I was all excited to get a rifle set up the other day with a new optic.  I mounted a Razor 1-6 in a low PRI mount.  I was a little put off when I found that the 'recoil lug' that locates the mount longitudinally doesn't come close to taking up the whole space of the Picatinny slot.  Reminded me of an EoTech in similarity--the EoTech (and this PRI mount) can slide fore and aft quite a bit, until the mount is tightened down.  I'm sure there is a reason for this, and that the tightening method of the mount is plenty strong to keep the optic and mount from moving, but why wouldn't the manufacturer make the recoil lug the same size as the Pic. slot?  It's not like the dimensions are a secret.  Is there enough variance in Pic. dimensions that mount companies feel it might limit what receivers it could go on?

If I remember correctly--proper mounting for something like this is to slide the mount as far forward in the Pic. rail slot as possible before tightening, correct?

Thanks

Yes, it’s due to tolerances that the lugs aren’t an exact fit. Simply push forward while installing, like you mentioned, and you’ll have no issues.

There is a "spec", but I have seen handguard rails side by side and the differences were quite significant.  It is more noticeable on a longer rail, after about 15 slots and there is a 1/4" difference in the overall length

Optics people don't want to be hamstrung on their mounts so they err to the minimum.

A narrow lug lets them fit Weaver as well as Picatinny bases.  Ideally there would be separate mounts for each, but most manufacturers choose to use one mount/ring for both.

It does not matter really. I have a Larue on an AR that shoots .7 MOA average at 300 yards. The mount does not give a damn if you mount it fore or aft. It ends up in the middle after a shooting session. It still shoots to the same POA and groups range from .5 to .9 MOA at 300-400 yards.

If the mount clamps hard enough the mount won't need the recoil lug because it won't move.

I prefer mounts like the Reptilia AUS that only have 1 recoil lug. Very seldom will 2 recoil lug mounts both make contact with the rail.

And the LT204 from Larue. It has a single recoil lug at the rear. It should make contact and be seated forward before clamping. If you see that the lug is no longer contacting the rail after some shooting, you probably did something wrong.

100% FALSE

Go check your LT204 right now with any 20+ oz scope mounted. The rear lug is visible. There is a 99% chance it is not seated fore RIGHT NOW if you have shot any significant number of rounds through it. This is well known to happen with the LT204. I have multiple LT204 and LT104 mounts.

I have tested mounting the scope all the way fore and when I noticed the lug not all the way forward where I put it, I tested all the way aft and it did not stay there either nor did it change tbe behavior or accuracy of the rifle. I'm sure it happened with LT104 too, but I could not see the double lugs hidden under the mount.

It seems the recoil of the rifle firing pushes the scope mount forward, but the gun going into battery pushes the gun forward / the mount back. If you clamp the levers hard enough to stop the slow drift to center, you can't remove the scope mount without tools to get the lever to start moving. Even when I tune the lever tension to be right at the threshold of being able to remove by hand... after one or two range sessions my LT 204 mounts have drifted a tiny amount to center. like the width of a playing card or less from whichever lug I mounted against.

As I said before, it also seems that it does not matter. Rifle still shoots great to same POA.