.338 rum puzzle

[jabey9210]

Answer me this the other day I went out to test loads in a custom 338 rum with a 32" barrel and my start load was a 250 gr lapua with 92 grains of H-1000 and it shot consistently in the 2950's then I stepped up to 93 grains and it shot around 2915 which is 35 fps slower than my start load then with 94 grains it went up to 2975. My first thought was that I some how mixed up the loads. Then I went out again with 285gr hornady's and with 88 grains of h1000 it shot five shots at 2750 with an extreme spread of 11 fps then I stepped up to 89 grains and it again shot slower at 2720 with an extreme spread of 8 fps then went to 90 grains of powder and it shot back up to 2790.  I made dang sure today there was no way i mixed up loads so why twice would my middle load which is a full grain more of powder be slower than the start load it makes me very curious.

[Qaz]

 I think I would call Hodgen's instead of asking this question here.

[336SC]

I have had that happen with certain powders also but in my one .358 Winchester.  Sometimes when maximum pressures are achieved in a particular rifle, adding more powder does not result in more velocity, it is like a point of diminishing return.  The load used may be under the "book" maximum, yet
still could be maximum in a given rifle.  Sometimes an overly long barrel will upset the peak pressure curve for the powder charge used, and then the
velocities will start to drop.  A classic example of this is the 22lr.  The most efficient barrel length has been proven to be between 16 and 20 inches.  If you go much above 20" in barrel length, the velocity will sometimes begin to be slower than with the shorter barrel lengths.  I'm certainly no expert at
these things, just passing along some knowledge I've gained after 43 + years reloading.
336SC