I've seen this discussion crop up several times, so I thought I'd comment on it here for reference. I'm using the Alpha magazines to get slightly more room for longer loaded rounds. This is compared to the AICS magazines that have a spacer in the front of the magazine. There is a potential issue with using magazines that don't have this spacer. Without the spacer the tip of the bullet can be further forward relative to the rest of the rifle, assuming no other changes. This means the tip might catch on the bottom edge of the receiver if it has not been positioned to account for this. Many people simply notch out the receiver to let the round come up through the loading port. In my opinion, this is not a good idea.
This picture from Varmint Al shows a simplified finite element analysis (FEA) of a 243 round being fired. You can see where the stress is in the steel to contain the pressure. Now cut out some steel behind the lower lug abutment for a loading port and feed ramp. Now instead of a 0.473" diameter medium rifle round, make it a 0.555" WSM round. Now notch it out even more to use a different magazine...
For me I think it's a bad idea to cut that part of a receiver. That is a piece of high grade steel responsible, in part, for containing several tons of white hot fire a few inches from my face. I'd prefer to cut the rear part of the receiver that's holding much less force with much lower consequences if it fails.
Most disasters are a chain of factors that happen to combine in just the wrong way. Personally, I'd like to have as many links there as are easy and reasonable. Like safety glasses. I've never needed them, but I still wear them because it's an easy link to add to the chain. You could probably take 2 lug nuts off of each wheel on your car and have no problems. But I don't think this makes it a good idea, especially when there is no reason to do it. Why not just cut the rear of the receiver? This could be a feature and selling point for a brand of DBM (that moves the mag back to take advantage of longer COL) and would be another product to slightly enhance our hobby.
This picture shows what is, in my opinion, the proper way to use Alpha magazines, particularly with magnum rounds. Basically, the magazine was moved back relative to the receiver with a few modifications: the bottom metal was moved back by opening up the bolt holes off-center and filling with a steel washer/bushing, the REAR of the receiver was cut to give the clearance needed, the bolt stop was modified, and the inletting adjusted. The front of the receiver was NOT cut (green oval). This particular bottom metal (not naming names) was so far forward that even AICS magazines with spacers would hang rounds up under the receiver. You can see part of the original inletting. I would like to see manufacturers offer a bottom metal that fits like this to begin with. I've only ever seen one brand talk about this, but I would not recommend them for other reasons. Yes this is more work (particularly when starting with an improper bottom metal), but I believe it is a better solution.