The issues with large, very overbore cartridges and low density charges, especially with faster burning powders, is detonation. When there is too much air space in the case, if the primer flash is able to flash over the top of the powder in the case, instead of having the powder up against the primer flash hole, then instead of getting a controlled burn and expansion of pressure as the powder burns, a large portion of the powder all ignites at once, and instead of the controlled burn, it is an instantaneous detonation, with a massive pressure spike that, at a bare minimum, locks the rifle up and often breaks a few things, and on the other end, completely ka-boom's the rifle, and can cause major injuries to the shooter.What would be the major concern? Seriously
The other issue is it won't necessarily happen every time, you could develop a load at the range and have it working fine, but then you go out hunting with it, have the rifle pointing downward, the powder leaves air space between the flash hole and the powder, then you pull up to shoot and your rifle explodes in your hand.
The other place you see this happen sometimes is with reduced loads in standard rifles, such as a 30-06. I have seen evidence of people loading reduced loads in 30-06, .270 wins and other similar rifles using slow burning pistol powders. Some shot the load many, many times. There was recently a thread on here where a guy did just that, and was having a young kid shoot it, and nearly had a ka-boom, it locked up his rifle, and I believe broke his firing pin.
The case capacity of a 7 Allen Magnum is around 122 grains, according to this thread, post # 6 -
What is the big deal with the 7mm allen mag.? is it really that much better than the regular 7mm rem mag? can you help me with this, I love the coefficients for the 7mm. and i already own a 7mm rem mag in a browining abolt. is this new caliber really something i should look deeper into?
However with powder at least in reference to the detonation concern, you don't look at fill volume by weight in comparison to H2O, but case volume fill. 90% of the 122 grains would be 109.8 grains, however powder doesn't take up the same amount of volume by weight as water. You have to look at case volume fill, not weight, adding in another factor. All this is the reason I would advise against using a powder like H4350 in a 7 AM, there are proven safe powders that Kirby does recommend that get great velocity, the risk (possibly damaging your gun up to blowing yourself up, at bare minimum more barrel wear on a 7 AM) vs reward (maybe a velocity gain?) simply does not add up. In the stated words of Kirby Allen, "With conventional bullets, even lightweight bullets, it would be very dangerous."