A box magazine sets the effective limit on overall cartridge length. Getting a bullet to sit close to the lands when the round is chambered requires either some trickery in chambering specs or, mostly, bullet selection. However, with selection there is also limitation. For safety’s sake, no factory loaded round is going to approach lands-on seating structure. When a bullet touches the lands at rest, pressures will, not can, spike. All good, if it’s accounted for. This sort of “fine-tuning” is strictly a (careful and knowledgeable) handloader’s realm.
AR15 loaded magazine
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