Inaccurate measurements from Stoney Point OAL Guage

[firstshot425]

I have one of the older "Pre Hornady" Stoney Point OAL guages. It is the one with the curved handle and a heavy metal wire for moving the bullet forward. For years, I have been using this guage along with a sinclair comparator tool to measure dist to lands and have never questioned the measurement results when actually seating bullets. I just took the OGIV measurement and subtracted 0.010 and used the result for seating the bullets. All the time thinking I was seating the bullets 0.010 off the lands.

Just on a wim, I took a 120g NBT and put it in the OAL guage, pushed it out a ways, measured the OGIV with the comparator tool and got a measurement of 2.030

I then used a hot glue gun to anchor the bullet to the modified OAL guage case, unscrewed the case & anchored bullet all the way off the OAL guage and measured it with the comparator tool. Low and behold, I got a measurement of 2.290. A 0.010 difference. What the heck?

I was able to duplicate this with several other bullet types. 145 Nosler SPT, 140 Sierra SPT, 140 Nosler Accubond and 120 Sierra SPT. In each case the actual OGIV (case NOT on the tool) was 0.010 less than the measurement with the case on the OAL guage.

In trying to figure out why, I noticed that when measuring with the case on the OAL guage, the metal wire that pushes the bullet forward was preventing the rear blade of the caliper from measuring at the center of the case head. In other words the thickness of the metal wire was raising the rear blade up slightly so that it was actually measuring on a slight angle instead of perfictly centered on the case head.

So, I'm thinking that all this time when I thought I was loading bullets 0.010 off the lands, I was actually loading them 0.020 off the lands.

Does this make sense? Has anyone noticed this before? Can anyone out there with this type of OAL guage duplicate my findings?

Thanks
firstshot

 

[firstshot425]

I just measured DTL for a 140 Sierra SPT using the stoney point OAL/sinclare comparator and got 2.207. (BTW this is the exact meaurement I got when I previously measured 10 random bullets)

So instead of backing off 0.010 as I normally would, I seated the bullet in an empty case to exactly 2.207. I then used a black Sharpie to blacken out the bullet, loaded it in the rifle, and closed the bolt. As I closed the bolt, I didn't feel any resistance as if the bullet had hit the lands and upon close inspection under magnifying glass, there were NO rifling marks at all on the bullet.

 

[higgite]

Hornady sells a bullet comparator body that clamps to your calipers like the Sinclair does, but it is offset from center just enough to account for the offset in the OAL gauge. The Sinclair tool doesn't have that offset built in, so you're right, you do measure at an angle with the Sinclair comparator body.

 

[firstshot425]

higgite

Thanks for the info. Glad to hear that I'm NOT going crazy (at least not any more than I've always been). I suppose I could spring for one of the new Hornady set-ups, but as long as I know the Stoney Point measuring 0.010 longer than actual I can deal with it.

I'm going to do some more testing with other bullets but it looks like I can just seat bullets to the OAL guage measurement and be 0.010 off the lands.