Picatinny vs Weaver

I need advice on the difference between a Picatinny rail and a Weaver rail... are they interchangeable?

Q: I’ve got two questions:
1: What’s the difference between a Picatinny and a Weaver rail?

2: Are they interchangeable?

CHRIS PARKIN replies: Weaver rails are effective and well mannered but dimensionally different to a true 1913 military specification Picatinny rail. The size of the grooves across the rail, their exact spacings and location are not equal. Also, the claw feet on Picatinny rings are bulkier, often binding on the action of a Weaver rail-equipped rifle when its rail is thinner than its Picatinny rival. Even though the claw feet look identical, the recoil lugs on the underside of some night vision and thermal optics are too deep to fit the grooves on a Weaver rail or copies and, when tightening, this causes stresses in the scope’s base that can destroy your reliable zero retention. I prefer the Picatinny system because it is standardised and totally reliable from one international manufacturer to the next.

Weaver, and it’s companions, can be pretty much whatever they like, other than the width of the dovetails, and when microns matter with long-range optic precision, don’t make the mistake and make do. Even smaller items which have lateral bolts to anchor in the grooves for recoil security, will often not fit anything other than genuine 1913 Picatinny rails so be cautious and never assume they’ll fit.

When you spring the levers, or tighten the fastenings on any accessories, they should meet a fairly precise firm ‘thump’ as they bind up tight with little more than a half turn required to reach the appropriate torque (I use about 5Nm on Picatinny). If it feels as if you are squeezing something together, stop and check all tolerances because overtightening a Picatinny ring on a Weaver rail can rip its bolts out of the top of the action if the claws bind.