The truth is that the two styles of bases are very similar in some aspects and very different in others. Understanding those differences is the key to finding out which base style is the best choice for your application.weaver-styleWeaver-style
The Weaver-style base has been around for many decades and has proven itself as one of the top choices as a mounting platform by hunters and shooters the world over. A weaver-style base is a great option for shooters who want a very low profile mount. In a 2 piece, or 1 piece in certain configurations, the weaver style offers an affordable, yet rugged mounting base.
The Picatinny or MIL-STD-1913 rail is another time-tested design that offers its own benefits and drawbacks as well. The two major areas where the Picatinny rail differs from the weaver style are the number of slots, the slot placement, and the slot dimensions. the basic difference between the weaver and Picatinny slot dimension is that the Picatinny has a slightly wider, and slightly deeper slot. The Picatinny rail also has a standardized spacing of the slots, and the slots typically extend down the length of the rail. A Weaver-style base may have only one or two slots, and the placement of the slot can be anywhere on the base.
Any weaver style mount will work on both weaver style and picatinny rails, however, some manufacturers make picatinny specific mounts, and these mounts can only mount to a picatinny rail for a few reasons. Either the recoil lug that is being used is too wide for the weaver style slot, or there are multiple lugs that are spaced for the picatinny slot spacing, and they will not align with the weaver style slot placement. In short, the answer is yes, there is more forward and back play in a picatinny slot. The main thing to consider is that there is still a bit of play in a weaver slot, and typically picatinny specific rings also have a bit of play in a picatinny slot.
The groove width of the Picatinny rail is 0.206 in (5.23 mm)
versus 0.180 in (4.57 mm) of the Weaver rail/mount, and by contrast with the Weaver, the spacing of the Picatinny recoil groove centers is consistent, at 0.394 in (10.01 mm).
So you may notice some back and forth motion when items are mismatched. It's best of course to match the rail to the accessory for the best fit.