"...optimal headspace..." There's no such thing.
"...using data for IMR 4064..." Whatever for? There's IMR4166 data on Hodgdon's site. Use the 90 grain data. Three grains won't matter.
"...load to mag length..." Load to the max OAL given in your manual. Mag length is mostly a guess. 2.710" is MAX.
"... after 400 yards..." Every .243 bullet is headed down. Drop like bricks past 300.
I don't agree with a single line of this post.
1) There certainly is such a thing as optimal headspace for any given rifle type. Optimal headspace for brass sizing is simple - in a bolt gun, it's a shoulder position which closes easily, with slight resistance felt. Typically, this is 1-2thou sized from fireformed 2x or 3x fired cases. The bolt closes reliably and easily, but the brass isn't overworked or under length to produce inconsistent ignition or case separation. Pretty straight forward.
2) The Hodgdon 90grn data is for the Speer Hotcor - a VERY different bullet than the 87grn V-max. The respective load data for these different bullets can be considerably different - for example, the 87grn V-max data in Hornady's manual shows H4895 at 28.7-35.3grn, while Hodgdon lists H4985 for the 90grn Speer HotCor at 34.0-36.5grn - over a grain more for the Hodgdon data. For W760, Hornady lists 42.5grn as max for the 87grn V-max, Hodgdon lists 43.5grn as max for the 90grn Speer HotCor... Again, a grain more. Different bullets, different load data.
3) 2.710" is NOT the manual listed maximum for the 87grn V-max. 2.640" is the listed max. SAAMI max doesn't mean squat for real-world rifles. If a guy wants to seat the boattail to the base of the neck, you'll have the 87grn V-max seated to 2.690"-2.700" --> this length will give you .240" (full neck) of bearing surface contact. Find your leade, subtract your desired bullet jump, and seat the bullets to that length. Manual COAL's and SAAMI max's don't mean anything for real world rifles.
4) WTH? Drops like bricks past 300? I shoot the 87grn V-max 243win in a pistol a lot more than in a rifle, but in my full speed rifle loads, it's only a 1mil drop from 300 to 450yrds - that's a long ways from what I'd call "dropping like a brick." I'm only dropping 1/2 Mil per 50yrds out at 600yrds. I can still use a mil-dot reticle without dialing at all out to 775yrds (5Mil = the tip of the bottom thick wire). Dialing for range, the 243win is manageable past 1,000yrds, even with the lighter 87grn V-max. Even a moderately skilled shooter with a laser range finder should be able to get out somewhere around 600-700yrds before the gap between a rudimentary ballistic calculator and the real world drags them off of target - we're only talking 3-4 mils of drop out there (200yrd zero). Very manageable.
For the OP, I'll look forward to hearing how 4166 does for you. I've been feeding H4350 to my 243win for a while now, I'm thinking of trying some of these newer powders.
