Pressure signs with Norma brass

[badshot338]

So I had something interesting happen the other days. I was hoping to get some insight;

I have been working on duplicating the ballistic profile of the MK 248 Mod 1 cartridge in 300 WM primarily for the fun of it. I got ahold of a box and took apart ten rounds and measured and studied them to get an idea of how to start. Here is the info I got (Averages):

Cartridge weight: 543.28 gr
COAL: 3.462"
Length from base to Ogive: 2.8172" Brownell's comparator's used
Loaded neck diameter: .3332"
Powder Weight: 77.06 gr
Bullet Weight: 220.08 gr
Case weight: 246.14 gr (With unfired primer)
Case Weight: 245.76 gr (With fired primer)
Case length: 2.6162"
Case Volume: I measured in ml but I don't believe what I got was good measurements
Case neck thickness: .0138"
Variation of Neck Thickness: .0011"
Case manufacturer: Black Hills (Or whoever is making it for them)
Powder: H1000
Bullet: 220 SNK
Primer: Unknown

At the range I tested velocity with two rifles. No pressure signs were prevalent. Both rifles cycled smooth after firing and the case showed no signs of excessive pressure. I would be comfortable shooting either rifle with this factory load long term without issue. Atmospherics were very average. Temps in the 50 degree range with 50% humidity. Here is what I got with the rifles:

Ruger with 22" Lilja barrel and tight chamber with 5R rifling:
Avg: 2913
Max: 2940
Min: 2892
SD: 18.5

Remington with factory heavy barrel cut to 22" with average chamber specs:
Avg: 2786
Max: 2801
Min: 2786
SD: 13.4

These drastic differences in velocities was expected as the Ruger always shoots faster than the Rem.

My goal was to match this cartridge dimensionally and with similar average velocity. I was using the same 220 gr SMK's and H1000 powder. The cartridge cases were Norma and the primers were CCI Large Rifle Magnum #250's. The cases and primers were the only deviation.


I used the Remington mostly to bring the powder charge up incrementally to see what I could get the velocity to. Once I got close I loaded ten with 76.5 gr and ten with 77 gr of powder. Here is what I got with the Remington:

76.5 gr.:
Avg: 2735
Max: 2745
Min: 2726
SD: 7.6

77 gr.:
Avg: 2746
Max: 2762
Min: 2745
SD: 17.3

So while I got close with the velocity I was not quite able to get it there. I pulled out my Ruger (with the tighter chamber) expecting to be really close. I put the 76.5 gr load in and fired. The bolt got stuck. I reluctantly put another through. Bolt hard to open again. Tried one more, nothing changed. all three cases showed serious pressure. I was too nervous to try again with that load, let alone the 77 gr load.

I always thought Norma brass had thinner case wall making larger case volume which would usually reduce pressure. I use Norma brass in my .308's and I always have to load them with around an extra gr of powder compared to using LC LR or FC brass to duplicate velocities. So it was a big surprise that I started getting such high chamber pressures out of brass that I was expecting to be able to load hotter. I am wondering of the brass metallurgy is softer than the Federal, Winchester, and Black Hills brass I was using before which allowed it to expand more?

Or maybe the CCI primers are hotter than the primers used in the factory MK 248 Mod 1 loads and this is contributing to it...

My Ruger has always been a very fast shooter and I do believe it is resulting from the chamber being pretty tight. I am worried that maybe the chamber is just too tight and I need to have my gunsmith kiss it with a reamer and open it up just a touch.

So I'm looking for your thoughts on this. Sorry for the long winded post but there is a lot of info to cover to give a full picture.