O.A.L. discrepancies

[Sth Oz Dan]

I've just revisited my O.A.L. measurements with Barnes 26gn VG bullets.
Previous length using the Hornady tool and Hornady modified case was 2.275" with the bullet touching the lands.
Today's measurement was 2.280"
No worries, I've read that throats erode a little over time and usage, so that's all good I suppose.

So I grabbed a case that's been fired in my rifle a few times and only ever neck sized, and seated the bullet at 2.285" (no powder or primer), chambered it, and there was absolutely no resistance on closing the bolt. Ejected - still 2.285". I expected there'd be a little difference between the Hornady modified case and my once/twice/thrice fired case, so I knocked the bullet out and did it again. O.A.L. 2.295" same result. Eventually reached the point where there was resistance on closing the bolt and the round measured 2.315"
So the discrepancy between the two methods was 35thou. Didn't expect that much difference.

Some of you may have come across this issue previously??, but I thought it was worth a mention.
I guess the difference lies around the neck and shoulder junction.
Shoulder diameter on the Hornady case is 0.357", and mine is 0.362"

I guess the best way to get an accurate measure, is to do as some of you guys do - take that fireformed case, and thread the head to fit the O.A.L. guage tool.

[Darkker]

It is the Ogive that contacts the lands, not the nose.
Yes there are variations, so the oal can move.

[Rick in Oregon]

I guess the best way to get an accurate measure, is to do as some of you guys do - take that fireformed case, and thread the head to fit the O.A.L. guage tool.

Dan, if you go that route, you'll need a 5/16"-36 tap....not a common critter. Usually found at tooling supply houses/machine shops. Unless you have a lathe to do the drilling/tapping, it's easier in the long run to just either buy a modified case, or if a wildcat, just send a fireformed case to Hornady for the work to fit the tool. JME

[Sth Oz Dan]

Thanks Rick - I searched back through posts and found this info (5/16 36) and you're right - it is rare.
The local tool guy had a big list of taps and it wasn't even on there. He rang his supplier and they said they have them and they only cost $50
So I bought a whole tap and die set for $40 and decided to re-thread the O.A.L. gauge. Seemed the cheaper option, as then I could tap all future cases for a common thread.



Seems I'm stronger than that gauge.





A lathe certainly would've been useful.



And something to hold the case properly would've helped too. It ended up way too far from concentric. Gave it another go by drilling a 10mm hole in some pine, sawing across that hole, then gripping the case in this with the bench vise. Much better, but the gauge itself was a little off centre too. It just wouldn't sit right in the chamber.

I ended up re-adjusting my neck sizing die to just grip the bullet without giving too much tension. Chambered it with the bolt and measured that a few times and came up with 2.292" O.A.L. consistently.
The 2.315" measure I got, was with a case that had proper neck tension, and so must have jammed the bullet.

So I've learned that not only do I need the $50 tap, but I also need a lathe - please sweetheart

[bazz]

just for the record are you using a comparator its easer to get consistence measurements cheers bazz

[Sth Oz Dan]

are you using a comparator

Yep. Every time I take a measurement, whether it's for max O.A.L. / testing different seating depths etc, I record both base to tip (O.A.L.) and base to ogive. I've started recording these two for each bullet too (no case, base of bullet to tip and ogive) to refer to in case accuracy changes with a new lot.

[Wrangler John]

Measuring to the lands with a Varmint Grenade is okay, nothing wrong with doing it, the problem comes with seating the bullet to the lands.

Barnes notes in it's data and my testing confirms that seating to the lands will not produce the best accuracy. In every instance with the 26 grain .204, 36 and 50 grain .224, and 62 grain 6mm Varmint Grenades, seating well off the lands produces the best accuracy, at least a .020" jump. I ran groups with each seated deeper in .010" increments until the sweet spot was found. Best accuracy with the .204 26 grain Varmint Grenade in my barrels is when they are seated at a COAL of 2.260". This applies to a Shilen 1:12" twist or Pac Nor 1:8.5" twist barrel (both 24" Savage Varmint contour, both cut with a standard SAMMI chamber reamer. Both barrels shoot the same sub .5" groups using the same load of 25.2 grains of Rl-10x in Winchester cases with the Federal 205M or Remington 7-1/2BR primer (cases are FL sized and only have their primer pockets uniformed). I use the RCBS Competition seating die with the sliding bushing and side window for seating the .204" bullets, otherwise I drop as many on the floor than I seat (hands like hams).

Because of variation in the length of Varmint Grenade bullets I do not measure COAL to the bullet tips. I select the shortest one (there are usually batches of three or four lengths in any box) and seat that one to the 2.260" depth measured to the tip, then measure the COAL to the ogive with a comparator, then seat one of the longest dimension with the same die setting, and check the comparator COAL, usually they will be the same. Thereafter, if I have to adjust the seating depth for any reason I use the comparator number to reset depth. Also, if the bullet tip touches the end of the seating stem, I drill it deeper, and polish the taper until it contacts the somewhere above the ogive.

This propensity for a finicky seating depth also extends to the Barnes monolithic copper bullets. I found the 80 grain TTSX .257 BT bullet would not shoot until seated at exactly .050" off the lands. Any variation on either side would dramatically open groups. Same with the 150 Grain 7mm TXS BT.

I have found that the Barnes Varmint Grenade bullet line is about the most accurate available. The astounding performance on varmint is awesome - that 6mm simply makes wet alfalfa filled sage rats completely vanish - traveling to a dimension beyond sight and sound.

[Sth Oz Dan]

Good info there WJ - thanks
I noticed yesterday when loading a few that there was as much as 4 thou difference in O.A.L. with same seater setting. I'm at about 2.266", or 1.966 to ogive (26thou jump) in front of 28.7gn BM2. Yesterday this shot 0.85 from 5 shots. If I only count the last 3, it was 0.27"!

I realise the 204 tends to like a big jump with pretty much any bullet. But while testing Trailboss, that depth wasn't working. I just wanted to see if the slower bullet preferred no jump. It did perform much better when almost touching the lands.

[Bodei]

Seems like alot of work. Glad my rifle likes everything at 2.26! So far...