Velocity variance in 450B?
Posted: Mon Apr 27, 2020 12:20 pm
I posted in the AR area that I recently picked up a 20" Bushmaster-branded 450B upper. I'm having some ejection issues that may be due to it being overgassed, but I'm also seeing what seems (to me anyway) to be large swings in velocity in my handloads.
Loads:
Starline Brass (new, run through sizer die and expander die before loading)
250gr GTX bullets
2.25" COAL
CCI SRP
LIL GUN (four rounds each with a charge range of 36.4 to 38.2 in 0.2gr increments)
0.478 taper crimp
Charges were thrown using an Autotrickler which has an accuracy of +/- 0.02gr.
For the ten charge weights described above, four shots had extreme spreads (fastest velocity - slowest velocity) as follows: 30, 98, 76, 45, 82, 77, 33, 76, 97, 94. Velocities were measured with a Magnetospeed chronograph.
The next day I measured a smaller range of charge weights (37.5, 37.6, 37.7) this time with five shot groups. This time I used a LabRadar chronograph. Extreme spreads for these were 87, 129 and 30. Note that the 129 in this group was the same charge weight as the 33 in the series above, which implies that the relatively small ES the previous day was just luck.
I'm admittedly not familiar with this caliber but these seem like really large spreads to me. By way of comparison, I did a similar test on a 6.8 SPC upper recently and got extreme spreads of 21, 3 ,31, 23, 38, 10, 21, 29, 34, 26. So the worst 6.8 ES was only 4 fps more than the best 450B ES, and the best 6.8 ES was 10% of the best 450B ES.
Similar results with two chronographs discounts the idea that the chrono is the problem, and both chronos have been very reliable in the past. Weather was basically the same on both days.
If the upper is overgassed, could that possibly cause these velocity swings? I plan on trying different springs and buffers in an attempt to get the ejection problem under control. For this testing I plan on loading a number of rounds with the same charge weight, and I will chrono to see if different buffer components helps. I wanted to start this thread preemptively however in case people had other ideas on a possible cause.
Loads:
Starline Brass (new, run through sizer die and expander die before loading)
250gr GTX bullets
2.25" COAL
CCI SRP
LIL GUN (four rounds each with a charge range of 36.4 to 38.2 in 0.2gr increments)
0.478 taper crimp
Charges were thrown using an Autotrickler which has an accuracy of +/- 0.02gr.
For the ten charge weights described above, four shots had extreme spreads (fastest velocity - slowest velocity) as follows: 30, 98, 76, 45, 82, 77, 33, 76, 97, 94. Velocities were measured with a Magnetospeed chronograph.
The next day I measured a smaller range of charge weights (37.5, 37.6, 37.7) this time with five shot groups. This time I used a LabRadar chronograph. Extreme spreads for these were 87, 129 and 30. Note that the 129 in this group was the same charge weight as the 33 in the series above, which implies that the relatively small ES the previous day was just luck.
I'm admittedly not familiar with this caliber but these seem like really large spreads to me. By way of comparison, I did a similar test on a 6.8 SPC upper recently and got extreme spreads of 21, 3 ,31, 23, 38, 10, 21, 29, 34, 26. So the worst 6.8 ES was only 4 fps more than the best 450B ES, and the best 6.8 ES was 10% of the best 450B ES.
Similar results with two chronographs discounts the idea that the chrono is the problem, and both chronos have been very reliable in the past. Weather was basically the same on both days.
If the upper is overgassed, could that possibly cause these velocity swings? I plan on trying different springs and buffers in an attempt to get the ejection problem under control. For this testing I plan on loading a number of rounds with the same charge weight, and I will chrono to see if different buffer components helps. I wanted to start this thread preemptively however in case people had other ideas on a possible cause.