, #9 Heavy Varmint at 27" long
Action is a Defiance Deviant Tactical Long action w/ Lapua Bolt face.

I have attached a picture of the 3 fired cases as well as one of the fired cases next to an unfired case. The loads are in new unfired Norma factory brass.

 

[Steel head]

What speeds are you getting?

 

[JW74]

@Steel head

I hadn't attached my chrono yet for those first three shots unfortunately.