Odd velocity results with 155 Scenars

[fusiachi]

I was ladder testing possible BLC-2 loads with 155 gr. SMKs and Scenars. The velocities for the SMKs loaded to 2.775" were consistent with QuickLoad's predictions, but those for the Scenars loaded to 2.840" were over 100fps. higher than predicted for the three lightest loads; I decided to investigate this before proceeding with the rest of the test. Identical brass (Federal on its 4th firing). Can anyone shed some light on what might be going on here? Is the longer bearing surface of the Scenars in the case neck something that might contribute to higher pressures?

 

[LR-WSM]

Re: Odd velocity results with 155 Scenars

fusiachi,

Even though the Scenar is much longer than the SMK, I believe the bearing surface is shorter. A secant ogive will allow you to load this bullet and other VLD types at a longer COAL than with Tangent ogive bullets of the same weight.

Have you measured your distance to the lands with a Stoney point / Hornady tool? Get a .30 cal insert and adapter body for your callipers and measure base to ogive on all loaded rounds. I'd stay away from tip to base measurement as much as possible due to tip deformation. The loaded bullets ogive realationship / distance to the lands along with more bullet in the case would be my guess as to the vellocity increase.

Weight and bearing surface with the Scenars has been very consistant for me, can't speak about the SMK's as I havent shot any for comparison.

I wouldnt be too worried about speeds untill you find your max pressure charge weight with each bullet you plan to shoot from that rifle.

Good luck

 

[Unsichtbar]

Re: Odd velocity results with 155 Scenars

http://www.appliedballisticsllc.com/index_files/FULLBOREBulletUpdate.pdf

This might help explain

 

[Lt. Arclight]

Re: Odd velocity results with 155 Scenars

Lapua has a harder jacket.

 

[fusiachi]

Re: Odd velocity results with 155 Scenars

Outstanding information; thanks guys. I'll take a little time to digest it and probably be back with a few follow-up questions.

 

[fusiachi]

Re: Odd velocity results with 155 Scenars

OK. I adjusted the "Burning Rate Factor Ba" in QuickLoad to rectify my QL output with my chrono results and created a new powder file for this combination. As luck would have it the barrel time for the charge at the lowest rung on the ladder looks to be optimal, and 2700 fps. out of the 18" barrel on my DPMS should be fine to get out to 1K at 3000' on a warm day. The adjusted data for the loads in the top half of the original ladder was a positive reinforcement for my decision to stop after three rounds when velocities were higher than expected.

I'd still be interested in hearing from anyone who can shed some more light on this; for now my working theory is that engraving the harder jacket (did not know that previously) into the rifling is in some way responsible for the higher than expected pressures.