I can not offer much as far as an exact explination on this but I can tell you this as I have tested it and proven it. When I started developing my wildcat chamberings, before my match grade test barrels arrived, I just could not wait and I ordered in a couple lower priced, in-stock barrels just to get some bullets in the air and to form cases.
Two specifically were the 257 AM and the 7mm AM. The barrels I used for these were A&B 24" barrels. They actually shot pretty well to be honest but that is beside the point.
With the 257 AM, I was doing load testing with a 100 gr ballistic tip using WC872. Once I got to 92.0 gr with WC872, velocity stopped increasing, in fact, it actually leveled out and started to decline past this charge weight in this shorter barrel length. Velocity topped out at 3998 fps at 92.0 gr but dropped to 3923 fps at 95.0 gr.
I also noticed that I was being hit by something with loads higher then 92.0 grains. It was pretty obvious what it was but I wanted to make sure so I spread a big white shower curtain over the ground and laid down at the back of it and shot over it. It was hard to tell with just one shot but after 4-5 shots, you could easily see the ball powder on the curtain.
Switching to a 130 gr bullet stopped this totally. It also caused velocity to increase with every increase in powder charge telling me that the powder was being burnt inside the bore.
With the 7mm AM. IT was similiar results. Using the 140 gr BT, I loaded up with the same WC872. Velocity topped out at 3585 fps in the short barrel and stayed there inspite of increasing the powder charge another 4 grains up. It did not however loose any velocity.
I repeated the test with the white curtain and again, the same results. Only difference this time was that there were some pieces of powder that actually burned a black mark on the sheet. I could not tell if the powder had moved from where it landed after I moved or from the muzzle blast but it had obviously left a black burn mark on the curtain.
I wanted to know is the powder was burning outside the barrel but I could not tell for sure with further testing if it was or not. I did not however find any black marks with a remaining powder granual there with the black mark.
Again, with a 180 gr bullet, this stopped completely and velocity increased throught the test.
This tells me several things:
1. In a very low expansion ratio chambering, bullet weight is critical for making then cartridges powder charge burn relatively efficently.
2. Barrel length is also critical, especially with lighter weight bullets, even more so then with heavy weight bullets.
3. In some situations, unburnt or burning powder will exit the muzzle. That I have proven to myself.
I was once told the same thing with big bore revolvers, that all the powder was burnt inside the cylinder before the bullet ever crossed the cylinder gap. I actually believed it until I was shooting with a group of guys and one shot off a round from his 357 Mag S&W 66, Be hit the ground in pain and when we got him to open his eyes, there were three pieces of Blue Dot powder burnt to his eyeball.
These were not excessive loads, standard top end, from the book loads using 125 gr XTPs and Blue Dot.
Another test that has proven this to me was when I was testing my front ignition system which worked extremely well, just very time and labor intensive. When used on my 7mm AM, the bore really never warmed up, even after firing 10 rounds in a relatively fast string. With conventional ignition, three would have made the barrel so warm you would not want to hold it.
I also found that the fired cases were much hotter then a conventional case...
From this I deduce that the powder is burning inside the case, not in the bore.
Just what I have seen playing around.
I think there is a certain limit with any bore diameter where you will get to the point where powder will blow out the end of the barrel. This is greatly dependant on barrel length, bullet weight and powder burn rate but it will happen.
