Min & Max start loads

[Birdman]

Morning,

Firstly thank you for accepting me on the forum.

I am from South Africa and been using our local Somchem S365 powder for the last 20 years. Unfortunately the manufacturer seems to be closing down ....
. I please need advice on some starting min & max values on the following two.
Thanks in advance

Calibre : 243 Win
Bullet : Nosler Ballistic Tip 90gr
Cartridge O.A.L. : 68.5 mm
Powder : Vitha Vuori N550

Calibre : 270 Win
Bullet : Nosler Ballistic Tip 140gr
Cartridge O.A.L. : 82.5 mm
Powder : Vitha Vuori N550

 

[noslerpartition]

Welcome to the forum!
Did you take a look here:
https://www.vihtavuori.com/reloading-da ... rtridge=10

The powder is listed for both calibers and bullet weights

 

[Birdman]

Thanks, yes I did but unsure what to align the Ballistic Tip to. Have also emailed Vitha Vuori but they did not want to provide a recommendation.

 

[bdbrown66]

I would say the Sierra 90gr FMJ would be close enough. As always, start low and watch for pressure signs as you go up.

 

[DrMike]

Thanks for the post, Birdman; and welcome to the forum. A good general rule is to begin at least 5% below maximum charge. Some recommend starting at least 10% below maximum charge. The idea is simply to provide a level of safety in the event you have a lot of powder that deviates from expectations. In a standard case (e.g. the .270), I attempt to drop the load to allow for a velocity in the starting charge to give a meaningful difference--say at least 50 fps less velocity.
Slower powders usually require somewhat great charge drop; faster powders a somewhat less drastic charge to see velocity differentials.

In the .270, using a 140 grain BT, a starting charge of 51. grains (3.305 grams) of VN550 and building to a maximum charge of 53.5 grains (3.467 grams) is roughly a 5% drop from maximum charge. It should give a velocity range of ~150 fps and a pressure differential of more than 10K psi which allows a margin of safety.

In the .243, with a 90 grain BT, a starting charge of 41.5 grains (2.689 grams) of VN550 and building to a maximum charge of 43.5 grains (2.819 grams) would allow you to work within a safe range of ~150 fps and a pressure differential of >50K psi.

 

[noslerpartition]

DrMike:
I have no experience with the N550, but those loads are above all max loads on the page.
What are they based on?
Bullet selection?
You probably loaded more than I ever will, so I am curious to learn.

Gesendet von meinem Pixel 3a XL mit Tapatalk

 

[DrMike]

DrMike:
I have no experience with the N550, but those loads are above all max loads on the page.
What are they based on?
Bullet selection?
You probably loaded more than I ever will, so I am curious to learn.

Gesendet von meinem Pixel 3a XL mit Tapatalk

I checked the loads as generated on QL. To be certain, the loads I have listed are higher charges than those listed on the Vihtavuori site. The Vihtavuori loads are pressure tested, and consequently would be considered to be safe. The loads I have listed are generated by QL given the parameters Birdman provided. The QL projections have this going for them--the underlying data is recent. I did use a 609.6 mm barrel rather than the 620 mm barrel Vihtavuori used, but that wouldn't change the maximum chamber pressure. The differences are greater than I would have anticipated if appealing to lot-to-lot variation alone. The firing platform would contribute some variation, but not as much as suggested. The projection generated for the 243 Win is well below the Pmax set by SAAMI, as is the Pmax for the 270 Win.

It is again a good demonstration of the importance of reducing the initial charge to work within a safe range. It is a reminder that regardless of the data provided, each shooter is responsible to assess his/her load to ensure it is safe. Thanks for asking a very thoughtful question.

 

[noslerpartition]

I had a few powders that were far off in QL.
I sent some loads for pressure testing from time to time and there were some surprises.
Others were almost spot on.
Granted - the test barrels have minimum tolerances and therefore usually higher pressure, but it shows QL can only be as good as the data provided...

Gesendet von meinem Pixel 3a XL mit Tapatalk

 

[Birdman]

Morning Gentlemen, Than you all for the valued advice and input. Glad I have joined this forum.

 

[DrMike]

Glad to have you joining in the fun. Trust it will be a source of good intel for you.

 

[Joec7651]

Welcome to the group Birdman. You’ll find many fine people here with a plethora of first hand expertise. I’ve drawn on it many times myself. Enjoy our little hideaway here and again, I bid you a warm welcome.