What's the difference between weaver and picatinny rails?
The first thing to point out about weaver vs picatinny rails is what's the same. They have the same profile. If you were to look at them face on or in a cross section, they are identical.
Well if the cross sections are the same what's the difference?
Unlike dovetail rails, the picatinny and weaver rails use notches to help fix an accessory in place and keep it from sliding around. The difference between the two systems is the size and spacing of those notches.
Weaver rails are the older mounting rails from which the standards were created. Weaver rails had a notch/grove with a width of 0.180". The notches were placed wherever they were needed based on a specific accessory. They were usually being designed with a specific accessory in mind rather than to be a Rail Interface System (RIS).
The term 'Picatinny' refers specifically to the US military specification Mil-SD-1913 adopted in 1995. This spec also became the NATO spec STANAG 2324. This standardized the notch width and spacing so that any accessories could be easily mounted or moved from platform to platform. Picatinny Rails have a notch width of 0.206", just slightly larger than the weaver. The notches are also uniformly spaced with a distance of 0.394" from the center of one notch to the next. the term 'weaver' then can refer to any rail with the same profile but not to the precise military specs for notch size and spacing.
What's all this mean for paintball?
Accessory rails on paintball guns really didn't get popular until 2006 or so. This is well after the adoption of the Mil-SD-1913 Picatinnly rail standard. So, just about every rail you see on a paintball gun now is a Picatinny rail.
Many people are confused about the true difference between the picatinny and weaver rails. And they do have the same profile so many accessories will work on both. If you're looking at paintball markers or paintball acessories, regardless of whether they say 'weaver' or 'picatinny', they are MOST LIKELY designed to the picatinny spec.
If you're looking at firearms, and accessories originally designed for firearms, then you may need to look a little more closely at what you've got. For at least one manufacturer I know, they specifically refer to all of their rails and acessories as weaver. They are all designed to the picatinny spec and are compatible with the picatinny system BUT, because they haven't had the products 'certified' to military specifications, they don't want to say 'picatinny' and imply that they are certified. At the same time a manufacturer might call their rail a weaver rail because their notches are no where close to the picatinny standard spacing and truly are weaver rails.
As for accessories . . . will it fit? If an accessory description says it's for a weaver rail then it will probably WILL fit on weavers or picatinny rails. Even if it uses the thinner notch of a weaver you'll be able to get it to mount to the wider notching on a picatinny. As for spacing, most current accessories will use the picatinny spacing. If the accessory description says it's for a picatinny rail then it probably WILL NOT fit on a weaver rail. The fatter mounts will not fit in the thin notches and the spacing will probably not match your specific accessory designed weaver rail.
If you're looking for accessories the real question is what rail do you have on your gun or marker. If you're looking for a new gun or marker then you can fairly safely assume that whether the rail is called weaver or picatinny, if it seems to have evenly spaced grooves then it's designed to be a picatinny rail.
Need some visuals? Watch our video explanation using a Tippmann Cronus paintball marker.