300 Win Mag Unexpected Muzzle Velocity/Pressure

[E_T_G]

I was looking for a load for 190gn SMKs. I wanted a powder with a little more top end than H1000 or 4350. Since I couldn’t find any Reloader I decided to try N560. It looked good on paper (Sierra loading manual). I did find a recommended load of 67gn which is .3gn below starting load. I loaded up a few rounds to try out. Components were: 190SMK, N560, once fired Federal Mk 248 brass, and FGGM magnum primers. The group and velocities were pretty close until the last shot where I shot my suppressor cover – that dropped the velocity 200fps. Average mv was 2869.5fps. According to the manual this is what I should be getting at 72.6gn. Primers were flattened but still had rounded edges and no cratering. I loaded up 10 rounds of 67gn and a ladder going up .3 grains per step. This time I brought a new 300WM that had about 40 rounds through it - basically the same barrel – both 23.6”. This time the average mv was 3126.6, ES 10.8, and SD 3.5. The primers were flattened and slight firing pin cratering but no wiping or heavy bolt lift/extraction. 3126fps is higher than any published load in the Sierra manual. Needless to say, I didn’t shoot the ladder. The 10 shot group was 1.2” including 2 fliers. Minus the fliers the group was .34” @ 100yds. Unless it was due to more resistance from the barrel, I haven’t a clue why the velocities were that high or why the difference between the two guns. My other 300 has less than 200 rounds through it so I wouldn’t think it would make that much difference. Since it is military brass I should be able to go 1gn below starting load without worrying about a flash detonation unless that is what is causing the primer flattening and cratering. Does anyone have experience with N560? Velocities, SD, and ES were measured with a Garmin C1 Pro. Thanks E_T_G

 

[Slides]

Engaging the rifling sooner in the new barrel?

Assuming you aren't catching the case mouth in the freebore. Bolt closes easy on them?

I had issues with pressure from factory hornady 300winmag in a new custom barrel, ADI handloads of 2217 (H1000) were fine.

 

[E_T_G]

Cartridges chambered easily and all passed the case gauge. One barrel had 200 rds and the other 40 - I don't really think there would be much barrel wear but chamber could be slightly different. On the slower speed the temp was 10 degrees higher but doubt that would make 250fps difference. I don't know why the velocities are so much higher than the book. I do think FGGM primers are softer than CCI or Remington.

 

[Aftermath]

Where is the bullet in the chamber...jump or jam?

 

[E_T_G]

Measuring off case base to ogive for a round as loaded versus a jammed round, there is a gap of .021". Very close to my other gun.

 

[918v]

300WM chambers vary in length and girth. What’s mild in a Sako TRG will be too hot in somebody’s tight match chamber. Measure the brass diameter at the expanded part of the base and shoulder and the shoulder length.