Thread: Seating depth measurement problems

I've got a 7 SAUM I am developing a 162 a-max load for. I am trying to work out the seating depth to get the projectile just off the lands. I started by seating a projectile in an empty case, hooking it behind the extractor and closing the bolt, progressively seating deeper until I couldn't see rifling marks on the projectile.

I then picked up a Hornady modified case to use with the Hornady LNL OAL gauge tool (thinking it would be more accurate). Using the Hornady tool the distance from the base of the case to the ogive of the bullet is 20thou shorter than using the first method.

I'm not sure why there would be such a big difference, any ideas?

The first method you tried is quite unreliable, it takes quite a bit of pressure to leave marks on the bullet and if the case isn't tight enough, sometimes the lands can grip the projectile and pull it out a little.
I found blacking the projectile with soot from a candle flame and seating shallower, looking for the land marks to be more reliable.
I would trust the OAL gauge more than the first method you tried but the candle method is also very good.
Which ever method you use, do it a few times to make sure your results are repeatable!

Last night I built my own Modified case. I drilled out a primer pocket to fit a cleaning rod, sized the neck of the case and than took to it with my Dramel and cut three cuts down to the shoulder, sized it again to be sure the neck was good.

I can now put my projectile of choice in the case by hand(but still firm). I then insert the case, with projectile to the chamber and gently push the case in with the cleaning rod, then put the rod down the primer pocket hole and insure the projectile is alway the way forward. Gently remove the case(with projectile) and measure the overal length.

I'm consistancely get the same measurement to 0.003".

I've also use the cleaning rod(with a square/flush end) down the bore to the bolt face, mark the rod at the end of the barrel. Then place a projectile gently into the chamber and do the same thing with the rod. Measure between the two marks on the rod and that should be the total overall length. I got a very simalar length with this method and was very repeatable once I got the method fine tuned.

I'll get some photos.

Marks on the bullet CAN be differcult to define sometime.

Check your mag length too!

I get very consistent measurements with the Hornady tool. But 30 thou is a reasonable amount and should leave a good sized mark on the projectile. I have done the same process with my 7mm mag and the two methods agreed within a few thou.
I did notice with the 7mm mag that with the Hornady tool the projectile seemed to stick a bit and could then be pushed a bit further. I am not noticing this with the 7 SAUM

Its because amaxes are rubbish. Ill take them off your hands if you like.... lol

Give Gunworks a ring, they are supposed to be getting some more in later this year.

I made a budget version of the Hornady OAL gauge out of a fired case and an old telescopic car aerial and a nut from a dyna-bolt as my comparator.

With it I get near 100% repeatable results...and that's all that really matters - Its just a way of comparing one seating depth against another when I'm developing a load, as long as the measurements I get from it can be used to compare one loaded round V's another that all that matters to me.

Basically I use it to establish a baseline and then alter from there (reduce OAL by .005", .0010", .0020" or what ever) and shoot a group until you find the optimum OAL that gives you the best result. Then always use the same tool/s to determine it again in the future if you need to measure it again. I normally keep a few dummy rounds aside loaded to the optimum OAL to reference later on if need be for setting up dies etc..

Once you have your rounds shooting well the distance from the lands becomes irrelevant, the OAL from base to ogive is the most important measurement to know.

kj

It's interesting that you get the same results with the 7mm Mag but different with the SAUM.

Perhaps with the Hornady tool the bullet isn't stopping on actual rifling that will make grooves. Maybe fouling or other roughness in the throat ?

(This is logic, not experience)

Is the OAL of the modified case the same as a once fired one? Maybe enough to cause the .0030" difference in OAL between the two?

size your case and cut it with a hack saw a wee bit, until the proj is a firm fit, also chamber slowly so don't bounce the proj into the case

Originally Posted by Smiddy

size your case and cut it with a hack saw a wee bit, until the proj is a firm fit, also chamber slowly so don't bounce the proj into the case

+1, thats what i do, seems to work fine, with repeatable results. I do it 10 times and record the OAL to ogive of each then work out the average. I dont worry about removing the ejector or anything.

Originally Posted by kimjon

Is the OAL of the modified case the same as a once fired one? Maybe enough to cause the .0030" difference in OAL between the two?

I've wondered about this, will have to get the old eyeometer out when I get home, I tried your method of using a nut as a headspace gauge but don't have any that are taller than the neck.

Hi Stug - interesting to read this as I have had exactly the same problem as you discribe.

Each to their own and as you have read above, there are many ways to do this.

The method I use on my Model 700 is to firstly remove the ejector plunger and spring from the bolt face, take a fired case, push a projectile into it and close the bolt.

Measure BTO length gently, then seat projectile in sized case a few thou further out.

Polish bullet ogive with scoth brite verticaly leaving smooth dull polished vertical lines.

Chamber dummy round and look for marks from lands, progressivly test this and seat bullet 1-thou deeper at a time until marks no longer appear and just touch is found.

One you have your press set, repeat with a new case and projectile and test. Now you should have a perfectly sized dummy round that you can use for future comparative measurments.

Originally Posted by stug

I've wondered about this, will have to get the old eyeometer out when I get home, I tried your method of using a nut as a headspace gauge but don't have any that are taller than the neck.

Hi Stu

This is what I use to determine head space, pretty low budget - but once again 100% repeatable, so it does the job.

The nut was off a concrete dyna-bolt and just happens to sit firmly around the neck, and quite nicely on the shoulder of the case and is longer than the neck, allowing me to compare one against another when setting up my dies.

I've found the easiest way is to measure, then zero the vernier. From there it will show up as a variance (+ or -) in head space e.g 0.002'' etc..no maths required.

kj