perplexing OAL problem

[JustinP]

Gents, Ladies

I've struck a weird problem...

Today I set about establishing the OAL of loaded rounds in my new f-class,.223rem). Action is barnard 'P' and krieger match tube, chambered and fitted by benchrest gunsmith down here.

The two projectiles I looked at were the Nosler 80gr VLD and the Sierra 80gr matchking. Measurement were taken using a sinclair comparator on a mitutoyo digital caliper and a stoney point/hornady OAL gauge tool. Comparator insert and all other tools were undamaged and working just fine.

I took a random sample of 10 nosler projectiles and first measured them from base to ogive. Runout/variance was about .001". The same exercise with the Sierras gave a runout/variance of up to .004"

I then took OAL readings using the SP/hornady tool, using the same technique each time,and as things wore on, I got even more anal about doing it the same way each time...)

OAL readings for the Noslers were:

1.844"
1.843"
1.843"
1.843"
1.843"
1.843"
1.844"
1.845"
1.846"
1.845"

Not bad, given it's not an exact science. Then, onto the Sierras, which gave THIS result:

1.840"
1.827"
1.837"
1.824"
1.839"
1.833"
1.823"
1.839"
1.837"
1.828"

That is a variation/ES of .017"...! So, I pulled out 10 more and did the same thing - and the result was the same see-sawing measurements.

I then pulled out some more Noslers again, as an error check against my technique. Absolutely no issue, measurements were the same as the first sample.

Does anyone have any ideas on what the problem might be? The sierra projectiles did have a bit of variation from base to ogive,~.004") but I didn't expect to see such a seating depth variation....

Justin

 

[MSwickard]

It is a comparator. Each tool you use will not have the same diameter hole in which the bullet fits into. Therefore different measuremnts. Each comparator is referencing off a different location on the bullet. You would have to have the exact same bore on each coparator to get the same measurements. Remember a comparator is just a referencing tool.

Mike

 

[tripcrow]

Are the Sierras compressing the powder load any?

 

[MSwickard]

One aspect I forgot to mention is when using just one of your comparators and you are getting different measurements from one box of bullets....ie Sierra, more than likely you are getting bullets that were made in different dies and or different machines.

Mike

 

[Cheechako]

Justin

When you say that the OAL varies, do you mean the base to ogive length or the base to tip of bullet??

If you have consistent base to ogive that is really what is important.

On the other hand, if you are getting inconsistent OAL, then measue a few bullets and you might be surprised how much different they are in length because of inconsistent meplats. That could account for all of your problem.

Inconsistent meplats are fairly common, even on match grade bullets. Some shooters think it's not important while others will get a tool to uniform them.

Ray

 

[Bronsin]

I have used all of the tools for checking these dimensions and have gotten tired of all of the conflicting measurements. Now I just seat the bullet long and chamber it in the rifle. The rifling will put marks on the bullet where they touched. I then use 0000 steel wool to remove the marks, set the bullet seater .005 shorter than the measurement I got to the ogive, and re-chamber the round. If it still has marks, I repeat the process until it doesn't. That ogive measurement then becomes my zero.
If you are measuring to the tip of the bullet, the readings will be all over the place. Use your Sinclair comparator to check the length to the ogive and you will get much better results.

 

[JustinP]

Thanks for the feedback guys

I've taken two sets of readings:

The first was to check bullet base to ogive readings on both Sierra and Nosler projectiles, using a stainless .224 comparator insert attached to my calipers,0.0005" mitutoyo digitals). As stated, the Noslers were pretty uniform,ES ~ 0.001") and the Sierras were a bit more of a mixed bunch,ES ~0.004"). No real problems and everything according to plan...

I then used a modified .223 case on a hornady/SP OAL gauge, with said projectiles and measured from case head to bullet ogive. This is where everything went weird with the Sierras.

Case head to ogive measurements were uniform with the Noslers,within .003" across ten measurements) and all over the place with the Sierras,up to .017" across over ten measurements)

I was wondering if uneven meplat thickness had something to do with it?

Also, I've measured the OD of a sample of the projectiles and the diameter readings on the bearing surface sees a pretty uniform .2235" for the Noslers and .224" for the Sierras.

Perhaps the OAL gauge tool or modified case or my technique is for crap - but there was no problem with the Noslers...

AARGH