Reducing load as bullet seated deeper .243AI

[Tictac]

Hi,

I currently run a .243 AI with 46.6gn's RS62, 87gn Bergers at 3280 - 3300 out of a 24" barrel. This is quite warm and on a hot day I've been uncomfortable with this level. I'm thinking of reducing the load to 46.0 but I also want to play with seating depth. I'm currently .040 off and want to try .030, .060 & .080 initially. I don't see an issue with the .030 jump, but I am interested in peoples experiences on the effect on pressure with the case capacity reduction.

i.e. Going from a warm load at 46.6gn's .040 jump to 46.0gn's .080 jump. my .6gn's powder reduction made up for in less case capacity and my pressure not going to actually reduce?

Paul

 

[Straightshooter1]

Hi,

I currently run a .243 AI with 46.6gn's RS62 (equivalent of IMR4350) 87gn Bergers at 3280 - 3300 out of a 24" barrel. This is quite warm and on a hot day I've been uncomfortable with this level. I'm thinking of reducing the load to 46.0 but I also want to play with seating depth. I'm currently .040 off and want to try .030, .060 & .080 initially. I don't see an issue with the .030 jump, but I am interested in peoples experiences on the effect on pressure with the case capacity reduction.

i.e. Going from a warm load at 46.6gn's .040 jump to 46.0gn's .080 jump. my .6gn's powder reduction made up for in less case capacity and my pressure not going to actually reduce?

Paul

In looking at my QuickLoad numbers for your example, it shows that the pressure will indeed reduced significantly enough to change things up. Keep in mind that MV is a function of pressure as is the harmonics of your rifle, so the result of that load could move things out of the accuracy node.

 

[Laurie]

QuickLOAD and the pressure / MV effects of COAL changes are extremely misleading if taken literally. The COAL that matters pressure-wise is that with the bullet ogive just short of the lands. That's because in a long throated set-up with lots of bullet jump to the lands, the bullet moves to the lands well before PMax is achieved. PMax is then determined by the effective combustion chamber volume at that position, not where it started from seated in the case. That's why excessively throated rifles don't produce QL calculated pressures and MVs.

Many years ago, Vihtavuori had a little table on the effects of bullet seating position on a 7.62 Nato cartridge in its loads data, seating the bullet progressively deeper in the case. Contrary to received wisdom, neither pressure nor MV rose with the first three or four deeper seating steps; then both reduced very slightly on the final COAL reduction. It is surmised that when jump reaches a certain point, the 'run at' the lands reduces the duration of the check the bullet the bullet receives on being engaged by the rifling and it moves forward more easily and quickly.

i.e. Going from a warm load at 46.6gn's .040 jump to 46.0gn's .080 jump. my .6gn's powder reduction made up for in less case capacity and my pressure not going to actually reduce?

As above the deeper seating shouldn't increase pressures, may even reduce them marginally although that's unlikely, whilst the 0.6gn charge reduction will definitely produce a small decrease.

What does increase pressures is seating the bullets into the lands. When you get to hard jam-seating, this produces a considerable peak pressure increase.

 

[Bill Norris]

Increasing jump to lands does not seem to increase pressures. I have heard this point argued within a local circle of shooters but have not seen it proven yet.

 

[mikecr]

You should back away from a powder node for seating testing anyway.
After all, you want to see results of one change at a time. So you do not need a collapsing powder node added to seating testing.
Then, with best seating found, do your powder development again.

 

[Doom]

I think your drop to 46 should keep you on the node for seating depth you currently use.

As for pressure vs seating depth, once you are away from the lands the variation in seating depth has a minor effect on pressure. For your range I wouldn’t worry but just watch for signs.