myers129: As you know I also have a .375. I use the standard Sav BP and have no powder chamber or recess in the BP. Now I do not shoot the heavy loads you do nor as heavy of bullets but I never had ignition problems with the various powders I used, one of which was 4350. The powder column in mine is quite long. I use 209 primers, I had used mag primers but had some sticking issues that the Wins corrected.
No problems with ignition but did have some problems with keyholing and other things. Keep us posted.
BINGO!!!
I have a feeling the powder chamber that we have put in our smaller caliber barrels has actually made powder ignition WORSE, not better in our "small" caliber SMLs.
How?
We all know that the burn rates of a burn change as we use the powder in different caliber barrels right?
4350 in a .416 is a very fast powder.
4350 in a .458 is a very slow powder.
4350 in a .375 with out a powder chamber would be an extremely fast burning powder.
Now cut a .500 powder chamber in a barrel that is 1.7 to 2.5" long, and use 4350...
With the .500 powder chamber in any barrel, 4350 has been turned from a very fast burning powder to a very slow burning powder when it's in the powder chamber.
Can 4350 or Retumbo be ignited in a powder chamber?
Of course, I can easily ignite it with my .490 powder chamber and a 209 primer.
But we are definitely slowing down the initial ignition burn rate of any powder by using a powder chamber.
A short and fat cartridge in a center fire rifle is not the same a short, fat powder column in a SML.
Why?
In a center fire cartridge with a LRM primer, the pressure the cartridge needs is built up by the UN-engraved bullet being forced into the rifling.
In our SMLs, we are shooting full formed or land rider bullets that build up minimal pressure by themselves, compared to the same bullet fired in a centerfire rifle.
In a SML using a LRM ignition system with SLOWER powders, or with a powder chamber, your load is being pressure starved, because the SML bullets doesn't help build pressure for the powder to work as efficiently as in a center fire rifle.
The LRM primer does not add any pressure to the load, only heat. Because in a centerfire cartridge, the LRM primer doesn't need to add any pressure to the load.
The bullet being forced into the rifling creates the pressure the load needs.
Now that I'm using a bigger orifice bushing .035"- .040", with the bigger PFC, the 209 Magnum primers do a much better job at igniting the SLOWER powders in any SML, than ANY LRM ignition system.
Why? How? you say...because the 209 Magnum primers add much more pressure to the load that the slower powders NEED to operate efficiently when using SLOW burning powders.
I've proved this time and again...my .458 209 guns need 20 grains LESS 4350 (slow powder) than a sealed system LRM gun to get the same velocity.
My .416 gets up to 100 fps higher than the sealed LRM guns do with slow powders. How do I know? They've told me.
I also have LRM systems for my guns, and people are finally starting to shoot the slower powers with their LRM systems and they are telling me that the exact same load is up to 100 fps slower than my 209M is with the same loading pressure.
The only difference is that mine has a 209 primer system that ignites the slower powders better, in any SML.
Think about this, who builds the ultimate small caliber SMLs?
Swinglock does. What type of ignition system do they have?
A straight 209M primer or...
A SMRP system with a BOOSTER built into the ignition module. (genius)
The only way the guys with the LRM ignition systems will get their guns to work with slow burning powders efficiently is to use a booster. Especially if you have a powder chamber.
Using a LRM system of any type works great with a FAST burning powder. Like 4198 in a .45... or 4350 in a .416
Heck I could ignite the 4198 powders by smacking two rocks together.
But when you use any LRM system with a slow burning powder, like 4350 in a .45 or Retumbo in a .416, especially with a powder chamber, your load is going to be pressure starved unless you use a booster.
In any LRM gun, if you don't cut a powder chamber in your small caliber SML that will help you out with ignition and pressure starving, because a smaller chamber must drastically increase the burn rates of powders in SML guns.
Right now my go to powder with my .45s is IMR4831.
It creates lower pressure with much higher velocity.
Just like Retumbo does in a .416
The next slow powder I'm going to try igniting in my .45s with the magnum 209 primers is IMR-DIRT, but it's all sold out at the shops around here. But at least I know the 209M primer will ignite it.
So bottom line, if you want to use a powder chamber in your LRM gun with a small caliber SML and you have to use a slow burning powder because you're using heavy bullets, you will need a booster powder.