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Why variations in a powder manufacture's Min/Max recommended loads for different but same weight bullets?

THE Flapjack

Private
Minuteman
Oct 12, 2019
38
27
75
England
I’m putting this out there because it’s dumber not to ask than ask a dumb question.

When looking at a powder manufacturer’s published Min/Max loads for the same powder, calibre and weight but different designs of bullets, presumably tested in the same barrel, what parameters produce the considerable variances in recommended Min/Max powder weights?
I use Vihtavuori because it is reliably available in the UK. Not necessarily so for US powders.
If I knew what bullet design factors produce these wide variances in Min/Max loads for same weight but different designs of bullets, it would be some guidance and I might be better placed to answer my own questions.
If only that was answered re this mammoth post, that would help!

Vihtavuori used 27.5" 1:10 barrel, Large Rifle Magnum primers, Lapua cases trimmed to 2.714
My Blaser factory match fluted barrel is 27" 1:10 and I’m using Lapua and Peterson cases with Fed GM215M primers.
Cases run through Widden click adjustable sizer bushing dies & micrometer seater. Dies made to my fire formed brass.


N570.jpg



So, for hand-fed, bumped 0.002" and seated 0.020" off the lands, where do I start with my two 300gr of choice?
300gr Hornady A-Tip BC1 0.863 OAL 1.865
300gr Parker Productions Alu-Tip BC1 0.960 OAL 1.930 (New owners of Rocky Mountain Bullets)

My plan is to use Dan Newberry’s method and assume 92.40gr as the max, back off 7% to 85.93 gr as a cold shot and then 1% increments to first signs of pressure.

Seems a reasonable plan?
But what about my:
265gr Cutting Edge LZR BC1 0.800 OAL 1.813
265gr Cutting Edge MTH BC1 0.780 OAL 1.913

For lower weigh bullets, Vihtavuori recommends N565 or N560 but the only figures available are:

N565.jpg


My plan is to start at 77.38gr (83.2 gr -7%) and then 1% increments.

Then for N560

N560.jpg


Even less information, so my plan is to start at 75 gr, because you have to start somewhere!
 
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Different bullets have different bearing surfaces... More surface contact= more resistance sliding down the bore= usually more pressure
 
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Ogjive profile affects how easily a bullet transitions onto the rifling.

Your plan will prolly blow a primer. You’re using a long secant ogjive bullet .020” off the lands with a load meant for a much shorter tangent ogjive bullet. Not smart.
 
Bearing surface of bullets makes a big difference in the pressure generated in the chamber.
I guessed as much but only Berger quote bearing surfaces.
Strikes me as strange that such a large data point for reloading is not quoted by other manufacturers.
 
Ogjive profile affects how easily a bullet transitions onto the rifling.

Your plan will prolly blow a primer. You’re using a long secant ogjive bullet .020” off the lands with a load meant for a much shorter tangent ogjive bullet. Not smart.
Noted and taken on board.
Another approach could be same loads but use SAAMI COAL, which for example, in my chamber for the 300gr Berger OTM Match will give a 0.1565" jump and the 265gr Cutting Edge LZR, a 0.2015" jump.

Your views would be appreciated.
 
Ogjive profile affects how easily a bullet transitions onto the rifling.

Your plan will prolly blow a primer. You’re using a long secant ogjive bullet .020” off the lands with a load meant for a much shorter tangent ogjive bullet. Not smart.
Something was niggling me and just remembered.
Cutting Edge in their box for the M/T/H say, "Use powder charges from any reloading manual for the same weight bullet. Preferred jump is 0.000"-005" or 0.020-0.030"
For the LZR, just says use same charges as same weight bullets and cannelures inside the case throat walls. (Gives diagram)
 
If you want to start at the lands then you need to start low and work up. I like to start .050” from the lands because that gives the bullet a running start and lowers start pressure. It also allows me to play with COAL later when I’m fine tuning.