Brass Volume vs Pressure/Velocity

[257Roberts]

Hey folks, thought Id share some experience reloading the "new to me" 350 Rem Mag. Hopefully I can learn something along the way here. I am attempting to work up a deer hunting load, and have been thru several iterations now, with multiple guns.

Due mainly to obsolescence, the 350RM is plagued by poor brass availability. One must either form brass from something else, buy expensive new old stock cartridges and shoot them to get the brass, or if you are really luck, stumble into some new old stock virgin brass. Ive been really lucky and have a few boxes of NEW Nosler brass as well.

To start with, in order to save my GOOD brass, I formed brass from once fired Federal 7mm Rem Mag. The process works very well, even neck turning, and I had easy to make shootable brass. I worked up loads in both guns, no pressure signs for either, but noticed the velocities were VERY high. Like 2700fps+ from 225gr bullets out of 21 inch barrels. By all load manuals, even 24 inch barrels should be 2600 or less.

So I decided to check my same load, 55.5gr H4895, 225gr SMK from once fired authentic Remington 350RM brass, and also the new Nosler brass. Both shot equally well, with progressively slower velocities, Remington then Nosler. Ive settled on the new Nosler brass for this hunting season at least, 225gr bullet at 2530fps. Much more realistic velocities, and right at MOA accuracy.

So I decided to weigh some cleaned cases and get some averages:

• Formed once fired FC 7mm Rem Mag = 222.4gr
• Once fired R-P 350 Rem Mag = 215.9gr
• New Nosler 350 Rem Mag = 203.1gr

So my questions,

Is it "normal" for ~20gr increased brass weight [reduced internal volume] to equate to roughly 170-200fps increase in velocity?

Is there a brand of brass [7mm Rem Mag] that is thinner overall and might better represent either the R-P or Nosler authentic 350RM brass? [Id like to put round count on something more readily available, and save my authentic brass for times when proper head stamp is needed]

 

[GLShooter]

Look at the percentage difference. Put some H20 in them and you'll see how huge the difference. Guys worry about minor variations in 223/556 brass and ir is nothing like what you are seeing.

 

[Rocketvapor]

You can even use powder to SEE the difference with that much variation.
Take a heavy case FILL with powder, card the top level.
Transfer to light case.
This is about 13% difference in case weight (smaller cartridge) than yours.
Heavy case with about 31gr gives same velocity as 32gr in the light case.
Yours is 6 to 9%

 

[jackieschmidt]

That much volume difference, yes.

This has always been a problem when shortening and forming a longer case to the dimensions of a shorter case. As you progress from the shoulder to the web, the case walls become thicker.

So naturally, you will have less internal volume.

There is no way to tell exactly what any given case will produce as far as pressure and velocity go, but you have proven to yourself that a 20 grn difference in the weight of the brass cases with all outside dimensions being correct WILL produce a considerable spike in pressure and velocity.

As to whether you are in dangerous territory depends on other factors. But personally, I think pushing a 225 grn bullet 2700+ fps out of a 21 inch barrel is at the upper end of the pressure curve.

 

[fkimble@charter.net]

Lower the powder charge in the faster cases so all cases are giving the same velocity. For such big cases, the difference in FC and RP cases is only about 3 %. As mentioned, its the internal case capacity that really matters. If the brass shoots accurately then just use it. The game can't read the headstamp.

Frank

 

[piie]

Weigh water to be sure, not the brass - the brass from different manufacturers may have different composition and therefore, different densities, not to mention that the different manufacturers may have different dimensions/thickness on rim / head / web.

 

[Doom]

Different grades of brass do have different densities but the difference is small. Between C260 and C240, the most common brass used for cases, the difference is about 1.6% or roughly 3 grains in a 200 grain case. Also, putting 20gr of brass capacity in terms of powder its about a maximum effect of 2gr of powder.

One of the most notorious differences in case weight occurs in Winchester 308 bcases where the Win cases weigh ~165 gr versus most others that are ~173 to 185gr. This case has approximately 2 gr additional powder capacity due to the ballon head design of the case head which is thinner than other cases. It was developed for use in Palma competition and made its way into production brass.

Ultimately though, case weight comparison between manufacturers is only a gross approximation of case capacity. In this case OP's case its safe to assume the Nosler brass has greater capacity but it would be questionable to assume that all the difference is case capacity.

Below is typical sample of average 308 case weights for information.

 

[Zero333]

Not only less volume = more velocity, but the brass elasticity makes a difference too.

I have Lapua and Winchester brass for 4 cartridges. H2O capacity is almost the same with Winchester barely having more. But Lapua brass gets more velocity and last nowhere near as many reloads as the Winchester brass.
I have Winchester brass that's been reloaded 30+ times and primer pockets are still tight and never had a split neck. I do anneal every 6 to 7 reloads. Same loads with Lapua brass had much more velocity and wore out after 5 or 6 reloads.

You did the right move by keeping it within printed data velocities. No headaches and brass will last much longer considering how scarce 350 RM brass is.

 

[Zero333]

Also remember some powders spit out the payload faster than others. With 225 gr bullets H-4895 would be my 1st choice. If you prefer a ball powder, Win-748 is the best accuracy wise, then H-335. Both those ball powders will get you more velocity as well. I LOVE win-748. Wish I could find some up here in this tiny country most call Canada.

 

[257Roberts]

Thanks, appreciate all the replies. Ive been reloading for 30+ years, and more than 15 different cartridges, and the is the first I've honestly noticed the difference in pressure/velocity. Usually, I just pick brass based on feedback and availability, work up a load, and then it stays with that gun until I need to replace it. Even with common cartridges, i separate out my range plinking ammo from my accuracy stuff, and i keep ammo with specific rifles. So Ive never really allowed a major brass volume change by doing that.

But in this case, I should have measured some more up front before shooting things. But when I saw such high velocities, safe or not, I figured something was wrong.

With regards to brass thickness overall, is there a specific brand thats thinner than others? I have some Starline 32-20 I use for forming 25-20, and its really thin. Does that hold true with their 7mm Rem Mag as well? Might have to try some neck time I form brass from scratch.