Some have asked if there is a difference on FPS with a light crimp vs a heavy crimp on soft lead bullets.
I have also wondered if it would up the fps if the bullet could stay in the case a bit longer, while the powder energy increased in the milliseconds that just happened.
Back in 5/25/2013 I did some test with my M49 38 snub nose with some soft Mag-Tech 158gr LRN bullets at 1.55" OAL.
x.x grs of Blue Dot with a medium crimp put out 812 fps.
.2 grs less powder with a heavy crimp put out 851 fps.
The x.x load with just .1 gr more powder and a heavy crimp was loaded into five cases.
Only three were fired, due to the recoil and fps printed on the chrony screen.
The three averaged 948 fps........ an 136 fps increase in fps.
Yes a HEAVY crimp on a soft lead bullet, can increase the fps on a load.
Use caution when at or near a FULL LOAD of powder or pressures.