Proximity for heavier load?

[GardDog]

If X powder works well for a projectile that weighs Y, is there a percentage for added powder that should work for a projectile that is 5 grains heavier?

I'm doing a ladder test for the 225 gr Hornady BTHP-M. I found load data (min & max) for a powder that performs well with a 220 gr projectile. Should a percentage over the max for 220 be okay for the 225? I just don't want to stop short, when a heavier charge could have been okay. Thoughts?

 

[MALLARD]

Re: Proximity for heavier load?

if its only 5 grains heaiver and is the "same" type of bullet i imagine you wouldnt have to reduce your charge by too much , couple grains.

but , if bullet y , which is only 5 grains heavier, has a very different shape , ( like FMJ to HPBT) , then the changes in pressure can be pretty big.

 

[GardDog]

Re: Proximity for heavier load?

Yeah, the published load was for a RN and the 225 gr is a BTHP. Probably going to have to do a little searching.

 

[EMorr]

Re: Proximity for heavier load?

As MALLARD alluded in his post a heavier bullet usually requires a lighter powder charge not a heavier one. You don’t mention what the exact caliber you are loading for is but if it’s one of the bigger 30’s you might be able to find data for a 240 grain BTHP and use that info for the 225 grain projectile. If you do use the data from the 220 grain bullet just follow the old standby rule of backing off 10% and working up slowly looking for pressure signs along the way. Also I’m not sure if you already know this but wanted to mention you will want to seat the BTHP to a longer OAL than the RN bullet data suggests.

To answer your question more directly from everything I’ve seen pressure curves are non-linear in nature and increasing a powder by 5 percent does not always yield a 5 percent increase in chamber pressure. Going over book max is sometimes perfectly safe but as I said earlier be careful and watch for pressure signs.

 

[MitchAlsup]

Re: Proximity for heavier load?

A) Bullet: 5 grains out of 220 grains is only 2%-ish.
B) Powder: changing from a 175 SMK to a 168 SMK changes the 45 grain powder charge by -0.5 grains (or 1%)
C) Start low and work your way up

When faced with a quandry such as this where no data is available; a linear interpolation is a good course of action.
Take the closet lighter bullet's weight[CLB] (of the same shape) and the closest heavier bullet's weight[CHB] against the change in min powder charges for CLB[MLB] and CHB[MHB]. This will be your starting point.

MinChargeWeight = MLB + ((MLB-MHB)/(CLB-CHB))

{The calculated value in the parenthesis should end up negative. The same 'math' can be done for max charges, but this end can be less reliable--and we really don't want that.}

Be extremely vigilent on pressure signs until a baseline establishes itself.
ONLY use this precedure if there really is NO load data--and look hard.