Can someone please tell me which is correct?

Double shotguns have a swamp in the barrels. Meaning there is a very slight curve down the barrel from the breach to about mid length. Then there is a slight curve upwards to the muzzles.

If you measure the drop at comb and at heel with a long precison straight edge that contacts the rib at the muzzles - not resting on the top of the bead, you will get a certain measurement for the drop at comb and at heel.

But if you use a shorter straight edge that only extends to the mid point of the rib on the barrels. You will get different measurements for drops at the combs and at heel. The measurements might be as much as 1/4 inch MORE at the heel.

I assume the long straight edge method is proper? Thanx.

NE - you assume correctly. The drop measure is from "line of sight." An easy way is to lay the shotgun upside down on a large, flat table and measure up from the table top to the stock.

Also make sure the front bead is hanging off the edge of the table so you don't get a false measurement

CJ

If the gun has a mid-bead, just prop the front and rear of the rib on a pair of wood blocks, measure from the comb and heel to the table surface, and subtract the thickness of the blocks.

Is line of sight at the base of the front bead or over the top?

I've always used the base of the bead/rib surface for the front and the rib surface at the reciever.

I've seen some ribs below the barrels at the muzzles, so laying it on a table would not give an accurate account of that type.

Also, I have checked my dinner table and breakfast nook table for flatness. Only certain parts of them are acceptably flat for measuring a gun for drop. It's amazing how curved some table surfaces are. Frankly, I like using a straightedge to get a good reading.

And once we've determined these numbers to our satisfaction, what do they mean?

Do you like talking in riddles?

I would say the these drop measuring procedures would be correct, but you said in your first post that there is a slight curve upward to the muzzles. Someone might want to correct me if I am wrong, but not all barrels bend upward. If I remember correctly, the Model 21s made prior to 1960 had a slight downward bend which made them shoot "dead on". This was changed in 1960. I don't know how many other makes were made in this manner other than the 21s. But I think the earlier 21s were.

Mike,
Msasurments like this are useful in comparative assessments by us in determining whether or not a gun will shoot well for a given shooter with known positive experience with a set of dimensions. However, there are some variables that the basic drop/LOP/cast don't account for. Granted, they are more subtle than the big basic dimensions above. But you knew all this.

None of my SxS's have a swamped rib.

Somewhere, I saw a picture of how to measure drop with a string. The basic method involved standing the gun vertical (unloaded of course) elevated off the floor a few inches on a couple books or blocks, tie a small weight (large nut or other weight) on a string, hold the string against the rib at the muzzles, adjust the angle of the gun until the string just bears against the rib at the breach end, and measure distance from string at comb and heel.

Sounds a bit difficult to me. I have a long straightedge that will do the job.

I have a Paul Hodgins drop gauge (he's a stock maker in Utah and apprenticed at H&H). His gauge is similar to others you see advertised for sale - a long vertical flat bar of aluminum with drop down scales at the stock end for measuring drop.

On the underside of the gauge, along the length that would generally fit against the rib, there is a grove into which the beads would go, allowing the gauge to fit flat on the rib.

Regardless of whether or not the barrels are swamped, you measure drop from the surface of the rib.

As it turns out, the Cabelas in Ft Worth, TX has one of his gauges in the Gun Room and it may be that others do too, for anyone close by a Cabelas that might like to see one.