Sometimes this just isn't possible depending on a number of factors. Generally speaking, a rifles magazine is designed to at the least accommodate a cartridge loaded to the max cartridge overall length as per SAAMI specs (i.e. 2.80" for the 308 family, or 3.34" for most "long action" cartridges). Conveniently, most of the "typical for caliber" bullets will be under the max COL when loaded within this spec. Most rifle mass producers will cut their chambers so that the leade/freebore is sufficient to safely accommodate all bullet weights that are considered typical for the cartridge. In other words, when they design the reamer to chamber rifles for, say, 30-06, they have to consider that the owner might shoot factory loads up to 200 or even 220 grains, so they design the chamber to ensure that a long 220 grainer will chamber without hitting the rifling. They also cut a generous freebore because the greater freebore then lower peak pressures will be, giving an additional safety margin.

So with a factory cut chamber, you may have a difficult time seating out close to the lands unless you use a heavy for caliber bullet, and then you can easily run into the problem of not enough space in the magazine. In addition is may be impossible to reach the rifling with a mid to lightweight bullet because the bullet won't be seated far enough into the neck to hold it securely.

Depending on you rifle's chamber, you may or may not be able to reach the rifling and still fit the magazine, or maybe not at all with your chosen bullet. I lot of the competition AR shooters shoot the long 75-80 grain .224 cal bullets, and they have to single load the rifle through the ejection port because there is just no way to fit them into the magazine. If this is the case you might be left with two options, either just load the heavy for caliber bullets out and be satisfied with a single shot rifle, or you could have the barrel turned in and the chamber recut with a custom reamer with less freebore suited to the type and weight of bullet that you want to use. The problem with option 2 is that this might cause you to have problems chambering longer bullets and might cause pressure problems if you use factory ammo.

Mike